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Frequency and intensity of defoliation, dry matter production and net photosynthesis in grass and legume… Vickery, Peter Joseph 1970

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FREQUENCY AND INTENSITY OF DEFOLIATION, DRY 'MATTER PRODUCTION AND NET PHOTOSYNTHESIS IN GRASS AND LEGUME FORAGES  PETER JOSEPH VICKERY University of Sydney, i960 M.RUT.Sc., University of New England, 1964 Be So. Agr.  f  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy in the Division of  Plant Science We accept this thesis as conforming to the required standard  THE UMIVERSJTY OF BRITISH COLUMBIA January 1970  ii  In presenting this thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t f r e e l y available f o r reference and study.  I further agree that permission for extensive copying of this  thesis f o r scholarly purposes may be granted by the Head of my Division or by his representatives.  It i s understood that copying .  or publication of this thesis f o r financial gain shall not be allowed without my written permission.  P. J . VICKERY Division of Plant Science The University of B r i t i s h Columbia Vancouver 8, Canada  ABSTRACT The investigations reported in the thesis were designed to discover the importance of leaf area remaining after defoliation for subsequent forage regrowth.  In. f i e l d t r i a l s with a wide range of temperate  grasses the effects of differing frequencies and intensities of defoliation on dry matter production were examined.  The results of  these experiments showed that lenient defoliations at 3 inches (designed to leave photosynthetic leaf area after defoliation) did not in fact result in increased forage production compared with defoliations at 1 inch.  The low production with defoliation at 3 inches was accomp-  anied by a depression of the clover oontent in the stand. For more detailed investigations of the light use, leaf area index (LAI) and net photosynthesis relationships,orchardgrass ( l 2 a c t y l i s glomerata L.) and white clover (Trifolium repens L.) were chosen.  Growth and net photosynthesis, of the two selected species,  were measured under a number of temperature regimes with other major environmental factors at nearly constant and optimal levels.  Both  species grew optimally at 25°C; while net photosynthesis was  ,<  maximal at or below 15°C in orchardgrass and maximal at 20°C in white clover. The two species were established in pure.and mixed stands in the f i e l d and subjected to defoliation managements, similar to those i n the original  experiments.  The yields again confirmed the earlier result that defoliations at 3 inches, compared with 1 inch, resulted in lower yields and.  :  reduction in the clover content of the stands.  iv Samples of the forage stands and attached s o i l were removed to the l a b o r a t o r y f o r the determination of net photosynthesis at a number of l i g h t energy  ( 4 0 0 - 7 0 0 nm) l e v e l s .  The LAI and dry matter of these  forage stands and t h e i r components were then determined.  Prom these  data net photosynthesis at v a r y i n g LAIs was compared between stands of c o n t r a s t i n g managements and b o t a n i c a l composition. The r e s u l t s of these l a b o r a t o r y i n v e s t i g a t i o n s showed that there were d i f f e r e n c e s i n the net photosynthesis-LAI response between the d e f o l i a t i o n managements only i n the pure orchardgrass stands.  With  orchardgrass stands d e f o l i a t e d at 1 inch the r e l a t i o n s h i p between net photosynthesis and LAI was l i n e a r up to an LAI of 1 7 .  However, with  orchardgrass stands d e f o l i a t e d at 3 inches the response xras c u r v i l i n e a r and the optimum LAI was about 1 2 .  In the pure white c l o v e r stands  there were no c l e a r l y defined optimum LAIs or d i f f e r e n c e i n response between the stands with d i f f e r i n g managements.  The g r a s s - c l o v e r forage  stands showed an optimum LAI of 1 0 and the presence of some c l o v e r may have n u l l i f i e d an e f f e c t from the d e f o l i a t i o n managements. The data show that the l i g h t  i n t e r c e p t i o n - L A I theory was a p p l i c a b l e  to grass stands under l e n i e n t d e f o l i a t i o n .  The lower leaves, l e f t  after  a d e f o l i a t i o n , d i d not c o n t r i b u t e g r e a t l y to the stands a b i l i t y to a s s i m i l a t e carbon dioxide and f i n a l l y they became " p a r a s i t i c " r e s u l t i n g i n the stand having an optimum LAI of about 1 2 .  When the d e f o l i a t i o n  was heavy there was no evidence of leaves becoming " p a r a s i t i c " . Frequency of d e f o l i a t i o n at 1 inch had l i t t l e  influence on y i e l d despite  d i f f e r e n c e s i n the increase i n net photosynthesis f o r a u n i t increase i n LAI.  V  Carbon dioxide f i x a t i o n e f f i c i e n c y was c a l c u l a t e d from the l i g h t energy response data.  The e f f i c i e n c i e s were s i m i l a r to those published  f o r s i m i l a r material and showed furthermore only s l i g h t d i f f e r e n c e s i n l i g h t energy use between the forage stands of pure grass, pure c l o v e r and t h e i r mixture.  The d i f f e r e n c e s between the dry matter p r o d u c t i v i t y  of the forage stands caused "by the management treatments appeared to be a r e s u l t of an i n a b i l i t y of the c l o v e r to produce a s u f f i c i e n t l y high LAI to compete under the l e n i e n t  defoliation.  vi TABLE OF CONTENTS 1.  INTRODUCTION  2.  REVIEW OF LITERATURE 2.1  Page no. 1 .  Frequency and I n t e n s i t y of D e f o l i a t i o n and the Dry Matter Y i e l d of Forage Stands 2.1.1  Leaf Area and Light Energy Interception .......  2.1.2  Carbohydrate Reserve Status a f t e r Defoliation Observations of Dry Matter Production of Forage Stands .............  2.1.3  2.2  Net Photosynthesis and L i g h t Energy Conversion i n Forage Stands 2.2.1  2.2.2 2.2.3  2.2.4  3 ' 3  10 15  24  Factors Influencing Net Photosynthesis of Leaves w i t h i n Forage Stands  24  Methods of Measuring Net Photosynthesis i n Forage Stands  30  Measurements of Net Photosynthesis and Production from Forage Stands „  36  The E f f i c i e n c y of L i g h t Energy Conversion by Forage Stands  3.  3  41  FIELD EXPERIMENT WITH DEFOLIATION BY ANIMALS ..............  45  3.1  M a t e r i a l s and Methods  45  3.2  Results...  48  3.2.1  Production of I n d i v i d u a l Pastures .............  48  3.2.2  Management E f f e c t s on Dry Matter  3.2.3  Species and Grazing Management Interactions ...  54  3.2.4  Dry Matter and Leaf Area Index R e l a t i o n s h i p s ..  54  3.3  Discussion  57  V l l  TABLE OF CONTENTS (continued)  Page no,  THE PRODUCTIVITY AND NET PHOTOSYNTHESIS OF ORCHARDGRASS AND WHITE CLOVER UNDER CONTROLLED ENVIRONMENTS ...............  62  4.1  62  4*2  M a t e r i a l s and Methods  4.1.2  Growth Measurement  4.1.3  Net Photosynthesis Measurement  Results 4.2.1  4.2.2  at Four Temperature Regimes .. ' 6 4 ..................  o-  «  - 67  The Influence of Temperature on the Production of a Vegetative Unit  67  The Influence of Temperature and R e l a t i v e Humidity on Net Photosynthesis  4.3  66  71  Discussion  75  4.3.1 4.3.2  75 81  Dry Matter Production Net Photosynthesis  FIELD DEFOLIATION MANAGEMENT EXPERIMENT WITH CONCURRENT MEASUREMENT OF NET PHOTOSYNTHESIS AND DRY MATTER PRODUCTION.. 83 5.1  M a t e r i a l s and Methods  83  5.1.1  Techniques and F i e l d Design  83  5.1.2  Pasture Sampling and the Measurement o f Net Photosynthesis  5.2  86 91  Results 5.2.1  Field  5.2.2  Light Energy Response of Net Photosynthesis  I n v e s t i g a t i o n s ............................  91  i n the Forage Stands ............................  95  5.2.2.1  Pure Grass Swards ......................  98  5.2.2.2  Pure Clover Swards  112  5.2.2.3  Mixed Grass - Clover Swards  118  viii TABLE OF CONTENTS ( c o n t i n u e d ) 5.3.1 5.3.2  5.3.3  5.3.4  5.3.5  The I n f l u e n c e o f G r o w i n g S e a s o n E n v i r o n m e n t on N e t P h o t o s y n t h e s i s and P r o d u c t i o n  131  D r y M a t t e r P r o d u c t i o n and t h e I n f l u e n c e o f F r e q u e n c y and I n t e n s i t y o f D e f o l i a t i o n  133  The I n f l u e n c e o f F r e q u e n c y and I n t e n s i t y o f D e f o l i a t i o n on N e t P h o t o s y n t h e s i s o f t h e Forage Stand  138  The I n f l u e n c e o f B o t a n i c a l C o m p o s i t i o n o n Net P h o t o s y n t h e s i s o f t h e F o r a g e S t a n d  140  An Assessment  of the E f f i c i e n c y of Light  E n e r g y Use "by t h e F o r a g e A s s o c i a t i o n s 6.  7.  Page n o .  142  SUMMARY AND CONCLUSIONS ..................................... 145 6.1  The F i e l d E x p e r i m e n t w i t h D e f o l i a t i o n b y A n i m a l s  145  6.2  C o n t r o l l e d Environment S t u d i e s  146  6.3  N e t P h o t o s y n t h e s i s and D r y M a t t e r  Production  LITERATURE C TTED ............................................ 150  8.1  S o i l P h y s i c a l and C h e m i c a l D a t a C a n a d i a n E x p e r i m e n t s ... 168 8.1.1  8.2  Methods  G l o s s a r y o f M a j o r Terms  168  172  ix L I S T OP TABLES T a b l e no. 3.1 3. I I  Page n o . Mean y i e l d s o f s e v e n p a s t u r e s t a n d s  49 50  ' R a i n f a l l , monthly t o t a l s  3. I l l  Seasonal  3 . IV  The i n f l u e n c e o f g r a z i n g management on p a s t u r e  3.V  The i n f l u e n c e o f s e a s o n pasture production  3.VI 3. V I I  51  i n t e r a c t i o n s w i t h s o ™ g r a s s p r o d u c t i o n ...  and g r a z i n g management o f 53  The i n t e r a c t i o n s o f p a s t u r e s t a n d s and g r a z i n g management o n d r y m a t t e r y i e l d .....................  ;  55  Mean d r y m a t t e r y i e l d p e r g r a z i n g i n l b . / a c r e and L A I a t g r a z i n g f o r d a t a f r o m August 1 9 6 3 t o J a n u a r y 1965 t o g e t h e r w i t h c o e f f i c i e n t s f o r r e g r e s s i o n e q u a t i o n s r e l a t i n g d r y m a t t e r y i e l d t o L A I .........  '  4.1  T e m p e r a t u r e s i n °C o f e x p e r i m e n t a l r u n s  64  4. I I  Regressions temperature  4.Ill 4.IV  5.1 5. I I  5. I l l  5 . TV  5.V  and c o r r e l a t i o n c o e f f i c i e n t s and n e t p h o t o s y n t h e s i s  56  (R) r e l a t i n g 73  The n e t p h o t o s y n t h e s i s o r c h a r d g r a s s and w h i t e c l o v e r as i n f l u e n c e d b y r e l a t i v e h u m i d i t y  75  M u l t i p l e r e g r e s s i o n s o f t e m p e r a t u r e ( X ^ ) and v a p o u r p r e s s u r e d e f i c i t (X2) o n n e t p h o t o s y n t h e s i s (Y) i n o r c h a r d g r a s s and i n w h i t e c l o v e r ...................  76  S e e d i n g d e n s i t i e s o f g r a s s and c l o v e r f o r t h e e i g h t f o r a g e swards  84  The 1 9 6 7 above g r o u n d p r o d u c t i o n o f o r c h a r d g r a s s and w h i t e c l o v e r swards w i t h d i f f e r i n g s e e d i n g compositions  92  The i n f l u e n c e o f d e f o l i a t i o n management on t h e I 9 6 7 above g r o u n d p r o d u c t i o n o f o r c h a r d g r a s s and w h i t e c l o v e r p a s t u r e s and on t h e i r components  93  The i n t e r a c t i o n o f d e f o l i a t i o n management w i t h t h e pasture- swards i n t o t a l p r o d u c t i o n , g r a s s p r o d u c t i o n and c l o v e r p r o d u c t i o n ...................  94  M e t e o r o l o g i c a l d a t a d u r i n g t h e 1967 season  95  growing  X  LIST OF TABLES ( c o n t i n u e d ) Page no.  T a b l e no. 5.VI  The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g n e t p h o t o s y n t h e s i s ( Y ) t o L A I (x) i n o r c h a r d g r a s s swards a t s a t u r a t i n g , and f o u r o t h e r l i g h t e n e r g y l e v e l s .....................................  104  The e f f i c i e n c y o f l i g h t e n e r g y u s e i n CO2 u p t a k e i n o r c h a r d g r a s s swards a t f o u r l i g h t e n e r g y l e v e l s t o g e t h e r w i t h t h e i r CO2 u p t a k e and optimum L A I a t t h e s e e n e r g y l e v e l s ...............................  106  The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g t h e i n i t i a l s l o p e (Y) o f t h e l i g h t r e s p o n s e c u r v e s i n o r c h a r d g r a s s swards t o t h e i r L A I ( x )  108  The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g n e t p h o t o s y n t h e s i s (Y) t o L A I (X) i n w h i t e c l o v e r swards a t f o u r l i g h t e n e r g y l e v e l s ................  115  The e f f i c i e n c y o f l i g h t e n e r g y u s e i n CO2 u p t a k e i n w h i t e c l o v e r swards a t f o u r l i g h t e n e r g y l e v e l s t o g e t h e r w i t h t h e i r CO2 u p t a k e and optimum L A I at t h e s e e n e r g y l e v e l s  116  The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g n e t p h o t o s y n t h e s i s ( Y ) t o L A I (X) i n mixed o r c h a r d g r a s s w h i t e c l o v e r swards a t f o u r l i g h t e n e r g y l e v e l s ..  125  The e f f i c i e n c y o f l i g h t e n e r g y u s e i n CO2 u p t a k e i n m i x e d f o r a g e s t a n d s c o m b i n i n g o r c h a r d g r a s s and white c l o v e r , at f o u r l i g h t energy l e v e l s t o g e t h e r w i t h t h e i r CO2 u p t a k e and optimum L A I at t h e s e l i g h t e n e r g y l e v e l s .....................  125  8.1  A n a l y s e s o f 40 c o r e s o i l  170  8. I I  Field description  8. I l l  L a b o r a t o r y a n a l y s e s o f p r o f i l e s i t e s ....  5.VII  5.VIII  5.IX  5.X  5.XI  5.XII  composites  o f p r o f i l e s i t e s .................  171  172  xi L I S T OF FIGURES F i g u r e no. 4.2.1  4.2.2  4.2.3  4.2.4  4.2.5  Page n o . A d j u s t e d mean w e i g h t o f o r c h a r d g r a s s t i l l e r s and w h i t e c l o v e r nodes as i n f l u e n c e d b y t e m p e r a t u r e ...  69  A d j u s t e d mean a r e a o f o r c h a r d g r a s s t i l l e r s and w h i t e c l o v e r nodes as i n f l u e n c e d b y t e m p e r a t u r e  69  A d j u s t e d mean number o f l e a v e s , o n o r c h a r d g r a s s t i l l e r s and w h i t e c l o v e r r o o t e d n o d e s , as i n f l u e n c e d by temperature  70  A d j u s t e d mean w e i g h t o f o r c h a r d g r a s s and w h i t e c l o v e r l e a v e s , as i n f l u e n c e d b y t e m p e r a t u r e .......  70  A d j u s t e d mean a r e a o f o r c h a r d g r a s s and w h i t e l e a v e s as i n f l u e n c e d b y t e m p e r a t u r e  72  clover  4.2.6  A d j u s t e d mean w e i g h t s a r e a r a t i o o f o r c h a r d g r a s s and w h i t e c l o v e r l e a v e s as i n f l u e n c e d b y  4.2.7  The r e l a t i o n s h i p b e t w e e n t e m p e r a t u r e and n e t p h o t o s y n t h e s i s i n o r c h a r d g r a s s and i n w h i t e  4.2.8  The i n f l u e n c e o f v a p o u r p r e s s u r e d e f i c i t and t e m p e r a t u r e on n e t p h o t o s y n t h e s i s o f u n i t l e a f a r e a i n w h i t e c l o v e r and i n o r c h a r d g r a s s  4.2.9  5.1.1  5.2.1  ...  , ..  77  The i n f l u e n c e o f v a p o u r p r e s s u r e d e f i c i t and t e m p e r a t u r e on n e t p h o t o s y n t h e s i s o f u n i t l e a f w e i g h t i n w h i t e c l o v e r and i n o r c h a r d g r a s s ........  78  D i a g r a m a t i c r e p r e s e n t a t i o n o f chamber and a p p a r a t u s u s e d t o measure CO2 u p t a k e and r e l e a s e r a t e s b y h e r b a g e - s o i l c o r e s ...............  88  The i n f l u e n c e o f l i g h t e n e r g y o n n e t p h o t o s y n t h e s i s per metre^ l a n d .  99  5.2.2  The r e l a t i o n s h i p b e t w e e n L A I and days o f r e g r o w t h s i n c e t h e l a s t mowing f o r p u r e o r c h a r d g r a s s  5.2.3  The r e l a t i o n s h i p b e t w e e n L A I and w e i g h t o f herbage from u n i t a r e a o f l a n d f o r pure o r c h a r d g r a s s swards  101  xii  L I S T OF FIGURES ( c o n t i n u e d ) F i g u r e no. 5.2.4  5.2.5  5.2.6  5.2.7  5.2.8  Page no, The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e ^ l a n d and L A I i n o r c h a r d g r a s s swards at s a t u r a t i n g l i g h t energy l e v e l s  102  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e ^ l a n d and L A I i n o r c h a r d g r a s s swards a t s a t u r a t i n g l i g h t energy l e v e l s s u b j e c t t o a (9-3) d e f o l i a t i o n management .......................  103  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e ^ l a n d and L A I i n o r c h a r d g r a s s swards a t f o u r l e v e l s o f i n c i d e n t l i g h t e n e r g y ...............  105  The r e l a t i o n s h i p b e t w e e n t h e i n i t i a l s l o p e , o f the l i g h t energy response curves o f orchardgrass s w a r d s , and L A I o f t h e swards u n d e r t h r e e d e f o l i a t i o n managements ............................  107  The r e l a t i o n s h i p b e t w e e n n e t a s s i m i l a t i o n  rate  (STAR) o f l e a v e s i n o r c h a r d g r a s s swards and L A I  110  5.2.9  The r e l a t i o n s h i p b e t w e e n t h e l i g h t e n e r g y r e q u i r e d t o p r o d u c e 5 O 7 0 o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s i n o r c h a r d g r a s s swards and C v a l u e of t h e e x p o n e n t i a l equations f i t t e d t o t h e l i g h t energy response c u r v e s I l l  5.2.10  The r e l a t i o n s h i p b e t w e e n L A I and days o f r e g r o w t h s i n c e t h e l a s t mowing f o r p u r e w h i t e c l o v e r ' swards . 113  5.2.11  The r e l a t i o n s h i p b e t w e e n L A I and w e i g h t o f h e r b a g e from u n i t a r e a o f l a n d f o r pure white c l o v e r  5.2.12  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e ^ l a n d and L A I i n w h i t e c l o v e r  5.2.13  5.2.14 5.2.15  swards .........  114  The r e l a t i o n s h i p b e t w e e n t h e i n i t i a l s l o p e o f the l i g h t energy response curves o f white c l o v e r swards and t h e i r L A I  117  The r e l a t i o n s h i p b e t w e e n n e t a s s i m i l a t i o n r a t e (NAR) o f l e a v e s and p e t i o l e s i n w h i t e c l o v e r swards a n d L A I  119  The r e l a t i o n s h i p b e t w e e n l i g h t e n e r g y r e q u i r e d t o p r o d u c e 50?° of t h e maxim r a t e o f n e t p h o t o s y n t h e s i s i n w h i t e c l o v e r swards and t h e C v a l u e of t h e e x p o n e n t i a l equations f i t t e d t o t h e l i g h t energy response curves  120  X l l l  L I S T OF FIGURES  (continued) Page no.  F i g u r e no, 5.2.16  5.2.17  5.2.18  5.2.19  The r e l a t i o n s h i p between L A I and days o f r e g r o w t h s i n c e t h e l a s t mowing f o r m i x e d o r c h a r d g r a s s - w h i t e c l o v e r swards  122  The r e l a t i o n s h i p b e t w e e n L A I and w e i g h t o f h e r b a g e from u n i t a r e a o f l a n d f o r mixed o r c h a r d g r a s s w h i t e c l o v e r swards  122  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e ^ l a n d and L A I i n m i x e d o r c h a r d g r a s s - w h i t e c l o v e r swards a t s a t u r a t i n g l i g h t e n e r g y l e v e l s ...  123  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e ^ l a n d and L A I i n m i x e d o r c h a r d g r a s s - w h i t e c l o v e r swards a t f o u r l e v e l s o f i n c i d e n t l i g h t ' 124  5.2.20  5.2.21  5.2.22  The r e l a t i o n s h i p b e t w e e n i n i t i a l s l o p e o f t h e l i g h t energy response curves o f mixed orchardgrassw h i t e c l o v e r swards and t h e i r L A I .................  127  The r e l a t i o n s h i p between n e t a s s i m i l a t i o n r a t e (lIAR) o f l e a v e s and p e t i o l e s i n m i x e d o r c h a r d g r a s s w h i t e c l o v e r swards and L A I • ••0 a •• * • 0  129  The r e l a t i o n s h i p b e t w e e n l i g h t e n e r g y r e q u i r e d t o p r o d u c e ^dfo o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s i n mixed o r c h a r d g r a s s - w h i t e c l o v e r swards and t h e C v a l u e s o f t h e e x p o n e n t i a l equations f i t t e d t o the l i g h t energy response 130  8.1.1  - M o i s t u r e t e n s i o n r e l a t i o n s h i p s f o r s u r f a c e and s u b s o i l s i n t h e two p r o f i l e s i t e s  173  xiv ACKNOWLEDGEMENTS I w i s h t o acknowledge t h e a s s i s t a n c e graduate committee s and Dr.  D r . V. C. B r i n k  o f t h e members o f my  ( C h a i r m a n ) , D r . D. P. Ormrod"*  -  A. J.. Renney o f t h e D i v i s i o n o f P l a n t  B r i t i s h Columbia?  D r . L . E . Lowe, Department o f S o i l  U n i v e r s i t y o f B r i t i s h Columbia;  Science,  D r . D. J . W o r t , Department o f  B o t a n y , U n i v e r s i t y o f B r i t i s h C o l u m b i a and Dr. A g r i c u l t u r e Research S t a t i o n , Agassiz, The  Science, U n i v e r s i t y of  D. K. T a y l o r ,  Canada  B r i t i s h C o l u m b i a , Canada.  f i n a n c i a l a i d p r o v i d e d b y t h e Government o f C a n a d a  A i d O f f i c e t h r o u g h a Commonwealth S c h o l a r s h i p  External  and F e l l o w s h i p  Plan  award i s a c k n o w l e d g e d . The Mr.  .:  technical assistance  o f Mr. I . D e r i c s , Mr. D. P e a r s e and  D. A r m s t r o n g o f t h e D i v i s i o n o f P l a n t  Science U n i v e r s i t y of  B r i t i s h Columbia i s acknowledged. I n t h e r e s e a r c h a t t h e U n i v e r s i t y o f New E n g l a n d , New S o u t h Wales, A u s t r a l i a I acknowledge t h e a s s i s t a n c e  o f D r . B. R. W a t k i n ,  now  a t M a s s e y U n i v e r s i t y , P a l m e r s t o n N o r t h , New Z e a l a n d , i n p l a n n i n g  and  establishing the f i e l d  assistance Ghana.  I acknowledge t h e f i e l d  o f Mr. A. J . A g y a r e , now a t Ghana W o r k e r s B r i g a d e , A c a r a ,  The c o - o p e r a t i o n and f a r m l a b o u r o f t h e U n i v e r s i t y o f  New E n g l a n d , L a u r e l d a l e Finally and  experiment.  R e s e a r c h Farm s t a f f i s a l s o a c k n o w l e d g e d . -  I w i s h t o g r a t e f u l l y acknowledge t h e s k i l l e d  typographic assistance  technical  o f my w i f e , w i t h o u t t h i s h e l p t h e t h e s i s  w o u l d n o t have b e e n p o s s i b l e ,  1  Now Department o f H o r t i c u l t u r e U n i v e r s i t y o f G u e l p h  Ontario.  1.  INTRODUCTION The adjustment of defoliation in managing forage swards to achieve  maximum production, i s a well recognised objective.  However, in a broad  sense published results show no consensus in the optimal defoliation system.  Undoubtedly the reason for the lack of consensus i s the complex  nature of the problem as i t encompasses an entire ecosystem which, within i t s e l f , shows many interactions. During the past 30 years a number of hypotheses have been put. forward to account for the regrowth response of forage swards following different defoliation treatments.  The hypothesis of leaf area  and light interception regulating photosynthesis, and hence dry matter production, i s a comparatively recent contribution Monsi, 1954).  (Kasmaga and  The research reported in this thesis has been designed  to test this hypothesis in both the f i e l d and controlled environments. The f i r s t section of research work completed in Australia, was a f i e l d scale defoliation management t r i a l on grass-legune forage swards using sheep for the defoliations. "leaf area-light  The t r i a l was designed to test the  interception-photosynthesis hypothesis" by compar- .  ing the yields; from defoliation managements which varied both in the frequency and intensity of defoliation.  The results from the above .  experiment did not agree well with the hypothesis.  Investigations, i n Canada, were undertaken to c l a r i f y the situation. Two of the species used i n the Australian research v i z . orchardgrass Dactylis glomerata L. and white clover Trifolium repons L. were used and their growth and net photosynthesis was examined under controlled conditions,  The two species were grown alone and together i n forage  swards which were subjected to the same defoliation frequencies and intensities as i n the i n i t i a l experiment i n Australia.  The objectives  were three a) to compare the dry matter yield response to the management treatments i n an environment d i f f e r i n g substantially from the. Australian one, b) to provide material from known defoliation treatments so that the progress of leaf area, dry matter and net photo^ synthesis could be followed between defoliations and c) to follow the changes i n the net photosynthesis-light response curve as the swards regrew after defoliation.  This approach allowed further testing and  examination of the "leaf area-light interception-photosynthesis hypothesis" and provided possible reasons for the differences in the performance of the defoliation treatments.  3  2  2.1  J'-MJ^l  0 F  LITERATURE  Frequency and Intensity; of De f o l i a t i o n and t he^ Re growth Y i e l d of Forage Stands Regrowth of forage stands i s influenced "by the frequency and  i n t e n s i t y of previous defoliations(Brougham? 1955,1956).:  However,  u n l i k e environmental f a c t o r s which also influence regrowth r a t e s , frequency and i n t e n s i t y of d e f o l i a t i o n cannot e a s i l y "be described i n terms of optimal l e v e l s .  Undoubtedly t h i s i s a r e f l e c t i o n of the  i n t e r a c t i o n s of frequency and i n t e n s i t y of d e f o l i a t i o n with environment,and t h e i r influence regrowth r a t e s .  The extensive l i t e r a t u r e on  t h i s topic can be d i v i d e d into two major areas of concern a) the  ;  e f f i c i e n c y with which the l e a f area of the plant community i n t e r c e p t s l i g h t energy f o r regrowth and b) the carbohydrate reserve status of the plants. 2.1.1  Leaf Area and Light Energy Interception The r e l a t i o n s h i p between l e a f area, l i g h t i n t e r c e p t i o n and  subsequent growth i n plant communities has been based on the f a c t that the primary synthesis of dry matter i n most ecosystems r e s u l t s from the photosynthetic a c t i v i t i e s of the p l a n t ' s leaves i n u t i l i z i n g l i g h t to f i x carbon dioxide.  energy  The e f f i c i e n c y of t h i s process must depend, to a  l a r g e d e g r e e , on t h e s i z e , synthetic  s h a p e , p o s i t i o n and  Watson s u g g e s t e d t h a t a u s e f u l i n d e x o f t h e  c a p a c i t y o f a p l a n t community w o u l d be the r a t i o  photosynthetic  the L e a f A r e a Index (LAI)p  o f t h e l e a f a r e a o f t h e community t o t h e a r e a o f l a n d  i a t e d w i t h t h a t community.  The  communities  Brougham  (1955? 1956)  of s h o r t r o t a t i o n r y e g r a s s , r e d and w h i t e an i n i t i a l  95?°  interception in  H  developing  and  T_. r e p e n s L . ) , o b s e r v e d  i n c r e a s e d , growth r a t e  i n c i d e n t l i g h t was  i n t e r c e p t e d ; he  increased  a t t r i b u t e d the  i n growth r a t e s i m p l y t o the f a c t t h a t w i t h a g r e a t e r L A I the was  a b l e t o " t r a p " more l i g h t e n e r g y f o r p h o t o s y n t h e s i s .  phase f o l l o w e d where t h e g r o w t h r a t e became c o n s t a n t further increases increase The  i n the LAI,  and  a third  models, d e s c r i b i n g l i g h t S a e k i , 1953)  and  community  A second  regardless  of  and f i n a l phase where  an  (1955)*  i n t e r c e p t i o n by a p l a n t community  r e l a t e d net p h o t o s y n t h e s i s  agreed w e l l w i t h the steady  r e p o r t e d b y Brougham  The  and  1=1 I = light I k  = light  ( K s a n g a and  d e c l i n i n g phases o f  M o n s i and  a t t e n u a t i o n by a p l a n t community was  where  rise  i n the L A I caused a d e c l i n e i n growth r a t e .  ( M o n s i and 1954)  stand  ( L o l i u m perenne L. x L. m u l t i f l o r u m Lam..).,  c l o v e r ( T r i f o l i u m J^ratejise L.  a  assoc-  examining growth i n a forage  phase where, as t h e L A I of  i.e.  i n d e x has b e e n u s e d e x t e n s i v e l y s i n c e  e s p e c i a l l y i n s t u d i e s of l i g h t  until  photo-  organs.  I n 1947  1947  s t r u c t u r e of the  Saeki  (1953)  growth model f o r l i g h t  b a s e d on B e e r s Lax/ so  that2  (1.1)  exp  i n t e n s i t y b e n e a t h an L A I o f i n t e n s i t y at the crop  Monsi,  F.  surface.  = t h e e x t i n c t i o n c o e f f i c i e n t o f t h e community.  F = L e a f a r e a i n d e x above the o b s e r v a t i o n  point.  5 W i t h t h i s r e l a t i o n s h i p i n mind K a s a n g a and M o n s i (1954) p r o p o s e d  that,  as p l a n t s b o t h r e s p i r e and p h o t o s y n t h e s i z e t h e r e s h o u l d be an o p t i m a l L A I a t w h i c h i l l u m i n a t i o n o f t h e l o w e s t l e a v e s i n t h e community w o u l d be j u s t above t h e i r l i g h t c o m p e n s a t i o n p o i n t . at  They a l s o s t a t e d  that,  t h i s L A I , t h e r e was maximum u s e o f l i g h t f o r p h o t o s y n t h e s i s b y t h e  community f o r f i x i n g c a r b o n d i o x i d e .  The c o n c e p t o f " m a r g i n a l  c o m p e n s a t i o n a r e a " as p r o p o s e d b y D a v i d s o n and P h i l i p (1958) a l s o  1  c o n s i d e r s t h i s s i t u a t i o n and i s a n a l a g o u s t o t h e c o n c e p t o f optimum L A I . D o n a l d (1961) h a s r e p o r t e d t h a t i n c r e a s e s i n L A I above t h e optimum p l a c e d t h e l o w e r l e a v e s o f t h e community b e l o w t h e i r l i g h t p o i n t so t h a t t h e y e f f e c t a n e t l o s s i n c a r b o n d i o x i d e  compensation  fixation.  D o n a l d s t a t e d t h a t t h e s e l o w e r l e a v e s c o n t i n u e d t o r e s p i r e u n t i l t h e ,. p l a n t r e a c h e d a p o i n t where t h e l o w e r l e a f d e a t h r a t e e q u a l l e d t h e r a t e o f u n f o l d i n g o f new l e a v e s .  T h i s p o i n t he c a l l e d t h e c e i l i n g L A I a n d ,  i n t h e c a s e o f f o r a g e c o m m u n i t i e s , t h i s was a l s o t h e c e i l i n g  agric-  u l t u r a l y i e l d , where t h e a g r i c u l t u r a l y i e l d c o n s i s t e d o f p h o t o s y n t h e t i c parts.  However, a t t h e c e i l i n g L A I a s s i m i l a t i o n was s t i l l  p o s i t i v e as  n o t a l l t h e l i g h t e n e r g y a s s i m i l a t e d was r e q u i r e d f o r c o n t i n u i n g expansion.  The s u r p l u s e n e r g y was u s e d t o i n c r e a s e t h e s i z e o f n o n i  phot o s y n t h e t i c o r g a n s u n t i l r e s p i r a t i o n b a l a n c e d a s s i m i l a t i o n . In  leaf  1961, D o n a l d , r e v i e w i n g t h e above s e q u e n c e , o b s e r v e d t h a t t h e r e  were o n l y two p u b l i s h e d a c c o u n t s o f an o p t i m a l L A I u n d e r  field  c o n d i t i o n s , one b y Watson (195°") u s i n g a r t i f i c i a l l y t h i n n e d stands- o f k a l e (Br_aj^sica o l e r a c e a v a r . a c e p h a l a ) and a n o t h e r b y D a v i d s o n and D o n a l d (1958) u s i n g s u b t e r r a n e a n c l o v e r ' ( T r i f o l i u m s u b t e r r a n e u m L . ) . w o r k e r s have been u n a b l e t o d e m o n s t r a t e an o p t i m a l L A I . for  example,  examined  Some  Brougham  t h e growth r a t e o f a r y e g r a s s - c l o v e r f o r a g e ,  (1956)j  6 s t a n d and f o u n d t h a t w h i c h was  i t s growth r a t e r o s e t o an asymptote a t L A I 5  m a i n t a i n e d t o L A I 10,  Brougham a l s o measured l i g h t  inter-  c e p t i o n and f o u n d t h a t  95/^  agree w i t h the c o n c e p t  o f the optimum L A I t h e r e s h o u l d have "been a  decline  w  a  s  i n t e r c e p t e d when the L A I r e a c h e d 5<  i n growth r a t e between L A I 5 and  10.  (1962) a t t e m p t e d t o s t a b i l i s e the L A I o f  L a t e r Watson and F r e n c h  a k a l e c r o p a t an optimum by r e p e a t e d t h i n n i n g . growth r a t e was optimum LAIp season.  not  o b t a i n e d duo  i t appeared  that  Seasonal v a r i a t i o n  f o r the unexpected  results  shifted  to errors  However, a maximum  i n e s t i m a t i o n of the  t h e optimum v a r i e d  i n o p t i m a l L A I may o f Brougham  growth p e r i o d some c r o p s may not because  through the  growing  w e l l have b e e n t h e  (1956).  C l e a r l y over a  of environmental f a c t o r s  t h e optimum L A I and maximum y i e l d  that  w h i c h the p l a n t community i s c a p a b l e o f s u p p o r t i n g .  ":  o f an optimum L A I i s assumed t o be v a l i d ,  s t a n d s when s u b j e c t t o l i g h t  b o t h a n i m a l and m e c h a n i c a l  maximum growth r a t e s ,  (1964)  i n promoting  have shown t h a t  s t u d i e s , s u c h as t h o s e o f Brougham  Consequently  (1959)  such because  Begg and • efficient  g r a z i n g management  and B r y a n t  d e f o l i a t i o n s do not produce  as heavy d e f o l i a t i o n s .  probably  t h e s e young l e a v e s a r e t h e most  f u r t h e r s h o o t growth.  have shown l e n i e n t  However,  d e f o l i a t o r s t e n d t o remove t h e younger  l e a v e s , as t h e y a r e i n v a r i a b l y a t t h e t o p o f t h e canopy. Wright  control  i n monospecific forage  and f r e q u e n t d e f o l i a t i o n s .  d e f o l i a t i o n s have r a r e l y p r o d u c e d  may  has  t o a l e v e l f a r above  o f L A I a t an optimum i s p r o b a b l y most p r a c t i c a l  reason  specific  r e a c h an optimum L A I whereas o t h e r s  a p a r t i c u l a r combination  If the concept  To  and B l a s e r  (1961)  growth r a t e s as h i g h -  7 The maintainance of an optimal LAI also appears d i f f i c u l t f o r several other reasons.  Brougham (195°) has shown a diurnal variation  in optimal LAI, and Stern and Donald (1961) have shown that the optimum varied with the amount of solar radiation received. The latter observation may have i n part caused the seasonal changes i n optimal LAI suggested by the results of Watson and French (1961).  When multi-  specific forage stands are considered the optimal LAI w i l l vary with the species composition,  For example, Brougham (i960) has shown that,  at LAI 3, the light interception of a white clover sward was $2fo and of a short-rotation ryegrass sward 75$ and of a mixture of the two  8Cfc>.  These results are similar to those of Stern and Donald (1962) f o r Wimera ryegrass (Lolium rigidum Gaud.) grown alone and with subterranean clover.  In these forage stands the inclination and position of the  leaves i s complex and influences the LAI required to produce 95$ '' interception of the incident light ("Critical LAI' i s the term used by ;  Brougham (1958) to describe this particular LAI).  Brougham (1958)  reported that the C r i t i c a l LAI for the white clover sirards was 3.5? while for the short-rotation ryegrass sward i t was 7-1 and for the mixture of the two 4»8.  Warren Wilson (1965) has shown the importance of these  variations i n foliage inclination i n influencing the light attenuation of forage stands.  Further, Pearce  et a l . (1.967) have shown  that these differences i n foliage inclination do i n fact result i n differences i n net photosynthesis of the forage swards. The theoretical studies on light penetration in foliage by Anderson (1964) and Monteith (1965) have shown that an additional reason for failure of the optimum LAI light interception concept was the use ;  of the analogy with Beer's law. Beer's law accounts f o r attenuation of  8 a parallel beam of monochromatic light passing through a uniform solution and, as neither of these conditions are found i n a plant community", the analogy with Beer's law i s not j u s t i f i e d .  (1965) has  Monteith  therefore proposed the use of a binomial expression to express  light attenuation by foliage; thus for a given stratum:  I = Io (S + (1 - S) T.).  .  .....  .(1.2)  where S i s the fraction of the total incident light energy not intercepted by the forage stratum and T i s the transmission coefficient- of "Verhagen  et a l .  (1963) have  (1964) and  Monteith  (1965) to  the leaves in that stratum. the objections of Anderson  shown that equation ( l . l )  can be overcome by modifying i t so that k varies with LAI and with depth into the foliage stand. necessary as Anderson  However, further modification appears  (1966) has  demonstrated that k w i l l d i f f e r in  diffuse and direct l i g h t . Notwithstanding these d i f f i c u l t i e s with light attenuation models, i t is clear, as shown by Smith et a l .  (1964),  that the LAI i s the most  important single variable accounting for variation in the dry matter yield of forage swards.  However, not a l l forage stands show an  optimum LAI; some show an asympotic growth rate at high LAIs. research since 1961 two categories.  Much  has been concerned with the investigation of those  Black  (1963) has  confirmed the optimum LAI for sub-  terranean clover, but has shown that i t varied between 4 nd 7 depending a  on the level of solar radiation.  Similarly Harper  (1963) has  shown  that the optimum LAI for a crop of potatoes (Solanum tuberosum L.) varied with harvest date from  2.5  to  4.0.  However, Rees  (1963)  working  with o i l palm (Elaeis guinconsis^ Jacq.) plantations found an optimal  9 LAI of 3 from a number of samplings over a 54 day period.  For  graminaceous species tho optimal LAI i s frequently higher than those previously quoted. For example Pearce et_ a l .  (1965) found  that the  5«5  while Cooper  (1966)  optimum LAI in an orchardgrass sward was  found the optimum LAI in a forage sward of Italian ryegrass (Lolium multiflorum Lam.)  was  7-  Murtagh and Gross  (1966) have  published a  number of optimum LAIs for stands of t a l l fescue (Festuca arundinacea Schreb.) and rice (Oryza sativa L.).  In the rice communities the  optimum LAIs declined with planting density suggesting that at high plant densities the rice stem became photosynthetically important to the plant.  If allowance for this stem effect i s made the optimum LAI  for both t a l l fescue and the rice stands was about 11. Schibles and Weber  (1965) found  of a crop of soybeans (Glycene max while Williams  et_ a l .  no depression in the growth rate  (L.) Merr.) between LAI 3 and 6, .  (1965) observed  that the growth rate of a .crop  of corn (Zea mays L.) rarely became asymptotic.  However, they observed  that the magnitude of the increase in rate was substantially reduced between LAIs of  5  and  16.  Wilfong  _et a l .  (1967) have  examined forage  swards of a l f a l f a (Medioago sativa L.) and Ladino white clover and . found that the optimum LAI occurred over a broad, rather than a narrow range.  In both species an increase i n the LAI above 3 caused only  slight changes in the crop growth rate.  F i n a l l y Anslow  (1967)  was  unable to find a relationship between the mid summer growth rate of a L  sward of perennial ryegrass (Lolium perenno L.) and LAI. Despite the fact that some forage swards appear to have an optimum LAI while others do not, the association between LAI, light interception and photosynthesis i s well established. Further support comes from the  10 work of Brougham (i960) and of Hunt and Cooper  (1967) who  have both  shown very high correlations between chlorophyll content per unit area and crop growth rate with a wide range of species and genera.  Additional  discussion of the relationship between crop growth rates, LAI and net photosynthesis i s given in section 2.1.2  2.2.2.  Carbohydrate Reserve Status after Defoliation One of the more controversial topics of forage agronomy i s the  question of the importance of carbohydrate reserves for regrowth following defoliation.  Although this topic does not constitute part  of the experimental section of the thesis, a review of the carbohydrate reserve literature i s necessary because of i t s close association'with leaf area, light interception and photosynthesis.  The aim of this  review is to delineate those situations where carbohydrate reserves are important in forage regrowth as distinct from those situations where light interception and leaf area are important. It i s f i r s t necessary to define the terms, reserve and carbohydrate.  Reserve compounds are organic compounds which are elaborated  by the plant, stored, either passively or actively, and u t i l i z e d by the plant at some later date for maintenance and/or growth.  In forage  plants the most important reserve carbohydrates are sugars, fructosans and starches.  However, Davidson and Milthorpe  (1965) have  pointed out  that these reserve carbohydrates are in equlibrium with a pool of l a b i l e structural and nitrogenous compounds.  Inclusion of these nitrogenous  compounds agrees with the original concept for reserve compounds; proposed by Graber  et a l , (1927)4  11 Numerous i n v e s t i g a t i o n s , s u c h as t h o s e o f S u l l i v a n and  (1943),  (i960)  Albcrda  and d e l Poso  (1963),  Sprague  have shown f o l l o w i n g  d e f o l i a t i o n o f f o r a g e p l a n t s , t h e r e i s always  an a s s o c i a t e d d e c r e a s e  i n the carbohydrate r e s e r v e s of the p l a n t s .  These o b s e r v a t i o n s have  l e a d t o the g e n e r a l b e l i e f t h a t a l a r g e p r o p o r t i o n of the r e s e r v e s are i n c o r p o r a t e d i n t o new c o n c e n t r a t i o n of the  regrowth t i s s u e of the p l a n t .  ' r e s e r v e ' c a r b o h y d r a t e s h o u l d be a p r e r e q u i s i t e  f o r high regrowth rates. c a r b o h y d r a t e was  Therefore a high  first  T h i s h y p o t h e s i s t h a t r e s e r v e s were m a i n l y  advanced b y G r a b e r jet a l .  (1927) f r o m  an  e x t e n s i v e r e v i e w o f l i t e r a t u r e , and more r e c e n t l y has b e e n s u p p o r t e d i n  (1952).  a reviexv b y Weinraann In 195° experiment  May  and- D a v i d s o n o b t a i n e d r e s u l t s f r o m a d e f o l i a t i o n  w h i c h s u g g e s t e d t h a t t h e d r o p i n c a r b o h y d r a t e s was  uence o f c o n t i n u e d r e s p i r a t i o n and t h a t t h e y had l i t t l e i n i t i a t i n g regrowth.  May  (i960) r e v i e w e d  t h a t " I t seems somewhat p r e m a t u r e ,  a  conseq-  o r no e f f e c t i n  t h e l i t e r a t u r e and  therefore, to a t t r i b u t e  concluded  special  s i g n i f i c a n c e t o a c o r r e l a t i o n between r o o t c a r b o h y d r a t e l e v e l s and behaviour of the tops a f t e r Concurrent r e s e r v e problem  defoliation  w i t h May's r e v i e w r e - a s s e s s m e n t occurred.  c l a r i f i e d the problem  of the  carbohydrate  T h i s r e s e a r c h has, t o a l a r g e e x t e n t , >  f r o m an a g r o n o m i c s t a n d p o i n t , and, has  a p o s s i b l e r o l e of a l a b i l e pool of r e s e r v e s . pool i s u n c l e a r but  The  suggested  exact nature of the  i t appears t o i n c l u d e p r o t e i n a c e o u s m a t e r i a l ,  s t r u c t u r a l and s t r u c t u r a l  the  non-  carbohydrate.  An e a r l y c o n t r i b u t i o n t o t h e r e c e n t r e s e a r c h came f r o m Ward and Blaser  (1961) who  f o u n d t h a t t h e r e g r o w t h o f o r c h a r d g r a s s p l a n t s depended  b o t h on t h e l o a f a r e a r e m a i n i n g a f t e r d e f o l i a t i o n , as w e l l as t h e  12  carbohydrate reserve l e v e l .  Other workers, Baker and Garwood (1961),  found that these two factors were, however, important at different times of the year.  Eastin  _et a l . (1964) has confirmed the action of a  growth regulator acting with the carbohydrate reserves to control; the rate of re-growth.  They found the level of a growth regulator in. the  apex of brome grass (Bromus inernies Leyss.) appeared to suppress, tillering.  Once the apical dominance was established, usually at,  internode elongation, further t i l l e r i n g and regrowth did not occur even i f the carbohydrate reserves wero sufficient. In orchardgrass research indicates that the stubble fructosans are the principal reserve compounds (Sullivan and Sprague 1953 Blaser I 9 6 I , (1962)  Baker and Garwood 1 9 6 1 ) .  9  Ward and  Other authors, Reynolds and Smith  and Smith ( 1 9 6 2 ) , have included some root and or rhizome portions  in the carbohydrate reserve structures of other forage plants. Notwithstanding these differences in the reserve structures, and the interplay of other factors such as leaf area, a l l the above mentioned authors have shown a common pattern for the carbohydrate reserve concentration in plants following defoliation.  I n i t i a l l y after defoliation the  reserve concentration declined and this continued for at least 10;days. Following the decline phase the concentration of carbohydrate reserves increased with the rate of increase back to the original level being slowest under the most severely defoliated treatments.  In many forage  plants re-growth comes from established structures as well as crown or t i l l e r buds.  Thus in white clover Sasaki and Fukuju (1964),have  shown  that root carbohydrate reserves were used mainly to i n i t i a t e re-growth from the crown, while the stolon reserves were used for further r e growth from the stolon buds.  ;  They found that the most active phase of  use of a l l reserves was up to the 10th  day f o l l o w i n g d e f o l i a t i o n .  has been confirmed by Hoshino and Oizumi Brown and B l a s e r before new  (1964) have  This  (1968).  shown that the l e n g t h of the phase  carbohydrate reserves began to regenerate depended on the  growth a c t i v i t y of the p l a n t s , i n that environmental c o n d i t i o n s predisposed to high growth r a t e s tended to keep the carbohydrate reserve l e v e l low.  Thus, i n orchardgrass and Kentucky 31 fescuo they found  that the carbohydrate reserves c o u l d begin to r i s e as e a r l y as 10 days a f t e r d e f o l i a t i o n , but i f n i t r o g e n were applied, and or r a i n f a l l were near optimal the reserve carbohydrate l e v e l would not begin to r i s e f o r 40 days.  The r i s e i n reserve carbohydrate was  g e n e r a l l y accompanied by  a slowing of the growth r a t e together with maturity of the p l a n t s , lower a i r temperatures there was  and reduced n i t r o g e n supply.  Under these c o n d i t i o n s  an energy surplus i n the plants which was conserved as reserve  c arb ohydrat e s. Davidson and Milthorpe  (1965? 1966)  examined the r e l a t i o n between  d e f o l i a t i o n , the carbohydrate reserve l e v e l and the carbon balance of orchardgrass p l a n t s . respiration  By measuring continuously photosynthesis and ,  of the plant leaves together with root r e s p i r a t i o n and con-  c u r r e n t l y sampling f o r the carbohydrate l e v e l s and the dry matter production they were able to construct a carbon balance sheet f o r the plants as they regrew from a d e f o l i a t i o n . found that the photosynthesis of the new balance the r e s p i r a t i o n  With severe d e f o l i a t i o n they leaves was  i n s u f f i c i e n t to  of the plant and up to the 4th  drew on s t o r e d r e s e r v e s .  day the plants  However, from t h e i r measurements of the  ;  carbohydrate l e v e l s they were unable to account f o r a l l the carbpn used and t h e r e f o r e they suggested that other substances, probably p r o t e i n s ,  14 were being u t i l i z e d .  By varying the carbohydrate reserve level of the  plants they were able to show that, during the f i r s t two days of regrowth, i f the reserves were high they accounted for 80$ of growth and respiration.  On the other hand i f the reserves were low they were  hardly used and other substances made up the carbon d e f i c i t .  They  therefore questioned the universal role of carbohydrate reserves, especially as the environmental conditions which produce high levels of reserve carbohydrate (see Brown and Blaser and Smith and Jewiss 1966)  1964)? Vengris .ot a l . 19665  are those which r e s t r i c t regrowth per.se.  This type of situation may well have been the reason for the poor correlation found between carbohydrate reserves and regrowth rates found i n experiments such as those of Davies ryegrass.  Davidson and Milthorpe  (1966) with poronnial  (1966) concluded from their study  that carbohydrate reserves were important in controlling regrowth rates •4.  *  of forage in that they were part of a l a b i l e energy pool. Notwithstanding the above evidence some workers, for example Alberda  (1966) insist that non structural carbohydrates per se-are of  very great importance in determining regrowth rates of forage swards. Clearly more analyses are necessary to describe the l a b i l e pool proposed by Davidson and Milthorpe  (1965). The evidence of both Ward and..Blaser  (1961) and Davidson and Milthorpe (1966) i s i n agreement in that reserve compounds are u t i l i z e d during the f i r s t week of regrowth of orchardgrass and that later regrowth is dependant on leaf area and photosynthesis. Tropical graminaceous forages also show this dual dependancc; Humphreys and Robinson  (1966) using green panic (Panicum maximum var. triehoglume  (K.Schum) Eyles) and buffel grass (Cenchrus c i l i a r i s L.) found that, regrowth i n both these species was dependent on both carbohydrate reserves  15 and leaf area l e f t after defoliation  Humphreys also suggested that  the two factors may i n t e r a c t . Despite the general pattern described above there may be differences i n the agronomic response to carbohydrate reserves with a particular cultivar, strain or environment.  Eagles (1967) has shown  that high latitude populations of orchardgrass accumulate  carbohydrate  in response to low temperatures while populations from lower latitudes do not.  Colby  et a l . (l$66)  have shown that high carbohydrate  reserves are necessary for orchardgrass to withstand both high and low temperature stress conditions. F i n a l l y from an agronomic point of view i t i s necessary to consider the influence of the carbohydrate reserves on ruminant growth and nutrition.  As pointed out by Blaser _et a l . (1966) many forages  are low in u t i l i z a b l e energy, and f e r t i l i z e r nitrogen applied to increase the yield results in additional lowering of u t i l i z a b l e energy. On the other hand imposition of grazing managements to produce high, carbohydrate reserves and good ruminant nutrition can cause reduced dry matter yield.  Thus improved animal nutrition may have to be balanced  against reduced output per acre.  In this context the exact nature of  the reserves i s of smaller significance. 2.1.3  Observations of Dry Matter Production by Forage Stands The early work on the effects of frequency and intensity of  defoliation on herbage production by forage stands has been reviewed by Brougham (1959)» he observed that these reports showed d i f f e r i n g yields with similar managements.  In his experiments Brougham showed that  intensive grazing and long spelling maximized production from a short  16 r o t a t i o n r y e g r a s s sward.  Frequent  reduced  y i e l d w h i l e f r e q u e n t and  yield.  Bryant  experiments swards  i n t e n s i v e g r a z i n g s , however s e v e r e l y  l a x g r a z i n g s gave an  (1961 I968) c o n d u c t e d  and B l a s e r  5  frequent  and most  and  t h i s review recent  l a x treatment  gave t h e p o o r e s t  and  examine them t o see  in this  yields.  s e c t i o n t h e r e f o r e i s t o p o i n t out  literature  grazing  i n t e n s i v e treatment  the h i g h e s t y i e l d under g r a z i n g c o n d i t i o n s but frequent  similar  orchardgrass-alfalfa  w i t h o r c h a r d g r a s s - l a d i n o c l o v e r and  Again the l e a s t  intermediate  produced  experiment  The  purpose  such d i f f e r e n c e s  i f t h e r e i s any c o n s i s t e n t ;  The  p r o d u c t i o n o f some s p e c i e s may  even w i t h  be  depressed  i n f r e q u e n t d e f o l i a t i o n , , Garo-Costas  c u t a number c f s p e c i e s o f t r o p i c a l a t 10  inches.  Over a two  not  pangola  purpurascens)  grass  and n a p i e r g r a s s  a  >  b and c, 1961)  on n a p i e r , g u i n e a ,  However, w i t h t h r o e o t h e r  inch cut.  a l s o found  w i t h gamba g r a s s  grasses  Vicente-Chandler  i n v e s t i g a t i n g the e f f e c t  (Panicum  the h i g h e s t annual  c u t t i n g e v e r y $0  et a l .  o f f r e q u e n c y of. c u t t i n g  grasses, found  f r e q u e n t c u t t i n g s ( e v e r y 9 0 days f o r t h e f i r s t  and b)  3  (PenniBetum purpureum Schum.) c l o s e  p a r a and p a n g o l a  f o r the l a t t e r ) produced  grass  b y c u t t i n g at  ( D i g i t a r i a decumbens S t e n t . ) , p a r a g r a s s  c u t t i n g always o u t y i e l d e d the 10 (1959  of molasses  (Panicum maximum J a c q . ) y i e l d s ^ w e r e  i n f l u e n c e d by c u t t i n g h e i g h t .  tested,  (1961)  f o r a g e g r a s s e s e v e r y 2 months a t  year period y i e l d s  By c o n t r a s t g u i n e a g r a s s  by c l o s e c u t t i n g  and V i c e n t e - C h a n d l e r  ( M e l i n i s m i n u t i f l o r a Beauv.) wore s e v e r e l y r e d u c e d inches.  of  in this  pattern.  3 and  the  t h a t tho  t h r o e and  yields.  least  e v e r y 60  days  Oyenuga ( 1 9 5 9  &  days o p t i m a l f o r n a p i e r g r a s s , b u t  (Andropagon t e c t o r u m  Schugj.) he f o u n d  days gave g r e a t e r y i e l d s t h a n e v e r y 9 0 °r 20 d a y s .  c u t t i n g every  It appears  from  :  50  17  t h i s work t h a t the t r o p i c a l forage grasses n a p i e r , para, guinea and pangola behave i n much the same way as the temperate grasses i n t h a t they g i v e t h e i r maximum y i e l d under i n t e n s i v e but i n f r e q u e n t d e f o l i a t i o n . Other s p e c i e s such as molasses and gamba grasses do not behave i n t h i s manner as the former produces the g r e a t e s t d r y matter y i e l d w i t h l a x d e f o l i a t i o n w h i l e the l a t t e r appears t o have an optimum d e f o l i a t i o n frequency of every 7 weeks. Research on forage p l a n t s from temperate c l i m a t e s can be c o n v e n i e n t l y c o n s i d e r e d i n throe c a t e g o r i e s v i z , a) grass swards, b) legume swards and c) mixed grass-legume  swards.  Some o f the  s t u d i e s were p u r e l y p r a c t i c a l w h i l e others are aimed at e l a b o r a t i o n of the concept or the e v a l u a t i o n of d i f f o r e n c o s between herbage measurement techniques.  Because of these d i f f e r e n c e s i n aim, i t i s necessary t o  take them i n t o account when comparing the v a r i o u s s t u d i e s .  Studies  aimed at p r a c t i c a l v e r i f i c a t i o n of L A I concepts were made by Lambert (1962,  1964).  By t a k i n g concurrent LAIs and d r y matter y i e l d s of pure  swards of timothy (Phleum pratenee L.) and meadow fescue (Festuca JPjrfitansjLs L . ) , Lambert found t h a t frequent c u t t i n g , every 4 weeks., reduced y i e l d s compared w i t h a hay aftermath management where cuts wore at 10 week i n t e r v a l s .  The l e a f area f o l l o w e d the same p a t t e r n so t h a t  the hay c u t swards had a h i g h e r i n t e g r a t e d l e a f area than the 4 week cut swards.  Dewey ( 1 9 6 1 ) used s i m i l a r d e f o l i a t i o n treatments p l u s an  intermediate frequency and found t h a t w i t h orchardgrass swards the most i n t e n s i v e frequency gave the lowest y i e l d i n a l l three years of the trial.  Again the hay management produced the g r e a t e s t y i e l d except i n  one year when i t was equal t o the i n t e r m e d i a t e frequency.  18 However, increased cutting frequency is not necessarily associated with reduced yieldsfRobards and Leigh (1967) examined "barley grass (Hordeum leporinum Link.) pastures and found that cutting once a month gave a greater accumulated yield than cutting every 6 months while intermediate frequencies gave intermediate yields.  Clipping height  too, can influence the outcome of the clipping frequency trials...  Holt  and Lancaster (1968) examined the effects of both frequency and intensity of defoliation on the production of coastal bermudagrass (Cynodon dactylon (L.) Pres.).  They found that with cutting at 13 cm there  were no differences in yield between the three clipping frequencies. However, when clipped at 5  c  m  the usual pattern appeared and the least  frequent clippings gave the highest yield.  Clipping height per pc  influenced yield and the close clipping at 5 cm gave the highest,yield. With intermediate wheatgrass (Agropyron intermedium Host.) and pubescent wheatgrass (A. spicatum var. pubescons Elmer.) Baker and Hunt  (l96l)  found plants clipped at  2 inches  produced significantly more  herbage than did plants clipped at 4 inches. clipping height shows no general pattern.  Unlike clipping frequency,  Davis  (i960)  working with  reed canarygrass ( j ^ a l a r i s arundinacea L.) found that a 5 inch clipping height gave greater yields than a 1 inch clipping height. •rorkers(Drake  Other'  et a l . 1963) found that orchardgrass swards  clipped at 3 inches consistently outyielded swards clipped at 1.5 inches, no matter what the date of the f i r s t cut or the nitrogen.level. Matches  (1966) also  found similar trends with Kentucky 31 t a l l fescue  where 4 inch clipping outyielded 1 inch clippings. Clearly many factors influence the outcome of the clipping height experiments.  Scott  (1956) has stressed  the importance of the position  19 of the basal t i l l e r buds with reference to defoliation height and management. However, stage of growth at which defoliation takes place w i l l also influence the outcome of defoliation experiments.  Sheard  and Winch (1966) examined swards of timothy, brome grass and orchardgrass a l l clipped to a height of 2.5 inches with the clippings r e - i n i t i a t e d every 2, 4 or 6 weeks, when there was 8O/& or 95T° light interception and at pro or post elongation.  They found that for, these  species therowere no harvest c r i t e r i a which consistently produced the maximum yield.  Thus, although morphological features and phonological  events play a part in determining the outcome of defoliation practice they do not always show consistent trends. When the defoliation is by animal grazing, the selective nature of tho animals, especially at moderate stocking pressures, frequently negates the potential extra gains available from surplus herbage and moderate stocking  Roe  _et a l . (1959) subjected Bothrichloa ambigua  S.T. Blake^pastures to three stocking rates with rotational or continuous grazing and wero unable to detect any consistently s i g n i f icant difference between the grazing treatments in thoir effect on the pasture or on the l i v e weight gain of the sheep. With leguminous forages wide differences between results of defoliation experiments exist,  Davies  et. a l . (i960) found that Grimm  and Du Puits a l f a l f a cut 3 times a season gave greater yields than when cut only twice.  Ridgman (i960) working with Provence a l f a l f a , found  cutting at ground level severely reduced yield potential while cutting at any other stubble height between 2 and 6 inches produced no differences in y i e l d .  Opposite trends were obtained with L.adino clover  by Gervais (1960)5 he found that cuttings at 1-|- inches produced a  20 g r e a t e r c u m u l a t i v e d r y m a t t e r y i e l d t h a n d i d c u t t i n g s at 3 i n c h e s 2 cuts per  s e a s o n gave a g r e a t e r t o t a l y i e l d t h a n f o u r c u t s p e r  w h i c h i n t u r n was reported  g r e a t e r t h a n 6 c u t s per season.  season  R o s s i t e r (1961)  a s i m i l a r t r e n d w i t h s u b t e r r a n e a n c l o v e r whore one  a greater  cut  t o t a l y i e l d than 3 cuts.  g r a s s e s and  i t i s i n these forage  f r e q u e n c y and  mixtures t h a t the  influence  i n t e n s i t y o f d e f o l i a t i o n shows i t s maximum  complexity  a r i s e s when, as  o f t h e g r a s s and  legume t o d e f o l i a t i o n d i f f e r s . .  complexity.  In s u c h  o f t e n g i v e one  forage  of  i s almost always the case, the  a p a r t i c u l a r d e f o l i a t i o n management can  reaction  associations  or the  other  species  a competitive  The  r e s u l t i s t h a t t h e managements o f t e n r e s u l t i n c o m p a r i s o n s  net  the p r o d u c t i v e swards. it  a d v a n t a g e so t h a t i n t i m e i t d o m i n a t e s the.; s w a r d . of  c a p a c i t y of i n d i v i d u a l species r a t h e r than of the'mixed  T h u s , when c o m p a r i n g d e f o l i a t i o n managements w i t h m i x e d ' s w a r d s  i s n e c e s s a r y , as w e l l as c o m p a r i n g y i e l d s , t o compare t h e changes i n  the b o t a n i c a l Gervais  composition. (i960) examined t h e e f f e c t s of b o t h c u t t i n g h e i g h t  f r e q u e n c y on t h e y i e l d o f l a d i n o c l o v e r - t i m o t h y b r o m e g r a s s swards.  and  t o t a l production  C u t t i n g f r e q u e n c y i n f l u e n c e d c o m p o s i t i o n o f swards  c o m p o s i t i o n of the swards.  3 inches.  l e s s grass  when c u t 4 o r 6 t i m e s t h a n when c u t t w i c e .  they found that c u t t i n g height  p u r e swards and  and  l a d i n o c l o v e r - .-  i n t h e e x p e r i m e n t s i n t h a t t h e y y i e l d e d more clover,, but  who  has  gave  Legumes, however, a r e f r e q u e n t l y grown i n a s s o c i a t i o n w i t h  The  and  d i d not  i n f l u e n c e the  Thus i n t h i s i n s t a n c e  t h e c l o v e r c u t s at 1.5  inches  and  However,  botanical  species reacted  as  o u t y i e l d e d the c u t s  at  S i m i l a r r e s u l t s have a l s o been r e p o r t e d b y R o i d  found c l o s e c u t t i n g of p e r e n n i a l ryegrass-white  clover  (1966, and  in  1968)  21 orchardgrass-white c l o v e r , produced  t h e maximum y i e l d  In b o t h t h e f o r a g e s t a n d s examinod b y G c r v a i s ( i 9 6 0 ) c l i p p i n g tended  t o produce  ition for light  as o u t l i n e d b y D o n a l d  The  was  (l96l)  a b l e t o show t h a t Du P u i t s  t o competet. a l . ( 1 9 6 6 ) .  and b y B l a k e  o f some c u l t i v a r s  swards.  the i n f r e q u e n t  g r a s s dominance p r e s u m a b l y due  competitive a b i l i t y  et. a l . ( i 9 6 0 )  i n both  differs  too.  a l f a l f a was  Davies  able to  produce  and s u r v i v e i n a s s o c i a t i o n w i t h t i m o t h y , p e r e n n i a l r y e g r a s s o r meadow f e s c u e even w i t h o n l y 2 d e f o l i a t i o n s per season, was  not  season.  able to s u r v i v e i n these They a l s o found that  grass i t s a b i l i t y t o produce frequency.  however, Grimm  associations with 2 d e f o l i a t i o n s  i f S 1 2 3 r e d c l o v e r was was  not  g r a s s swards. They a l s o f o u n d  By c o n t r a s t Ward  persistence  of a l f a l f a ,  cuts per season The Smith  ert _ a l .  or t i m o t h y  et a l . ( i 9 6 0 )  p r e s e n t compared t o  that three d e f o l i a t i o n s per  (1968)  compared w i t h 6 c u t s p e r  found  than 2 d e f o l i a t i o n s greater y i e l d  per  and  w i t h brome g r a s s ,  s p e c i e s ; Wolf  and  orchardgrass  T h e y f o u n d , w i t h 4 0 0 l b . / a c r e n i t r o g e n a p p l i e d and  a 3 cut  the a l f a l f a dominated both the timothy  and  However, w i t h a 5 c u t p e r s e a s o n management a 5 O 2 5 O g r a s s s  c l o v e r b a l a n c e was  obtained.  Orchardgrass  a b l e t o compete w i t h t h e a l f a l f a  and  on t h e o t h e r hand was  showed a 5 0 s 5 0  grass-legume  at, 3 c u t s p e r s e a s o n w h i l e at 5 c u t s p e r s e a s o n t h e r e was dominance.  -  season.  always t h e s u p p r e s s e d  examined a l f a l f a g r o w i n g  p e r s e a s o n management, t h a t bromegrass.  cither  i n a s s o c i a t i o n with orchardgrass, under f o u r  legume, however, i s n o t  (1964)  per  defoliation  In g e n e r a l a l l t h e swards examined b y D a v i e s  s e a s o n u s u a l l y gave a g r e a t e r c u m u l a t i v e y i e l d season.  grown w i t h  i n f l u e n c e d by the  showed s l i g h t l y g r e a t e r y i e l d s when t h e legume was t h e pure  alfalfa  better ratio  a grass,  .  In g e n e r a l a l l t h r e e swards gave a g r e a t e r c u m u l a t i v e , y i e l d  22 with 3 cuts per season than with 5<  Holliday and Wilman (1965) also  have found the greatest annual y i e l d at the lowest cutting frequency on timothy-meadow fescue-white clover swards. Under some conditions deviation occurs from the pattern of "highest yield with the lowest frequency".  Reid (1966, 1968)  used two cutting  frequencies, cutting swards of perennial ryegrass-orchardgrasswhite clover when 8 or 10 inches high.  With swards defoliated at 2.5  inches, there was no depression i n yield even though the 8 inch swards were harvested an extra two times.  Deviation from the'highest yield  with lowest frequency" pattern can also occur where tissue senesence in the stand is high.  Hunt (1965) has shown with infrequent cutting  tissue decay can occur within the sward, often without any accompanying new growth.  It seems l i k e l y that this occurred in the alfalfa-grass  swards of Davies  jet a l . (i960) where two cuts per season yielded less  than the three cuts per season treatments. As a result of the diversity of results from clipping experiments and the ultimate necessity, in many instances, to relate defoliation practice to animal production, many workers have turned to measurement of forage production in terms of animal production.  Although i t i s  not the purpose of this review to cover this topic i t is necessary to emphasise some of the d i f f i c u l t i e s in relating clipping studies to agricultural systems using grazing animals.  Plot clipping techniques  can he made to closely approximate practical harvesting of hay or silage and there i s no problem in relating plot management to commercial farm management practice.  With animal defoliation the problem is  complex because clipping studies cannot easily be made to approximate animal defoliation? missing from simulated pasture in plots are animal  23  s e l e c t i v i t y , animal t r e a d i n g and the r e t u r n of dung and u r i n e . Broadbent  ( 1 9 6 4 ) designed a grazing experiment to bridge the gap  between c l i p p i n g and commercial grazing.  By comparing r o t a t i o n a l  grazing systems where the sheep were moved to a new paddock when' the LAI had been reduced to e i t h e r 1 or 2 he found that lambs and ewes had s i g n i f i c a n t l y higher l i v e weight gains when an LAI of 2 was l e f t i n each paddock.  However, when t h i s l a t t e r treatment was compared with a  set stocked system there was no d i f f e r e n c e i n the l i v e weights. : Other workers (Ruane and R a f t e r y 1 9 6 4 ) found a 2 0 paddock r o t a t i o n superior to a 2 paddock one i n terms of u t i l i z a b l e s t a r c h equivalent produced, and of hay conserved but not i n milk production. To improve the r e l a t i o n between c l i p p i n g and grazing experiments a number of workers have used simple grazing management systems which c l o s e l y approximate c l i p p i n g systems. there s t i l l  However, even with such systems  appear to be d i f f e r e n c e s ? f o r example Frame ( 1 9 6 6 ) found,  with perennial ryegrass-white c l o v e r swards that sheep grazings outy i e l d e d c u t t i n g s i n both accumulated dry matter as well as d i g e s t i b l e organic matter.  Bryant and B l a s e r ( 1 9 6 1 ,  1 9 6 8 ) on the other hand found  .that c l i p p i n g c o n s i s t e n t l y o u t - y i e l d e d c a t t l e grazing of both white < clover-and a l f a l f a - o r c h a r d g r a s s forage swards.  In the experiment  of Frame the d i f f e r e n c e appeared to be a consequence of i n t e r a c t i o n s between the grass-legune r a t i o and the e x c r e t a l - , c l o v e r - and fertilizer-nitrogen.  Thus as Bryant and B l a s e r removed the manure  from t h e i r p l o t s , part of the d i f f e r e n c e i n r e s u l t between the experiments could have been a consequence of d i f f e r e n c e s i n technique. Matches  (1968)  has compared c l i p p i n g with g r a z i n g by sheep, and with  g r a z i n g by c a t t l e .  The y i e l d s from swards of t a l l f e s c u e - l a d i n o c l o v e r ,  24 smooth b r o m e g r a s s - a l f a l f a and K e n t u c k y b l u e g r a s s - b i r d s f o o t t r e f o i l were n o t  i n f l u e n c e d b y t h e method o f d e f o l i a t i o n .  grass-ladino clover ,  however, y i e l d e d more d r y m a t t e r w i t h  d e f o l i a t i o n t h a n w i t h c l i p p i n g b u t t h e r e was y i e l d s w i t h sheep o r  Swards o f  animal  no d i f f e r e n c e b e t w e e n t h e  cattle.  C l e a r l y t h e above e x p e r i m e n t s  support the general c o n c l u s i o n that  t h e r e s u l t s o f c l i p p i n g and a n i m a l d e f o l i a t i o n e x p e r i m e n t s necessary  orchard-  as s t a g e s i n g r a s s l a n d e v a l u a t i o n .  are  both  However, b e c a u s e o f  great v a r i a b i l i t y i n r e s u l t between a p p a r e n t l y s i m i l a r  the  experiments  the r e s u l t s are best f i t t e d to o p t i m i z i n g forage p r o d u c t i o n i n the r e g i o n of the experiment  r a t h e r than t o t h e o r e t i c a l c o n s i d e r a t i o n s of  o p t i m i z i n g forage regrowth  2.2  Wet *  i n general.  P h o t o s y n t h e s i s and L i g h t E n e r g y C o n v e r s i o n  in  * Forage Stands  2.2.1. F a c t o r s  I n f l u e n c i n g Net  P h o t o s y n t h e s i s of Leaves w i t h i n  Forage Stands Net  p h o t o s y n t h e s i s , L A I and l i g h t e n e r g y i n t e r c e p t i o n b y  s t a n d s has  a l r e a d y been d i s c u s s e d  (2.1.1). I t  forage  i s the purpose of  this  s e c t i o n t o b r i e f l y o u t l i n e the o t h e r f a c t o r s which i n f l u e n c e net photosynthesis The  i n forage  i n f l u e n c e of temperature  and t h e r e s p o n s e Beinhart  stands.  o f a number o f f o r a g e s p e c i e s has b e e n documented b y  (1962), M u r a t a  (1964) and  on n e t p h o t o s y n t h e s i s i s w e l l known  Hesketh  and  (1967).  Iyama  (1963), E l - S h a r k w a y  and  Hesketh  In these r e p o r t s the s p e c i e s w i t h the  e r r a t e s o f p h o t o s y n t h e s i s a r e t h o s e w i t h a t r o p i c a l o r i g i n and h i g h e r optimum t e m p e r a t u r e  f o r photosynthesis.  high-  a  T h i s d i f f e r e n c e between  25 species of t r o p i c a l  and  temperate o r i g i n i s g e n e t i c a l and  appears  a consequence o f t h e p l a n t s w i t h t h e h i g h e r optimum t e m p e r a t u r e s v i r t u a l l y no p h o t o r e s p i r a t i o n whereas i n t h o s e optimum t e m p e r a t u r e s  p h o t o r e s p i r a t i o n and p h o t o s y n t h e s i s may  simultaneously active.  Ih t h e t e m p e r a t e s p e c i e s t h e optimum  observed  f o r net p h o t o s y n t h e s i s  response  curves  s p e c i e s suggest  o f t h e two  i s the  processes.  integral  a r o u n d 15  T e m p e r a t u r e has  stage  in chlorophyll  wheat  ( T r i t i c u m aestivum  and N a y l o r  result  temperature ,  increases sharply  photosynthesis i n .  (1967) have shown t h a t i n  i n p r o l o n g a t i o n of a p h o t o s e n s i t i v e  s y n t h e s i s while temperatures L.)  so t h a t a t t h i s t e m p e r a t u r e c h l o r o p h y l l content  be  R e s u l t s presented f o r these .  a l s o b e e n shown t o a f f e c t  o f 16°C  lower  - 20°C.  i n o t h e r ways; M c W i l l i a m  maize t e m p e r a t u r e s  o f 28°C do n o t .  however, t h e r e i s no p r o l o n g a t i o n a t t h e maize p l a n t s g e n e r a l l y have a  t h a n t h e wheat.  They suggest  In 16°C  lower  t h a t t h i s may  account  f o r the f u n d a m e n t a l d i f f e r e n c e between the s p e c i e s i n a d a p t i o n t o d i f f e r i n g temperature  zones o f t h e  Among o t h e r t e m p e r a t u r e has  observed  t h a t the  season  measured^ t h e maximum i s l e s s i n net p h o t o s y n t h e s i s are (1966) has  be  having  o f the temperature  that the p h o t o r e s p i r a t i o n process  with temperatures  plants  p l a n t s w i t h the  to  ;  world.  e f f e c t s on p h o t o s y n t h e s i s H e s k e t h (1967) i n f l u e n c e s t h e maximum net i n winter than  i n summer.  i n f l u e n c e d by temperature  demonstrated that the d i u r n a l c y c l e  c a u s e a mid-day d e p r e s s i o n o f p h o t o s y n t h e s i s .  photosynthesis D i u r n a l changes  i n two  ways.  Gates  of l e a f temperature Other  authors  can  (Iyama  et a l . I964) have shown a c l e a r d i u r n a l rhythm i n p h o t o s y n t h e s i s u n d e r constant conditions.,  T h e y have f u r t h e r shown t h a t some f o r a g e s p e c i e s ,  f o r example D a l l i s g r a s s  (Paspalum d i l a t a t u m P o i r . ) and  sudan g r a s s  26 (Sorghum sudanense ( P i p e r ) S t a p h . ) showed almost net p h o t o s y n t h e s i s when k e p t  at a c o n s t a n t  strong diurnal It  a t t h e optimum t e m p e r a t u r e s temperature  d i u r n a l rhythm i n  f o r t h i s process  hut  b e l o w t h e i r optimum t h e y showed a  rhythm.  i s w e l l known t h a t n e t p h o t o s y n t h e s i s  on t h e COg  no  i n p l a n t s i s dependent  c o n c e n t r a t i o n o f t h e s u r r o u n d i n g atmosphere.  The  phenomenon has b e e n w e l l r e p o r t e d i n r e s e a r c h work s u c h as t h a t o f Decker  (1959)>  i n w h i c h he f o u n d  s y n t h e s i s w i t h d e c r e a s i n g COg t o the r i g h t  a curvilinear  concentration.  o f t h e o r i g i n a t a COg  (196I) were  a s s i m i l a t e COg  from  the f i r s t  t h i s p l a n t produced Forrester was  enhanced i n an  effect. was  In t h e Og  zero.  very l i t t l e  et_ a l . (1966  COg  themselves  free  a t v e r y low c o n c e n t r a t i o n s .  i n soybean  possess  1  (e.g.  (1967) have  p h o t o r e s p i r a t i o n and  g e n e r a have l o w e r "optimum t e m p e r a t u r e s The  and  results  a photorespiration ,  ( e . g . soybean) w h i l e  i n the l i g h t  no  maize  These  other  maize). shown t h a t  exhibit  o f about 35°C f o r n e t p h o t o s y n t h e s i s w h i l e t h e  photorespiration.  Later  assimilation  atmosphere t h e soybean behaved l i k e  only photosynthesis  do not  Moss,  COg  f r e e atmosphere whereas i n maize t h e r e was  In t h e Gramineae, Downton and Tregunna  temperatures  with a  i n the l i g h t .  t h a t COg  mechanism c o n c u r r e n t w i t h p h o t o s y n t h e s i s  genera  photosynthesis  T h i s i n d i c a t e d t h a t the l e a v e s of  l e d t o t h e c o n c l u s i o n t h a t some p l a n t s p o s s e s s e d  tropical  line.passed  compensation p o i n t ) .  l e v e l s o f COg  a and b) f o u n d  a b l e t o a s s i m i l a t e COg  plants possessed  photo-  t o r e p o r t t h a t maize p l a n t s c o u l d  a i r w i t h v e r y low  c o m p e n s a t i o n p o i n t o f almost  i n net  T h i s response  c o n c e n t r a t i o n where  and r e s p i r a t i o n were i n e q u l i b r i u m ( t h e COg et a l .  decrease  optimum temperate  f o r n e t p h o t o s y n t h e s i s and  absence o f a p h o t o r e s p i r a t i o n s y s t e m  the  show  appeared  27 a l s o t o be c o r r e l a t e d w i t h t h e 4-carbon p h o t o s y n t h e t i c Kortschak  (1965) a n d  et_ a l .  (1966),  Hatch and Slack  pathway o f  whereas t h e temp-  e r a t e members o f G r a m i n e a e a p p e a r e d t o show C a l v i n c y c l e Leaf  anatomy t o o i s c o r r e l a t e d w i t h t h e absence o f p h o t o r e s p l r a t i o n .  El-Sharkway and Hesketh  (1965) s u g g e s t e d  structure  s h e a t h was s u c h t h a t  of the bundle  i n t h e b u n d l e were p r o t e c t e d recycled. leaf  This  However,  the sites  Hatch  absence plants  Despite  of a photorespiration confers  those p l a n t s Recent plants  may l i k e w i s e  reasons.  on t h e p l a n t s  possessing studies  reduces  et a l .  (1967) h a v e  carboxidase  it  system i n t h e t r o p i c a l a superior  reductions  ability  the point  Moss  et a l .  and a l s o f o u n d a n a b r u p t moisture  there  for  high, grasses5  path. rather  is clear grasses  than that  the  and .other  t o f i x CO,, c o m p a r e d  with  at which water  (1961) h o w e v e r ,  (1966) s t u d i e d  so t h a t  decreased  o f maize s u b j e c t  decrease  in  1.0  was a d e c r e a s e  L.)  closure  photosynthesis found only  to a moisture  slight  tension  a number o f f o r a g e  i n net photosynthesis -  deficit,  w i t h sorghum (Sorghum v u l g a r e  deficit  e_t a l .  t e n s i o n o f about  photosynthesis  (I966)  c a p a c i t y w e r e made b y E l - S h a r k w a y  light,  increased water  Murata  ;  photorespiration.  i n photosynthesis  16.0 bars.  found  f o r biochemical  areas o f doubt  photosynthetic  before v i s i b l e . w i l t i n g .  respiration  in tropical  a r e a l i m i t e d t h e maximum n e t p h o t o s y n t h e s i s  of the stomata w i t h  a soil  these  (1964). 3h i n t e n s e  and H e s k e t h stomatal  be a b s e n t  t o determine  their  of  model o f Lake  enzyme n e c e s s a r y f o r t h e 4 - c a r b o n p h o t o s y n t h e t i c  structural  net  t h e c o n s t r u c t i o n and  a n d t h e CO^ r e l e a s e d was t h e r e f o r e  o f phosphoenolpyruvate  Photorespiration  -  that  agrees w e l l w i t h t h e analogue  respiration.  concentrations the  photosynthesis.  plants  generally  with  bars5 f o l l o w i n g t h e d e c r e a s e i n the moisture  content  of  in  of the  28 leaves.  These workers also found that dark r e s p i r a t i o n was  by moisture stress so that energy conservation took place.  depressed After  watering, recovery to the o r i g i n a l rate of net photosynthesis took between 1 and 12 hours.  Red clover and a l f a l f a were among the species  most r e s i s t a n t to moisture stress while I t a l i a n ryegrass, orchardgrass and maize were among the most susceptible species. Mineral n u t r i t i o n i s also known to affect the photosynthetic performance of plants. For example Murata ( l 9 6 l ) found positive correlations of net photosynthesis with nitrogen, phosphorus and potassium levels as well as chlorophyll content i n r i c e .  Clearly  mineral deficiencies which reduce chlorophyll content must also reduce net photosynthesis.  However, the r e l a t i o n between net photosynthesis  and chlorophyll content i s by no means simple? Sestak (1966) has shown that i n young leaves grown under high l i g h t i n t e n s i t i e s there i s often greater net photosynthesis than would be predicted from a normal 1 l i n e a r r e l a t i o n with chlorophyll content.  The relationship between net  photosynthesis and the mineral content of the leaves i s also nonKLinear. Peaslee and Moss (1965) and Moss and Peaslee (1966) have found a c u r v i l i n e a r r e l a t i o n s h i p between net photosynthesis and l e a f magnesium and potassium l e v e l s i n maize? above a c r i t i c a l l e v e l of the element there was no further increase i n COg  assimilation.  The findings of Tsuno and F u j i s e (1965  a  an(  ^ ^) v i z . that photo-  synthesis was a function of the rate of photosynthate removal taken, together with the observations of Hart  et a l . (1965) v i z . that  translocation rates of sugar from leaves were depressed when any one of the elements N, K, P, or Fe were d e f i c i e n t , suggests a mechanism by which low mineral n u t r i t i o n depresses net photosynthesis.  Similar  29 depressions of net photosynthesis with high sugar levels have also been reported in barren maize plants by Moss (1962). High levels of minerals can depress net photosynthesis5  (1966) has shown that  Schroder  9>000 ppm NaCl severely reduced net photo-:  synthesis in both pangola and bahia grasses (Paspalum notatum Flugge). However, in St. Augustine grass (stenotaphrum secundatum (Walt.) Kuntze). and coastal bermuda grass there was greater resistance to s a l i n i t y with the latter species showing no appreciable depression in net photosynthesis u n t i l a level of 1,800 ppm NaCl was reached in the s o i l . Increasing leaf age has been shown to be accompanied by reduced net photosynthesis.  For example, Jewiss and Woledge (1967) using the  5th leaf of a t a l l fescue t i l l e r found a progressive decline in CO^ assimilation with increased age,  Begg and Wright (1964) examined the  efficiency of top and basal leaves of reed canarygrass and, although they did not measure CO^ uptake, in terms of dry weight they found the older leaves showed a depressed weight gain. (1966) have measured C 0  2  Recently Brown  et a l .  assimilation in upper and lower leaves of  white clover and a l f a l f a plants.  At saturating light energy levels,' in  a four week old stand, leaves at the bottom of a l f a l f a plants were about half as efficient in f i x i n g C 0  2  as those at the top.  With white  clover there was a progressive decline in CO,, assimilation with age. Defoliation studies by the same authors showed that after leaves reached two weeks of age they contributed very l i t t l e to further regrowth.  In fact with reed canarygrass lower leaves appeared to be  "parasitic" in that they respired more carbon than they fixed. Within a particular plant species populations of d i f f e r i n g genetic origin, particularly from climatically d i f f e r i n g areas, often show  30 d i f f e r e n c e s i n photosynthetic c a p a c i t y .  For example, Bjorkman and  Holmgren (1963) found p l a n t s from shaded and exposed h a b i t a t s showed t h e i r greatest photosynthetic e f f i c i e n c y when growing i n l i g h t c o n d i t i o n s approximating those o f t h e i r h a b i t a t s .  S i m i l a r l y Milner  and Hiesey (1964) found i n c l i m a t i c races o f Mimulus c a r d i n a l i s  Dougl.  o v e r a l l r a t e s of photosynthesis were p r o p o r t i o n a l to the l e n g t h o f growing season i n t h e i r o r i g i n a l environment.  More r e c e n t l y Eagles  (1967) has shown that Danish populations o f perennial ryegrass had a lower optimum temperature  f o r net photosynthesis than populations from  Algeria. The c a p a c i t y o f the CO^ d i f f u s i o n process l i m i t s photosynthesis, not only by d i f f u s i o n r e s i s t a n c e s inside and at the l e a f surface but also by the r e s i s t a n c e to CO^ transport from the a i r above the crop^to the crop surface.  Taking a l l these r e s i s t a n c e s into account  Gaastra  (1962) has estimated that photosynthesis of c l o s e d crop surfaces i s l i m i t e d by d i f f u s i o n on most mid-summer days.  On such days the l i g h t  energy wasted due t o d i f f u s i o n l i m i t i n g photosynthesis i s about  40?°  of  that a v a i l a b l e f o r COg f i x a t i o n . Recently Sweet and Wareing (1966) have presented evidence which suggests that growth i t s e l f can l i m i t photosynthesis.  However, t h e i r  r e s u l t s d i d not show c o n c l u s i v e l y whether i t was a consequence o f a metabolic sink e f f e c t or an auxin l e v e l e f f e c t on photosynthesis. 2.2.2  Methods of Measuring Net Photosynthesis i n Forage Stands Measurements of net photosynthesis i n forage stands has u s u a l l y  been accomplished by measurement o f the CO^ exchange r a t e .  The most  convenient method f o r measuring the CO- concentration changes and hence  31 the exchange rates, i s by infrared absorption.  Alternatively "^CC^  radioactive tracer techniques may be used to estimate GO^ concentrations in the a i r and i t s incorporation into plants. two techniques w i l l be discussed separately.  The use of these  In addition, for  completeness, i t i s convenient to discuss the mathematical models for estimating net photosynthesis  in this section.  The use of infrared gas analysers (IRGA) to measure COg concentration in the a i r came into prominence in the early 1950s5 i t is now well known and needs no further elaboration.  However, the systems for  sampling the CO^ concentration of the a i r surrounding the plants needs discussion as there have been considerable changes and  refinements.,  The aim of a l l these has been to devise a system giving an authentic estimate of C0 convenient  2  exchange.  At present there is s t i l l debate on the most  and accurate system.  The early systems were generally only suitable for the measurements of exchange rates of plant parts.  For example, Parker (1953) enclosed  a small twig and leaves of Picea excelsa in a plastic tube and drew; a i r over the twig and leaves and then through an IRGA. He calculated the COg exchange from the difference in concentration between the inlet and outlet of the plastic tube.  By contrast Decker (1954) enclosed; the  above ground portion of scotch pine (Pinus sylvestris) seedlings.in a sealed chamber system and from the monitored rate of CO^ depletion between 270 and 330 ppm C0  2  he calculated ihe C0  2  exchange.  :  Orchard  and Heath (1963) have described an apparatus which can easily be modified to either a closed or open system.  The design of chambers for  small plant parts has been shown to be c r i t i c a l by the work of Avery  (1966), Hardwick et a l . (1966) and Etherington (1967). However,  32 these chambers are generally too bulky for use on leaves i n their natural f i e l d condition. For f i e l d studies of whole plants and groups of plants larger and more complex chambers are necessary so that whole plants can be enclosed i n their f i e l d state.  The systems cover the whole range  from the closed c i r c u i t of Decker Parker  (1953).  (1954)  to the open c i r c u i t of  In the closed c i r c u i t category Koller  and Samish  (1964)  designed a null point compensating system where additions or removals of COg, keeping a constant concentration, were measured.  The system  uses a polythene bag of suitable size to enclose the plants which although somewhat permeable to COg does not influence the results i f the n u l l point is kept at ambient levels of COg and i f a slight positive pressure is maintained within the bag, A semiclosed system was described by Musgrave and Moss (I96I)? j*  *  this has been used, and in some cases s l i g h t l y modified by Moss (1961), Baker and Musgrave (1964) and Baker (1965).  e_t a l .  Essentially the :  system was kept closed and the rate of COg depletion measured? however, they periodically metered in measured quantities of COg so that the concentration was kept at ambient +50  ppm COg.  I n i t i a l l y COg  assimilation was calculated from the rates of COg depletion. This was later modified so that the COg additions would maintain a smaller range and in this case the rates of COg uptake were calculated from the COg additions. The open c i r c u i t system has been used by a number of workers, Hesketh and Musgrave (1962), Hesketh and Moss (1963), Baker and Musgrave (1964)? Pearce et a l . (I965) and McCree and Troughton (1966). system, as CO^ uptake i s the product of flow rate and the C0  o  In this differ-  33 ential between the inlet and outlet, the flow rate is usually adjusted for a CO^ d i f f e r e n t i a l of not more than ambient +_ 25  ppm CO^.  1 (966)  larger d i f f e r e n t i a l the influence of the CO^ level on net must be taken into account.  McCree and Troughton  With a  photosynthesis corrected for  the COg d i f f e r e n t i a l by a linear relation, but a curvilinear one would be necessary i f the d i f f e r e n t i a l were great.  If the chamber volume  is kept large, compared to the foliage volume, a small d i f f e r e n t i a l can be achieved without high flow rates, which cause unnatural wind conditions.  Low flow rates, as shown by Takakura (l$66),  can result  in errors and under estimation of CG^ exchange due to diffusion problems. Takakura has shown that i f the chamber volume to foliage volume ratio is large, moderate flow rates give maximum CO^ exchange but i f the ratio is small, maximum exchange may never be observed even at extreme flow rates. A l l the systems described, apart from their own d i f f i c u l t i e s , have errors associated with the infrared analyses. have been stressed by Brown and Rosenberg  14 to infrared analyses  These analytical errors  1 (968. )  As an alternative  .CO,, may be introduced into closed chamber systems  and i t s rate of depletion monitored with a Geiger tube.  (196I)  Lister et_ a l .  have described such a system which also uses an IRGA so that-  levels of both ^ C 0  2  1 (967)  and "^CO,, may be followed.  More recently Austin and Longden  have described a small,  chamber which can be clamped to leaves in -fche f i e l d for short term 1  ^C0  2  exposure.  counting the  Here the C0  2  1 (967)  assimilation rates are calculated by  a c t i v i t y in the leaves.  Wolf  system for exposing whole plants in the f i e l d to C0 . 2  problems of quantity of isotope and contamination  has described a  However, here  of the s o i l become  34  important„ Infrared gas analysers can also be used to measure the COg gradient above a crop.  The COg moved into the area by the wind i s estimated  from the relative changes in the COg and wind profiles and together with an estimate of the COg liberated from the s o i l i s used to estimate the COg assimilation-  This system, because i t i s completely chamber-  less, imposes no a r t i f i c i a l environment on the crop. has been used by Inoue Szeicz  et_ a l .  The technique  (1958), Lemon (i960) and Monteith ..and  (i960). However, as pointed out by Monteith (1962) there i s  difficulty  in maintaining accurate records of the COg gradients over  long periods.  He has also shown that there is uncertainty about the  relationship between gradient and flux in the presence of temperature gradients.  These two reasons and a combination of them can, on  occasions, produce absurd flux values.  With care and with appreciation  of the limitations of the method worthwhile results can be obtained and often in situations which cannot be explored by the chamber methods.  For example, Wright and Lemon  (1966) have used this method  to examine the v e r t i c a l distribution of photosynthesis within a corn crop. Four mathematical models have recently been published to predict net photosynthesis in f i e l d crops? these are by Monteith de Wit  (1965), . i.  (1965),. Duncan et a l , (1967) and Idso and Baker (1967). The  model by Monteith  (1965) i s based on his binomial expansion for light  passing through a unit leaf layer, combined with two parameters describing the light response curve of single leaves and assumes that solar radiation varies sinusoidaly.  Net photosynthesis was then estim-  ated for any day length and solar radiation l e v e l .  He has shown that  35 t h e model's e s t i m a t e s rate experiments. profile,  model b y  e x p r e s s i o n but  height  de Wit  then  angles,  was  and  the  more complex? he u s e d a l i g h t  amount o f d i f f u s e  t h e r e s i s t a n c e t o COg  Most o f t h e s e  field  of  the l i g h t  observations.  l e a f a r e a and  Its overall  response curve  l e a f area,  leaf  angle  distribution  direct  geometric  extremely The  angle,  sky l i g h t  brightness.  good e s t i m a t e s model o f  o n l y w o r k e r s who  Idso  They allowed  light  and  electronic  The  and B a k e r  include a greater  f o r the  made b y  then a s c e r t a i n i n g the  the  leaves.  elevation, solar brightbeen shown t o  give  data.  (1967)  used the  light  It i s of note t h a t these  e n v i r o n m e n t was  b a s e d on m e t e o r o l o g i c a l  plant variables  of the  a n a l y s e d by  parameters.  c l a s s i f y i n g l e a v e s on t h e  The  and  temperature  authors  have i n c l u d e d t e m p e r a t u r e as a f a c t o r The  overall  Duncan et_ a l .  response curve  model has  those  suitability  t r a n s m i s s i v i t y and  f o r were sun  of a c t u a l  of p l a n t s .  p r e d i c t i o n models.  geometry was  were a b l e t o  position, reflectivity,  i n the p h o t o s y n t h e t i c  response curves  analyses  a i r to  problems  depends on t h e  sward.  at  light,>the  a knowledge o f t h e  i n the  thus  environment v a r i a b l e s allowed  n e s s and  attenuation  movement f r o m t h e  accuracy  e x p r e s s i o n and  u s e d a l a r g e r computer and  variations The  and  f a c t o r s represented  number o f f a c t o r s i n t h e i r model. of  expression.  T h i s model t o o , gave r e s u l t s w h i c h c l o s e l y a p p r o x i m a t e  of  (1967)  radiation  attenuation  have b e e n s o l v e d b y programming t h e model f o r an  computer.  growth  i n c l u d e d f a c t o r s f o r the d i s t r i b u t i o n of l e a v e s  the LAI,  o f t h e sun  the canopy. these  However, i t r e q u i r e s a knowledge o f t h e  l e a f t r a n s m i s s i o n and L A I f o r t h e l i g h t  The  various  are c l o s e t o a c t u a l d a t a from p u b l i s h e d  are  the  i n photosynthetic  energy  allowance  flow for leaf  plant f o r position  energy regime f o r each c l a s s .  The  model has  and been  36 used  on h y p o t h e t i c a l c r o p s t o p r e d i c t changes i n p r o d u c t i v i t y due t o  differences  2.2.3  i n leaf  geometry.  Measurements o f Net P h o t o s y n t h e s i s and P r o d u c t i o n from Forage Experiments  Stands  where b o t h d r y m a t t e r  have b e e n measured f o r f o r a g e s t a n d s ties  p r o d u c t i o n and n e t p h o t o s y n t h e s i s  a r e few^ due l a r g e l y t o d i f f i c u l -  i n measurement o f n e t p h o t o s y n t h e s i s .  de Wit's  I n 1962 S t a n h i l l  model f o r c a l c u l a t i n g p o t e n t i a l p h o t o s y n t h e s i s  used  (see S e c t i o n  2.2.2) and compared t h e s e r e s u l t s w i t h t h e measured d r y m a t t e r p r o d u c t i o n from  an a l f a l f a f i e l d .  :  The g e n e r a l s e a s o n a l t r e n d o f  measured and p r e d i c t e d v a l u e s was v e r y s i m i l a r , however, measured p r o d u c t i o n was c o n s i s t e n t l y l o w e r the l i g h t light  i n t e r c e p t i o n showed f o r a y e a r l y average  intercepted.  t h e p o t e n t i a l p r e d i c t e d from first  2  and McCloud (1962).  exchange u n d e r a r a n g e o f l i g h t  They u s e d  an IRGA i n  o f n e t p h o t o s y n t h e s i s o f 2.4g C 0  1 and 2 i n c h e s were n o t l i g h t i n response  showing more e r e c t l e a v e s .  Swards  /m / h r ,  1 while  s a t u r a t e d a t 7>000 f t - c .  appeared  L A I and a n g u l a r d i s t r i b u t i o n  2  intensities.  t o produce a s a t u r a t i o n  2  1  i n a forage  w i t h a chamber c o n t a i n i n g a s o d o f bermudagrass t o  c u t d a i l y t o 8 i n c h e s r e q u i r e d 5>000 f t - c .  difference  interception  production to reach  d i r e c t measurements o f n e t p h o t o s y n t h e s i s  a closed c i r c u i t  level  light  t h e p h o t o s y n t h e t i c model.  s t a n d were made b y A l e x a n d e r  measure t h e C0  o f d r y matter  Measurements o f  o n l y 33/£ o f t h e  This indicated that i n e f f i c i e n t  was a major r e a s o n f o r t h e f a i l u r e  The  than the p o t e n t i a l .  swards c u t t o This  t o be a consequence o f d i f f e r e n c e s i n  o f t h e l e a v e s , w i t h t h e 1 and 2 i n c h swards The l i g h t  A l l CO, a s s i m i l a t i o n r a t e s i n t h i s  response  c u r v e s o f bermudagrass  section are per unit  area of land.  37 swards a t L A I ' s between 14 and 25 were a l s o d e t e r m i n e d .  14  were l i g h t  those  saturated at  a t an L A I o f  2 1.2g nil  COg/m / h r .  25  5,000 f t - c .  were l i g h t  and f i x e d  saturated at  2.4g  Swards a t L A I  COg/m / h r . w h i l e  7,000 f t - c .  and f i x e d  The p h o t o s y n t h e t i c c a p a c i t y o f an 8 i n c h s t u b b l e ' was  when t h e L A I b e f o r e c u t t i n g was 20, b u t where t h e L A I was 14,  before  2 the s t u b b l e was c u t , t h e s t u b b l e f i x e d l , 2 g COg/m / h r . a t a l i g h t saturation level capable  4,000 f t - c .  of  2  The s t u b b l e b e l o w  i n c h e s was n o t  o f f i x i n g COg.  Billings  e_t a l .  (1966) measured  t h e COg a s s i m i l a t i o n o f a l p i n e  t u r f b y c o v e r i n g a s e c t i o n o f i t w i t h a chamber i n an open system w i t h an IRGA.  At a l i g h t  intensity of  2 2.1g  COg/m /hr.  r a t e cannot  fixed.  3,700 f t - c .  circuit they  However, as t h e p l a n t s a r e s p e c i a l i z e d  stands  this  be e v a l u a t e d i n terms o f L A I o r o t h e r p l a n t p r o d u c t i o n  measures. The  found  i most e x t e n s i v e i n v e s t i g a t i o n s  of net photosynthesis  i n forage  accompanied b y L A I measurements have b e e n made b y P e a r c e  (1965, 1967), Brown  et a l .  used  chamber s y s t e m w i t h an IRGA t o measure t h e COg  an open c i r c u i t  concentration d i f f e r e n t i a l . r e f r i g e r a t i o n and t h e l i g h t  (1966) and W i l f o n g  (1965) f o u n d  between 3 and 8. from  et a l .  The chamber t e m p e r a t u r e  With orchardgrass  the saturation l i g h t  The r e g r o w t h  weight  l e v e l was  They  was c o n t r o l l e d b y  i n t e n s i t y v a r i e d e i t h e r by c l o t h  o r b y u s i n g r e f l e c t o r f l o o d lamps. et a l .  (1967).  et a l .  stands  screens Pearce  4,500 f t - c .  f o r LAIs  o f herbage i n c r e a s e d l i n e a r l y .  8 t o 30 days a f t e r c u t t i n g w i t h 95$  light  interception after  15  2 days and a t a L A I o f 5.  A t a L A I o f 5 t h e swards f i x e d  however t h e r e was no c l e a r l y d e f i n e d o p t i m a l L A I as C 0  p  2.5g  COg/m / h r . ,  u p t a k e was  38 almost  t h e same o v e r t h e L A I r a n g e o f 3 t o 8.  regrowth  of d r y matter  L A I between 3 and 8,  also  incapable of u t i l i z i n g et a l .  i n t e r c e p t i o n and  i n d i c a t e d a s i m i l a r wide r a n g e i n o p t i m a l  Prom s t u d i e s o f n e t p h o t o s y n t h e s i s b e f o r e and  a f t e r c u t t i n g they found  Pearce  Light  t h a t l e a v e s belox* 4 i n c h e s i n t h e sward .were  the h i g h l i g h t  (1967) examined  L A I on t h e n e t p h o t o s y n t h e s i s  energies.  the influence o f l e a f  of a b a r l e y stand.  The d i f f e r e n c e s , i n  l e a f a n g l e were o b t a i n e d b y g r o w i n g t h e p l a n t s i n f l a t s at  v a r i o u s s l o p e s and t h e n r e t u r n i n g t h e f l a t s  p o s i t i o n f o r measurement.  L A I o f 11 t o i n t e r c e p t required  angle.  a t 18°  kept  differences i n  L e a v e s a t 90°  95/^ o f t h e i n c i d e n t  an L A I o f 7 and t h o s e  of s o i l  t o the h o r i z o n t a l  Above an L A I o f 2 t h e y f o u n d  n e t p h o t o s y n t h e s i s due t o t h e l e a f  a n g l e and  light  r e q u i r e d an  while those  a t 53°  r e q u i r e d o n l y an L A I o f 4.5'  At  2 an L A I o f 11 t h e s t a n d f i x e d  8g CC^/m / h r .  The d a t a f r o m  these  .* experiments light  was compared w i t h n e t p h o t o s y n t h e s i s  a t t e n u a t i o n model o f S a e k i  (i960)  and a c l o s e r e l a t i o n was  between t h e p r e d i c t e d and t h e e x p e r i m e n t a l Wilfong relationships these rate light  efc a l .  (1967) examined  in alfalfa  and L a d i n o  species they found as L A I i n c r e a s e d .  s a t u r a t i o n was r e a c h e d saturating light  the net photosynthesis c l o v e r swards.  Net p h o t o s y n t h e s i s  4>500 f t - c .  between  5,000 and  l e v e l s the Ladino  w h i l e t h e a l f a l f a sward f i x e d 7.0g t h e optimum L A I f o r t h e s e n a r r o w r a n g e viz„ L A I 2-5  while  and L A I  With stands o f  i n net a s s i m i l a t i o n  i n Ladino  c l o v e r swards was  i n a l f a l f a swards  7>000 f t - c .  c l o v e r sward f i x e d CC^/m / h r .  swards o c c u r r e d o v e r f o r Ladino  found  results.  a c u r v i l i n e a r decrease  s a t u r a t e d a t about  as p r e d i c t e d b y t h e  light  At t h e s e 3.0g  COg/m / h r .  The d a t a i n d i c a t e d t h a t a broad r a t h e r than  c l o v e r and L A I 2-8  for alfalfa.  3  In further work, the same group (Brown  ?  e_t a l . 1 9 6 6 ) confirmed  the above trends for swards of orchardgrass, Ladino clover and a l f a l f a although at saturating light levels, s l i g h t l y higher rates of net photosynthesis were reported.  They also examined bermudagrass swards;  with an LAI of 4 . 5 there was no light saturation of net photosynthesis even at 1 2 , 0 0 0 f t - c .  At 1 2 , 0 0 0 f t - c . the swards fixed 1 4 g C0 /m /hr., 2  a much higher rate than previously observed with other species. This agrees well with the evidence of Downton and Tregunna ( 1 9 6 7 ) that members of the ChlOrideae, such as bermudagrass, have l i t t l e or no photorespiration.  The earlier work, by Alexander and McCloud ( 1 9 6 2 ) ,  on bermudagrass indicated a lower C0  2  fixation rate.  This may have  been a low estimate as i t appears that they did not correct for COg evolution from the s o i l .  Monteith  from the s o i l can make up  2Cffo  e_t a l . ( 1 9 6 5 ) have shown that  CO^  of the carbon budget of actively grovring  grass swards. Net photosynthesis in a r t i f i c i a l forage stands has been studied by King and Evans ( 1 9 6 7 ) $ they grew stands of wheat, a l f a l f a and subterranean clover in phytotron cabinets and determined their 0 0 ^ uptake with an open c i r c u i t IRGA system.  These swards a l l showed a  rise in GO^ uptake with increases in LAI and then a plateau maximum. The LAI at which the maximum was reached increased with increasing intensity of illumination.  At the highest light intensity ( 3 , 3 0 0 f t - c  wheat fixed a maximum of 5 g C0 /m /hr. at an LAI of 5 2  While with  a l f a l f a stands the same rate as wheat was observed at an LAI of 1 0 , however, the response of these stands at 3 , 3 o o f t - c . indicated that maximum and plateau were outside the range of the experiment.  Sub-  terranean clover behaved in the same manner as a l f a l f a and showed no  40 2  plateau at LAI 5 and 3,300 f t - c . | here 3.2g COg/m /hr. were fixed. Leaf area indices above 10 i n a l f a l f a and 5 i n clovers rarely occur as the plants appear to self prune by senescence of the lower leaves. Speculation as to the behaviour of legume stands above these LAIs. i s therefore of dubious value. In a l l of the experiments described so f a r in this section there was no clear evidence of a decline in net photosynthesis above an "optimal" LAI.  The results of Alexander and McCloud (1962) vaguely  suggest such a behaviour above an LAI of 20 in bermudagrass and the results of King and Evans (1967) similarly suggest that this may happen about LAI 5 i energy.  n  subterranean clover given a very low level of light  Most of the evidence points to the wide rather than the narrow  '.'optimum" LAI.  This would indicate that l i v i n g lower leaves are not  often "parasitic" and often have a positive net assimilation rate;. The evidence that the lowermost leaves have no net COg assimilation does not conflict with the above as i t i s only necessary for them to show, zero net assimilation. Recently Hunt  r  e_t a l . (1967) have described a wind tunnel tech-  nique to measure net photosynthesis from profiles of COg, water vapour and wind velocity.  Prom these preliminary studies they observed a  2 maximum COg fixation of 7»lg COg/m /nr. for an a l f a l f a sward and 3«9g 2 2 COg/m /hr. in orchardgrass both at a light energy level 42cal/cm /hr.5 a level which should have light saturated the swards.  It i s of note  that the COg fixation levels from this method compare well with those reported for these species by Brown  et_ a l . (1966) and Wilfong (1967)  using the open c i r c u i t chamber technique.  41 2.2.4  The  E f f i c i e n c y o f L i g h t E n e r g y C o n v e r s i o n by F o r a g e  Stands  As has b e e n p r e v i o u s l y shown i n t h i s r e v i e w many f a c t o r s l i m i t photosynthesis  and  growth i n f o r a g e s t a n d s .  n u t r i e n t s , d i s e a s e and c o n t r o l by  When s u c h c o n t r o l s a r e s u c c e s s f u l t h e  growth o f t h e p l a n t s , s u i t a b l y a d a p t e d i n t e r c e p t i o n and C 0  discussed  i n S e c t i o n 2.2.1  intercept  a l l the l i g h t  2  to the p a r t i c u l a r c l i m a t e , ; a s s i m i l a t i o n b y t h e sward.  t h e L A I o f t h e sward must be  i f h i g h growth r a t e s a r e t o be  Assuming t h e r e f o r e a l l o t h e r f a c t o r s c l o s e t o o p t i m a l what t h e n  a r e o p t i m a l and  The  2  purpose of t h i s  t h i s q u e s t i o n , i n o t h e r words, how  sufficient  t h a t the LAI i s  energy use?  s e c t i o n i s to explore  much CO,, c a n be f i x e d  The  and w i t h what  answers o f t h e s e q u e s t i o n s a r e  as w e l l as p r o v i d i n g a s c a l e on w h i c h t h e e x t e n t a p a r t i c u l a r f a c t o r may In 1956  Niciporovic  e f f i c i e n c y of l i g h t At t h i s day.  be  put., f o r w a r d  of a growth  a g e n e r a l f i g u r e o f Ijfo  agricultural  crops  e f f i c i e n c y o f 14$ However, l o w e r  (1958)  and Blackman and B l a c k  CO,,.  was  9fo,  w h i c h was  w h i l e w i t h b a r l e y , Kamel  (1966)  (1959)  /  found  i t s efficiency  (1959)  found  an  v e r y c l o s e to N i c i p o r o v i c • s estimate.  e f f i c i e n c i e s have b e e n r e p o r t e d too. Rees  while Cooper  f o r the  2  w i t h o i l palm p l a n t a t i o n s f o u n d t h e e f f i c i e n c y o f l i g h t 1,2^/0  limitation  in fixing  t h a t d u r i n g t h e most a c t i v e growth phase o f s u g a r b e e t energy use  plant species  e s t i m a t e d t h a t t h e s e c r o p s c o u l d f i x lOOg C0 /m  More r e c e n t l y G a s t r a  of l i g h t  of  judged.  energy use by  e f f i c i e n c y he  to  achieved.  v a l u e as t h e y a l l o w t h e c o m p a r i s o n o f swards o f d i f f e r e n t  by  As  i s the p o t e n t i a l p r o d u c t i v i t y or c a p a c i t y o f  a f o r a g e sward t o f i x C 0 ?  e f f i c i e n c y of l i g h t  moisture,  i n s e c t s a r e a l l s u b j e c t t o some measure o f  agriculturalists.  depends on l i g h t  F a c t o r s s u c h as  net  (1962)  working  e n e r g y use, was  with p e r e n n i a l ryegrass pastures found  only a  42 2/o e f f i c i e n c y .  Both these workers found h i g h e r e f f i c i e n c i e s but these  were o n l y f o r t h e s e e d l i n g y e a r o f t h e c o m m u n i t i e s . p o i n t e d out t h a t e f f i c i e n c i e s  as h i g h as 20$  cultures  energy  level  s u b j e c t t o low l i g h t  Bonner  (1962) h a s  are possible with  algal  l e v e l s , b u t as t h e l i g h t  energy  i n c r e a s e s t h e e f f i c i e n c y d e c l i n e s t o about 5$ due t o s h a d i n g  between t h e c e l l s . decline  The same o c c u r s w i t h f o r a g e a s s o c i a t i o n s where t h e  i n efficiency  i s a c o n s e q u e n c e o f i n t r a - and i n t e r - l e a f  shading  as w e l l as mutual s h a d i n g between t h e c e l l s  and c h l o r o p l a s t s .  c l e a r t h e r e f o r e t h a t t h e lower  f o r e s t a b l i s h e d stands are  realistic,  especially  efficiencies  i f t h e y a r e s u b j e c t t o h i g h mid-summer  levels f o r a large proportion of their  (1963) have  Loomis and W i l l i a m s  radiation  growing p e r i o d .  estimated t h e upper l i m i t  p r o d u c t i v i t y b y e v a l u a t i n g t h e a b s o r p t i o n and u t i l i z a t i o n r a d i a t i o n on a quantum b a s i s .  It i s  of crop  of solar  They estimated that with a l i g h t  energy  2 i n p u t o f 222  cal/cm  / d a y and 8.64  jiEinsteins/cal.  t h e COg f i x a t i o n  2 c o u l d r e a c h 104g  COg/m / d a y w h i c h c o r r e s p o n d e d  e n e r g y u s e o f 12$,  i n c l u d i n g an a l l o w a n c e  f o r the inorganic n u t r i e n t  i v i t y was 77g  content g  organic material/m  for respiration.  / d a y i . e . 690  i s s u r p r i s i n g l y close to the e a r l i e r  (1956).  W i l l i a m s and Loomis  51g/m  / d a y and a l i g h t  r e c e n t l y Begg  energy  (1963) r e p o r t e d  lb/acre/day.  product-  This  ;  estimate by Niciporovic, that the highest  with a d r y matter  a s i m i l a r d r y matter  producer  productivity of  c o n v e r s i o n e f f i c i e n c y o f 6.7$.  (1965) r e p o r t e d  Correcting  their figure for potential  estimate  t h e y c o u l d f i n d was s u d a n - g r a s s  t o an e f f i c i e n c y o f l i g h t  More  production with  2 Bulrush m i l l e t  (Pennisetum  conversion e f f i c i e n c y of  (1968) s u g g e s t  typhoides^ o f  9«5$»  54g/m  I t i s o f note  /day with a l i g h t  energy  t h a t Downton and T r e g u n n a  t h a t b o t h t h e s e p l a n t s would have no p h o t o r e s p i r a t i o n .  43 Using the assumption that a maize stand was as efficient as Chlorella on a quantum basis Yocum, et a l . efficiency of light energy use of 12$.  (1964) estimated  a maximum  However, from their f i e l d  studies of CO^ exchange they found that the efficiency was only  5.1$.  Uncertainty as to the magnitude of dark and photorespiration makes estimation of the potential maximum CO,, uptake rate d i f f i c u l t , however, taking the overall figure of a 33$ loss for total respiration used by Loomis and Williams  (1963) and  their productivity estimate, the  average CO^ uptake would be 12.8g by Yocum  et a l .  (1964) for  CC^/m  /hr.  the maize with the  The average rate quoted  5*1$  efficiency was  2 9,.0g CO^/m  /hr. indicating a considerable margin available for  increased efficiency and uptake for this crop as well as the forage crops mentioned in Section Black  (1964)  2.2.3.  r  used a different approach.  With a knowledge of the  growth responses of subterranean clover to radiation and using a mean seasonal radiation level and an average c a l o r i f i c value of 4>0QO cal/gram he found a maximum efficiency of light energy use of  4.2$.  For the environment of Adelaide, South Australia he calculated a seasonal dry matter production of 65 metric tons/hectare (m.tons/ha) was possible compared with an actual average of 8 m.tons/ha.  He  demonstrated that the actual production was reduced by 32 m.tons/ha with summer drought and by 2 m.tons/ha with sub-optimal LAI and by 16 m.tons/ha with a super-optimal LAI.  With none of these factors,  2 limiting the maximum daily rate of production, would be 23 g/m /day and  2 the minimum lOg/m /day with a seasonal average efficiency of light use of  2.9$. Recently Hunt and Cooper  (1967) have  compared the efficiency of  44 l i g h t e n e r g y c o n v e r s i o n "by a number o f f o r a g e s t a n d s as t h e y r e g r e w f r o m 507° l i g h t i n t e r c e p t i o n .  R o u g h - s t a l k meadowgrass ( P o a t r i v i a l i s )  2 w i t h a g r o w t h r a t e o f 7g d r y m a t t e r / m / d a y showed a n e f f i c i e n c y o f  2 3.3$ w h i l e t a l l f e s c u e w i t h a g r o w t h r a t e o f 12g/m / d a y showed an efficiency  o f 7.2/0.  Prom t h e i r measurements o f CO^ u p t a k e under; g r o w t h  chamber c o n d i t i o n s K i n g and E v a n s (1967) f o u n d t h a t t h e e f f i c i e n c i e s o f l i g h t e n e r g y u s e f o r swards o f wheat, l u c e r n e and s u b t e r r a n e a n .  2 c l o v e r were 12.8, 13.5 a n d 9.8 a t g r o w t h r a t e s o f 30, 39 arid 20g/m / d a y respectively.  These a u t h o r s f o u n d t h a t t h e g r o w t h r a t e s were  by a r e l a t i v e l y h i g h r a t e o f d a r k r e s p i r a t i o n , 50$  of gross photosynthesis.  depressed  o f t e n b e t w e e n 40 a n d  The h i g h e f f i c i e n c i e s o f l i g h t e n e r g y  use,  b a s e d on g r o s s CO^ u p t a k e , t h e r e f o r e r e f l e c t t h e optimum CO,-, d i f f u s i o n c o n d i t i o n s and a r e t h e r e f o r e i n l i n e w i t h t h e l o w e r f i e l d where C0  o  a v a i l a b i l i t y c o u l d be l i m i t i n g i t s u p t a k e .  estimates  45  3.  F I E L D EXPERIMENT WITH DEFOLIATION BY ANIMALS The  of  experiment  t h r e e mentioned  evident from t h i s  reported i n this  section  i n the introduction: experiment  i s the f i r s t  The problems  are then i n v e s t i g a t e d  i n the s e r i e s  w h i c h became  i n the- two l a t e r  experiments.  3•1 A  M a t e r i a l s and Methods The e x p e r i m e n t a l a r e a was l o c a t e d  S t a t i o n at the U n i v e r s i t y Climatically,  on t h e L a u r e l d a l e  o f New E n g l a n d , New S o u t h Wales,  c o o l w i t h a J u l y mean maximum o f  o f 28.9  53.1°F,  summers a r e warm w i t h t e m p e r a t u r e s  of  inches.  Winters are  and mean minimum o f  80.0°F  commonly t o g i v e a J a n u a r y mean maximum  1948).  Australia.  t h e a r e a i s c h a r a c t e r i s e d b y summer r a i n w i t h h i g h  v a r i a b i l i t y and a mean a n n u a l r a i n f a l l  (Roe,  Research  and  80.0°F  90.0°F  Wisenboden/Black  earth  In F e b r u a r y 1962  (Stephens, grass-legume  et a l .  and  occurring  and mean minimum  The s o i l t y p e i n t h e e x p e r i m e n t a l a r e a i s a  c h o c o l a t e " , as d e s c r i b e d b y H a l l s w o r t h  32.7°F  55.1°F,  "normal  (1952), w i t h  some  1953). p a s t u r e s t a n d s were  established.  E a c h s t a n d h a d t h e same c l o v e r m i x t u r e and one o f t h e f o l l o w i n g g r a s s e s : - New Z e a l a n d P e r e n n i a l r y e g r a s s ( L o l i u m perenne  L . ) , Grass-  46  l a n d s A r i k i r y e g r a s s (L_. p e r e n n e x L_. m u l t i f l o r u m x L. p e r e n n e ) , Grasslands Cocksfoot  ( D a c t y l i s g l o m e r a t a L . ) , S143 C o c k s f o o t , S26  C o c k s f o o t , A l t a Fescue ( F e s t u c a arundinacea  Schreb.) and S170 T a l l  Fescue.  The s e e d i n g r a t e s were 1 0 l b . p e r a c r e f o r r y e g r a s s and f e s c u e s , a n d 8 l b , per acre f o r c o c k s f o o t s .  E a c h o f t h e above p a s t u r e g r a s s e s  d r i l l s o w n t o g e t h e r w i t h 2 cwt. p e r acre o f superphosphate,  were  2 l b . per :  a c r e o f New Z e a l a n d w h i t e c l o v e r ( T r i f o l i u m r e pens L . ) , l-§- l b . p e r a c r e o f Montgomery r e d c l o v e r ( T r i f o l i u m p r a t ens e L.) and 1-g- l b . p e r a c r e Broad r e d c l o v e r .  The f i e l d d e s i g n was a r a n d o m i s e d b l o c k - s p l i t  plot.  E a c h o f t h r e e b l o c k s were d i v i d e d i n t o s e v e n p l o t s and w i t h i n e a c h b l o c k t h e seven grass-legume stands plots.  were r a n d o m l y a l l o c a t e d t o t h e  E a c h p l o t was s p l i t i n t o 3 s u b - p l o t s f o r t h r e e g r a z i n g manage-  ments i n w h i c h sheep were u s e d as d e f o l i a t o r s . t h r e e g r a z i n g managements managements (1)  t o those  s u b - p l o t s was random.  The h e r b a g e grew t o 3 - 4 i n . and t h e n was g r a z e d t o a h e i g h t o f (3-l).  The h e r b a g e grew t o 9 - 1 0 i n . and t h e n was g r a z e d t o a h e i g h t ' o f 1  (3)  The g r a z i n g  weres-  1 - i n . , h e n c e f o r t h d e s i g n a t e d as management (2)  A l l o c a t i o n of the  i n . v i z . management  (9~l).  The h e r b a g e grew t o 9 - 1 0 i n . and t h e n was g r a z e d t o a h e i g h t - o f 3 i n . v i z . management  (9-3).  E a c h o f t h e 63 sub p l o t s measured 39 f t . x 6 3 f t . By A p r i l , 1 9 6 2 , t h e p a s t u r e s were w e l l e s t a b l i s h e d and r e c e i v e d a common g r a z i n g w i t h sheep. i n February,  1  From t h a t d a t e u n t i l t h e e x p e r i m e n t  began  1 9 6 3 , t h e p a s t u r e s c o n t i n u e d t o r e c e i v e a common g r a z i n g  The i n d i v i d u a l g r a s s - l e g u m e s t a n d s w i l l be r e f e r r e d t o b y t h e s t a n d a r d common name o f t h e i r sown g r a s s .  47 management d e s i g n e d t o k e e p them b e t w e e n 1 and 6 i n c h e s i n h e i g h t . D u r i n g t h i s p e r i o d t h e a r e a was The  fenced i n t o the a p p r o p r i a t e paddocks.  g r a z i n g managements were f i r s t  a p p l i e d i n F e b r u a r y , 1963  and  f r o m t h i s d a t e y i e l d s o f h e r b a g e and b o t a n i c a l c o m p o s i t i o n a t t h e o f g r a z i n g were r e c o r d e d g r a z i n g was  From A u g u s t ,  also recorded.  Records  s i t e a l s o commenced i n A u g u s t , managements w a 3 as f o l l o w s s  1963,  time  l e a f area index (LAl) at  of r a i n f a l l at the experimental,  1963.  The  procedure  when t h e average  f o r the g r a z i n g  height of the  herbage,  between t h e t h r e e b l o c k s o f a p a r t i c u l a r p a s t u r e and g r a z i n g management was  j u d g e d t o have r e a c h e d t h e g r a z i n g h e i g h t , as e s t i m a t e d b y m e a s u r e -  ment w i t h a s t a f f , s a m p l e s o f h e r b a g e were c u t t o d e t e r m i n e yield.  The  the  plot  sample c o n s i s t e d o f f o u r random 6 f t . x 1 f t . q u a d r a t s  cut  from e a c h p l o t a t a h e i g h t a p p r o p r i a t e t o t h e p a r t i c u l a r g r a z i n g , management  E a c h s u b - p l o t sample was  f o r d r y m a t t e r d e t e r m i n a t i o n (200  m i x e d , w e i g h e d and  sub-sampled  g) and b o t a n i c a l a n a l y s i s (50 g ) . '  B o t a n i c a l a n a l y s e s were b y hand s e p a r a t i o n o f t h e h e r b a g e i n t o t h r e e c a t e g o r i e s v i z 5 g r e e n g r a s s , g r e e n c l o v e r and "dead m a t e r i a l " of senescent  and dead g r a s s and c l o v e r .  consisting  After botanical analysis  and  b e f o r e d r y i n g , t h e l e a f a r e a s o f t h e g r e e n g r a s s and c l o v e r components o f t h e p a s t u r e were d e t e r m i n e d w i t h an a i r - f l o w p l a n i m e t e r ( J e n k i n s , 1959)' The  A l l d r y i n g o f h e r b a g e was  done i n a f o r c e d - d r a u g h t oven a t 90°C.  y i e l d o f h e r b a g e on e a c h s u b - p l o t was  then used  to estimate  t h e number o f sheep r e q u i r e d t o g r a z e t h e h e r b a g e t o t r e a t m e n t i n t h r e e days.  The  e s t i m a t e was b a s e d  on mean l i v e w e i g h t  height  of the  sheep^ as e x p e r i e n c e had shown t h a t t h e y w o u l d consume d r y m a t t e r e q u i v a l e n t t o a p p r o x i m a t e l y 2$ o f t h e i r body w e i g h t  e a c h day.  a p p r o p r i a t e number o f s h e e p were t h e n p l a c e d on t h e p l o t s .  The  Usually.  48 g r a z i n g commenced ensure  t h e day a f t e r t h e herbage samples were t a k e n .  that f e r t i l i t y  t r a n s f e r was m i n i m a l ,  t h e sheep were always  removed a f t e r t h r e e d a y s even i f s m a l l p a t c h e s consumed. of  The sheep were a l s o p r e g r a z e d  the experimental  The  herbage l e f t  a f t e r g r a z i n g was t o p p e d  g r a z i n g management  on a l l p l o t s were t a k e n  Results  3.2.1  Production of Individual P r o d u c t i o n o f herbage  the f i r s t  (Table  3-1).  The r a i n f a l l  a marked r e d u c t i o n i n p r e c i p i t a t i o n  essentially similar  while the remaining the y i e l d  records  (Table 3.II) also  i n the- s e c o n d  Dead f o l i a g e t h e S143  fescue  lower  than  show  year of the t r i a l .  o f t h e sown g r a s s components p a s t u r e s  i n both years.  S 1 7 0 tall  productive In 1964,,  f e s c u e and c o c k s f o o t  than that growing with the ryegrasses  a l s o showed d i f f e r e n c e s  senescence  was  (P<0.00l).  i n 1964, w i t h g r e a t e s t  and S 2 6 c o c k s f o o t and l e a s t  was  f e s c u e was t h e most  g r a s s e s showed i n t e r m e d i a t e p r o d u c t i o n .  o f the c l o v e r s growing w i t h the t a l l  significantly  of potash.  y e a r was c o n s i d e r a b l y l e s s  p r o d u c t i v e p a s t u r e g r a s s and p e r e n n i a l r y e g r a s s t h e l e a s t  in  area received  Pastures  i n the second  However, t h e p r o d u c t i v i t y r a n k  the p l o t .  t o a l l o w assessment o f t h e y i e l d s f o r  p e r a c r e s u p e r p h o s p h a t e and 0 . 5 cwt. p e r a c r e m u r i a t e  cwt  errors.  1 s t ) c u t s o f herbage ..  E a c h autumn t h e e x p e r i m e n t a l  3.2  in  transfer  w i t h a r o t a r y mower t o t h e  ( J u l y 3 1 s t and F e b r u a r y  two growth p e r i o d s p e r y e a r . 2  fertility  s i m i l a r to those  h e i g h t and t h e c l i p p i n g s d i s t r i b u t e d o v e r  E v e r y s i x months present  o f herbage were n o t  on p a s t u r e s  p l o t s t o f u r t h e r reduce  To  senescence  i n t h e S170  tall  swards.  The  p a s t u r e t y p e s d i d n o t show s i m i l a r t o t a l  p r o d u c t i o n i n t h e two  Table  3.1  Mean y i e l d s o f seven p a s t u r e stands i n lb./acre/annum o f d r y m a t t e r  S170 Pasture Component  Perennial Ryegrass  Ariki Ryegrass  Grasslands Cocksfoot  S143 Cocksfoot  S26 . Cocksfoot  Alta Fescue  Tall Fescue  S.E. i Means  1963 1227  1705  2005  1741  1962  1813  2179  136  Clover  929  829  977  1205  1037  819  754  99  Dead F o l i a g e  341  464  386  470  382  473  397  55  Grass  1964 Grass  232  315  334  338  390  515  744  50  Clover  472  526  245  401  254  175  213  44  Dead F o l i a g e  276  235  220  460  386  220  121  33  2998  2632  2933  142  •  Year  Total Production (grass + c l o v e r )  1963  2156  2553  2982  2946  1964  704  841  579  740  645 .  960 .  957-.  58  50  I n 1 9 6 3 t h e p e r e n n i a l r y e g r a s s p a s t u r e s showed  years.  t o t a l production ( P < 0 . 0 1 )  less  than the r e s t ,  significantly  while i n 1964, the  G r a s s l a n d s c o c k s f o o t p a s t u r e s showed s i g n i f i c a n t l y l o w e r t o t a l ion than the other pastures ( P < 0 . 0 l ) .  product-  F u r t h e r i n 1 9 6 4 t h e S170  tall  f e s c u e p a s t u r e s showed s i g n i f i c a n t l y g r e a t e r t o t a l p r o d u c t i o n t h a n t h e other pastures ( P < 0 . 0 l ) .  The p r o d u c t i o n o f t h e p a s t u r e s was  examined w i t h t h e y e a r d i v i d e d i n t o F e b r u a r y t o J u l y (Autumn - W i n t e r ) January  ( S p r i n g - Summer).  nificantly different  two g r o w t h s e a s o n s ,  and t h e o t h e r f r o m A u g u s t t o  i n t h e two g r o w t h s e a s o n s  (P<0.05  In 1963, t h i s  3. I I R a i n f a l l , monthly t o t a l s  Table  one f r o m  P r o d u c t i o n f r o m t h e sown g r a s s e s  P..< 0 . 0 1 i n 1 9 6 4 s e e T a b l e ' 3 . I I I ) .  also  was.sig-  i n 1 9 6 3 , and  interaction  with  i n inches.  Au±umn 'Winter  Feb,  Mar.  Apr.  May  June  July  Season Total  1963  1.25  5.46  1.96  7.78  1.68  1.13  19.26  1964  0.68  5.12  2.25  0.52  1.16  3.31  13.04  Spring Summer  Aug.  Sept.  Oct.  Nov.  Dec.  J an.  1963/4  1.98  0.10  1.94  2.76  1.78  6.84  15.40  1964/5  1.04  1.95  2.04  3.15  1.16  1.47  10.81  P.O.  2% m i l e s  1  T o t a l f o r Feb.  1 9 6 3 t o Jan., 1 9 6 4  34.66 ,in.  T o t a l _ f o r Feb.  1964 t o J a n . 1965  23.85  1  in.  The d a t a f o r F e b . t o J u l y I 9 6 3 i s f r o m • A r m i d a l e d i s t a n t from t h e e x p e r i m e n t a l s i t e .  Table Seasonal  Growth Season  Perennial Ryegrass  3.Ill  i n t e r a c t i o n s w i t h sown g r a s s p r o d u c t i o n , mean y i e l d s i n l b . / a c r e of d r y m a t t e r .  Ariki Ryegrass  Grasslands Cocksfoot  S143 Cocksfoot  given  . S26 Cocksfoot  Alta Pescue  S170 Tall Fescue  S.E. o f Means +  Grass 1963 AutumnWinter  1054  1344  1721  1422  1531  1399  1494  SpringSummer  1394  2065  2288  2060  2393  2227  2865  146  Grass 1964 AutumnWinter SpringSummer  354  366  180  336  291  331  561  110  264  488  340  488  699  927  293  90  Dead M a t e r i a l 1964 AutumnWinter  270  300  327  302  428  256  SpringSummer  283  169  113  617  343  184  1.  67  52 s e a s o n o c c u r r e d b e c a u s e o f h i g h p r o d u c t i o n b y t h e S170 t a l l ures  i n t h e s p r i n g - summer w h i l e  fescue  fescue  i n t h e autumn - w i n t e r p e r i o d  past-  tall  In 1964? the  p r o d u c t i o n was s i m i l a r t o t h a t o f t h e o t h e r g r a s s e s .  i n t e r a c t i o n w i t h s e a s o n was a c o n s e q u e n c e o f l o w autumn - w i n t e r  prod-  u c t i o n by the Grasslands  cocksfoot  and l o w s p r i n g - summer p r o d u c t i o n b y  the p e r e n n i a l r y e g r a s s ,  Dead f o l i a g e t o o , showed a h i g h l y s i g n i f i c a n t  i n t e r a c t i o n w i t h season i n 1964 ( P < 0 . 0 0 l ) ,  h e r b a g e s e n e s c e n c e was  uni-  f o r m d u r i n g t h e autumn - w i n t e r p e r i o d ; i n t h e s p r i n g - summer S 1 7 0 t a l l fescue  showed l i t t l e  3.2.2  Management  s e n e s c e n c e and t h e S 1 4 3 c o c k s f o o t much s e n e s c e n c e .  E f f e c t s on D r y M a t t e r  Production  The e f f e c t o f g r a z i n g management on d r y m a t t e r y i e l d was in both years  (Table  3 . TV).  The ( 9 - l ) management gave t h e g r e a t e s t  t o t a l herbage p r o d u c t i o n , the (3-1) (P<0.00l). for  The p a s t u r e  grass production  similar  intermediate  and t h e (9-3)  least  components a l s o f o l l o w e d t h i s p a t t e r n e x c e p t  i n I 9 6 4 where t h e ( 3 - l ) management gave  slightly  more g r a s s d r y m a t t e r t h a n t h e ( 9 - 1 ) . Table  3.XV  Pasture Component  The i n f l u e n c e o f g r a z i n g management on p a s t u r e given i n lb./acre/annum of d r y matter.  Grazing 3-1  Management 9-1  9-3  production,  S.E. o f Means +  1963  Grass  1921  Clover  992  Dead f o l i a g e  501  2423  IO69  78  1155  669  53  202  25  546  1964  Grass  601  530  98  32  Clover  315  529  136  30  Dead f o l i a g e  176  517  129  15  53 The s e a s o n o f g r o w t h a l s o i n f l u e n c e d t h e r e l a t i v e between t h e g r a z i n g managements ( T a b l e  3.V)..  differences  Between t h e (9-1) and  the  (3-1) managements, t h e d i f f e r e n c e i n p r o d u c t i o n  of grass,  and  dead m a t e r i a l was l e a s t i n t h e autumn and g r e a t e s t  clover  i n the spring,  however b e t w e e n t h e two i n t e n s i v e g r a z i n g managements and t h e (9-3) management, t h e d i f f e r e n c e s were g r e a t e s t the s p r i n g .  i n t h e autumn and l e a s t i n  The i n t e r a c t i o n was h i g h l y s i g n i f i c a n t ( P < 0 . 0 0 l ) i n 1964>  with n e g l i g i b l e production  f r o m t h e ( 9 3 ) t r e a t m e n t s i n t h e autumn.. -  W i t h dead f o l i a g e t h e i n t e r a c t i o n i n 1963 senescence i n t h e (9-3).  a  s  a  r e s u l t o f l o w autumn  However, i n 1964, i t was a r e s u l t o f v e r j r  low s e n e s c e n c e i n t h e (3-1) management  T a b l e 3»V  w  i n t h e s p r i n g ( T a b l e 3.V).  The i n f l u e n c e o f s e a s o n and g r a z i n g management production, given i n lb./acre o f d r y matter.  Growth S e a s o n Grazing Management  Autumn - W i n t e r  3-1  9-1  on p a s t u r e  Spr i n g - Summer  9-3  Pasture Compcnent  3-1  9-1  9-3  ;  S.E. of MeaiiE  1963 1643  1988  650  2209  2858  1488  95.'  Clover  303  371  183  1681  1939  1154  79  Dead f o l i a g e  232  222  60  769  871  343  40'  Grass  196"4 Grass  738  318  1  464  743  215  59  Clover  270  163  1  360  894  289  35  Dead f o l i a g e  340  446  146  11  588  112  44  54 3.2.3  Species The  and G r a z i n g Management  s p e c i e s x management  significant  a r e shown  Grasslands  cocksfoot  management  favoured  (P<0.01). production  was l o w e r t h a n  ryegrass  3.2.4*  while  whereas t h e  (9 l) -  strains of t a l l  the c l o v e r  fescue  fescue  and c l o v e r  i n the cocksfoot  and t a l l  ryegrass.  produotion  and t a l l  pastures  r e s u l t e d i n d i f f e r i n g senescence  fescue  subject to  showed a wide r a n g e o f g r a s s s c l o v e r  and L e a f  Area  ratios.  i n the pastures  Index R e l a t i o n s h i p s  examine t h e r e l a t i o n s h i p between t h e t o t a l d r y m a t t e r y i e l d and  the L A I t h e d a t a were r e d u c e d  into  'the c o c k s f o o t s ' and 'the f e s c u e s ' To  pastures  management  (P < 0 . 0 1 and P < 0 . 0 0 l ) .  Dry Matter  To  pastures,  the cocksfoot  g r a z i n g managements  (3-1)  the  gave s i m i l a r g r a s s  the grass production  -  years  1964?  statistically-  p r o d u c t i o n from the p e r e n n i a l  management  (9 l) g r a z i n g management  in both  In  g r a s s p r o d u c t i o n from b o t h  p a s t u r e s . However,  The  3.VI.  and S 1 4 3 c o c k s f o o t  The (3-1) i n both  i n t e r a c t i o n s w h i c h were  i n Table  showed t h e g r e a t e s t g r a s s  Interactions  the d a t a  i n these  s i x groups,  'the r y e g r a s s e s ' ,  and t h e t h r e e g r a z i n g  groups q u a r t i c polynomial  managements.  r e g r e s s i o n s were  fitted  w i t h t o t a l d r y m a t t e r as t h e dependant v a r i a b l e and L A I as t h e independant v a r i a b l e . linear  The s i g n i f i c a n c e  of f i t t i n g  were t e s t e d b y a n a l y s i s o f v a r i a n c e .  a q u a d r a t i c term produced a s i g n i f i c a n t These g r o u p s were c o c k s f o o t The  (P<0.00l),  terms h i g h e r  In t h r e e g r o u p s o f d a t a  r e d u c t i o n i n the e r r o r v a r i a n c e .  (9-l) (P<0.05),  r e g r e s s i o n s f o r t h e s i x g r o u p s were a l l h i g h l y  (P<0.001,  Table  3.VII).  (9-3) ( P < 0 . 0 5 ) . significant  The r e g r e s s i o n s show t h a t a t h i g h L A I s  was a r e d u c t i o n i n t h e i n c r e m e n t  than  o f d r y matter f o r each u n i t  there  LAI i n the  T a b l e 5.VI The i n t e r a c t i o n s o f p a s t u r e stands and g r a z i n g management on d r y m a t t e r y i e l d , i n lb./acre/annum.  Management  Perennial Ryegrass .  Ariki Ryegrass  Grasslands •Cocksfoot  S143 Cocksfoot  S26 Cocksfoot  Alta Pescue  S170 Tall Pescue  S.E. of Means +  Grass 1964 3-1 9-1 9-3  526 133 38  468 420 57  717 301 1  548 392 72  571 566 33  599 707 240  777 ) 1195 ) 261 )  85  268 478 17  173 251 102  149 ) 327 ) 160 )  79  616 347 183  461 772 185  323 ) 631 ) 238 )  67  285 685 188  106 481 73  69 ) 247 ) 48 )  67  C l o v e r 1964 3-1  9-1 9-3  603 604 209  445 874 258  179 396 160  386 771 477  Dead M a t e r i a l 1963 3-1 9-1 •  9-3  475 436 114  601 620 171  435 . 465 258  594 553 263  Dead M a t e r i a l 1964 3-1 9-1 9-3  165 605 60  193 426 85  167 321 172  246 855 279  T a b l e 3. VII  Mean d r y m a t t e r y i e l d p e r g r a z i n g i n l b . a c r e and M I a t g r a z i n g f o r d a t a from August 1963 t o January 1965 t o g e t h e r w i t h c o e f f i c i e n t s f o r r e g r e s s i o n equations r e l a t i n g dry m a t t e r y i e l d t o LAI i n the form d r y m a t t e r y i e l d ( Y ) = a + b.LAI + C.LAI 2  Treatment  LAI  Yield/ grazing  •  Regression c o e f f i c i e n t s a •b c  Ryegrass  1.24  1105  206.5  727.2  Cocksfoot  2.23  1133  11.2  618.0  T a l l Fescue  1.45  1109  305.9  3-1  1.42  837  9-1  2.52  9-3  1.51  1  Correlation Coefficients  No. o f Observation  0.849  78  -35.8  0.866  129  555.0  NS  0.775  87  231.6  390.8  NS  0.820  150  1804  691.3  604.2  -48.1  0.628  78  951  109.5  721.9  -64.1  0.822  66  NS i n d i c a t e s non s i g n i f i c a n t r e g r e s s i o n c o e f f i c i e n t .  57  two g r a z i n g managements was 9 i n c h e s .  w h i c h were d e f o l i a t e d  Further this effect  c o c k s f o o t as t h e g r a s s component. matter had  when t h e p a s t u r e  o n l y occurred i n the pastures  with  As t h e r e l a t i o n s h i p b e t w e e n d r y  and L A I i n t h e c o c k s f o o t p a s t u r e s was c u r v i l i n e a r t h e s e  a higher L A I at g r a z i n g than the ryegrass or t a l l fescue  Although  height  pastures  pastures.  t h e "b" c o e f f i c i e n t f o r t h e c o c k s f o o t p a s t u r e s was g r e a t e r  than t h a t of the t a l l  fescue pastures  i t d i d not r e s u l t  i n greater  y i e l d s per g r a z i n g because of the c u r v i l i n e a r i t y i n the d r y matter response.  The r y e g r a s s p a s t u r e s  but t h i s d i d not r e s u l t  showed t h e l a r g e s t  "b" c o e f f i c i e n t  i n a h i g h e r mean y i e l d p e r g r a z i n g compared  w i t h c o c k s f o o t and t a l l f e s c u e p a s t u r e s .  Thus d e s p i t e t h e d i f f e r e n c e s  between t h e r e g r e s s i o n s t h e t h r e e p a s t u r e t y p e s ly  LAI  t h e same mean d r y m a t t e r  a l l produced approximate-  y i e l d per grazing.  W i t h t h e management r e g r e s s i o n s t h e r e were d i f f e r e n c e s i n t h e r e g r e s s i o n c o e f f i c i e n t s w h i c h r e f l e c t e d t h e d i f f e r i n g y i e l d s p e r •• grazing.  The s l o p e s and c u r v i l i n e a r i t y o f t h e (9-1) and t h e (9-3)  r e g r e s s i o n s were s i m i l a r and t h e d i f f e r i n g y i e l d s o f d r y m a t t e r p e r g r a z i n g was a c o n s e q u e n c e o f t h e d i f f e r i n g amounts o f l e a f a r e a removed w i t h each g r a z i n g ( c f . the "a" c o e f f i c i e n t s ) . management showed a s u b s t a n t i a l l y  lower d r y matter  L A I ("b" c o e f f i c i e n t ) compared w i t h t h e o t h e r 3.3  The (3-1) g r a z i n g increment  per u n i t  managements.  Discussion The e x p e r i m e n t r e p o r t e d i n S e c t i o n 3 showed t h a t r a n k s  pasture types  i n r e s p o n s e t o the- t h r e e g r a z i n g managements  of the were t h e  same, r e g a r d l e s s , o f t h e a b s o l u t e s e a s o n a l y i e l d f r o m t h e p a s t u r e s . The r e s u l t s  of the second year c o n s t i t u t e d v e r y low seasonal  yields  58 and  i t has been n o t e d t h a t t h e l o w r a i n f a l l  these  y i e l d s ? t h e h e a v y c l a y s o i l was C o n s i s t e n t l y the  i n t h i s year c o n t r i b u t e d to  probably  an a d d i t i o n a l f a c t o r .  ( 9 l ) g r a z i n g management showed t h e  highest  _  t o t a l and component y i e l d s o f h e r b a g e e x c e p t f o r g r a s s h e r b a g e i n I964 when i t was  s i m i l a r t o t h e (3-1)  management i n y i e l d .  management a l w a y s showed t h e lotirest h e r b a g e y i e l d s . agree w i t h those ryegrass  i n a c l i p p i n g e x p e r i m e n t and  f o r cocksfoot (1959) i a  (7-3)  (9-1)  p u b l i s h e d b y C a s t l e and R e i d  na  those  The  (9-3)  These r e s u l t s  (1958), f o r p e r e n n i a l  of Bryant  i n a c l i p p i n g and g r a z i n g e x p e r i m e n t .  and B l a s e r  (1961),  However, Brougham  g r a z i n g experiment w i t h s h o r t - r o t a t i o n ryegrass  found that  g r a z i n g management gave h e r b a g e p r o d u c t i o n e q u a l t o t h a t f r o m a  management.  T h e r e was  a g r e a t d i f f e r e n c e between t h e  experiment  r e p o r t e d h e r e and t h a t o f Brougham, f o r i n h i a c x p c r i n e n t t h e number o f g r a z i n g s were c o n s i d e r a b l y h i g h e r .  C o n s e q u e n t l y t h e r e was  probably  l e s s l o s s o f h e r b a g e , by t i s s u e d e c a y b e l o w 3 i n . , i n Brougham's experiment.  I n Brougham's e x p e r i m e n t the  g r e a t e r number o f g r a z i n g s t h a n t h e  (7-3)  ( 9 - 1 ) , and  management p e r m i t t e d as a r e s u l t t h e y  a  showed  e q u a l y i e l d s d e s p i t e t h e f a c t t h a t t h e herbage g r o w i n g b e l o w 3 i n . , was never harvested. r e - g r o w t h was management.  s l o w and  o f new  the  t i s s u e decay o c c u r r e d below 3 i n . i n the  As a r e s u l t t h e h e r b a g e f o r e a c h s u b s e q u e n t  consisted mainly herbage l e f t  However, i n t h e e x p e r i m e n t r e p o r t e d h e r e  r e - g r o w t h and n o t o f c o n t i n u e d  (9 3) _  grazing  growth from  the  after grazing.  Prom c o n s i d e r a t i o n s o f L A I and  light  i n t e r c e p t i o n as p u b l i s h e d  by  Brougham (1956), D a v i d s o n and D o n a l d (1958) and D o n a l d (1961), t h e (9-3)  g r a z i n g management s h o u l d have been a d v a n t a g e o u s t o  pasture  p r o d u c t i o n b e c a u s e t h e r e - g r o w t h w o u l d have t o r e p l a c e l e s s L A I t o  reach  59 the  optimum and maximum re-growth rate.  was approximately 1000  The herbage l e f t below 3 i n .  lb./acre dry matter and should have provided a  reasonable leaf area index for re-growth.  However, obviously the data  reported here did not support this theory.  There are a number of  reasons which could account for the poor re-growth.  F i r s t l y re-growth  could have occurred up to the c e i l i n g LAI without reaching the treatment height. From the LAI figures this was unlikely unless there was an unusual arrangement of the grass leaves.  Secondly, the tissue l e f t  after grazing may have been unable to assimilate carbon dioxide even when placed in a favourable light regime as found by Alexander and McCloud  (1962).  Thirdly the t i l l e r s of the  (9-3)  management may have  undergone some jointing which could have permitted an apical dominance to suppressed pasture re-growth as found by East in Fourthly, intraplant competition remaining herbage of the (9-3) ited production.  et a l . (1964).  for s o i l moisture between the  management and new growth may have lim-  Of these reasons the second was probably the one  responsible for the poor performance with the (9-3) management. Generally the results did not show a low grass to clover ratio on plots subjected to the (3-1) Blackman  (1939) and  management as suggested by the work of  Willoughby  those observed by Brougham  (1954)?  (1959) and  similar species and managements.  but the ratios were similar to Wolf and Smith  Donald  (1956) has  (1964)  for  pointed out the  ultimate grass to clover ratio w i l l be a consequence of the interaction of f e r t i l i t y level, rate of nitrogen fixation, grazing management and rate of cycling of nutrients through the plants, animals and s o i l . Hence in this grazing situation where the clover could dominate the grass, this result could be balanced by high nitrogen a v a i l a b i l i t y , thus  60 maintaining The  an e q u a l  grass-clover  most p r o d u c t i v e  obtained. (1963)  except  The  and  fescue  however, t h i s o r d e r soil  and  i s of  type  the  little  on w h i c h i t  area  least  i n t h e same r e g i o n .  Production  i n a s s o c i a t i o n with ryegrasses, w i t h the t a l l  seems u n l i k e l y t h a t n i t r o g e n was  productive ryegrass  ryegrass  pastures  pastures.  fescues.  intermediate  Prom t h i s r a n k and  relatively  pastures  occurred  i n the c o c k s f o o t  by Brougham ( 1 9 5 6 )  the  was  obtained  and  the  tall  a l s o observed  ( 3 - l ) management f a v o u r e d  w h i c h showed i t s b e s t L A I and  by  poor g r a s s  in'the  growth  and  edaphically adjusted  fescue  pastures.  The  s l o w d e c l i n e i n autumn p r o d u c t i v i t y o f the r y e g r a s s e s  C o n s i d e r i n g s p e c i e s x management ion,  of the  less  From the g r a s s - c l o v e r f i g u r e s i t seems u n l i k e l y t h a t c l o v e r  suppression  noted  i n the  con-  l i m i t i n g g r a s s growth i n t h e  a result  with  components,•  F u r t h e r the h i g h c l o v e r y i e l d s  a p p e a r e d t o be  c o l o n i z a t i o n of bare spaces clovers.  was  f r o m the . c l o v e r  s i d e r i n g the r e v e r s e r a n k o r d e r o f p r o d u c t i o n f r o m t h e g r a s s it  least  o f p r o d u c t i v i t y a g r e e s w i t h the f i n d i n g s o f H i l d e r  showed g r e a t e s t y i e l d cocksfoots  tall  f o r t h e e n v i r o n m e n t and  order  f o r another  S170  s p e c i e s was  productive, perennial ryegrassp significance  balance.  a l l s p e c i e s except  i t was  on s p e c i e s x management  experiment.  i n t e r a c t i o n s f o r grass  p e r f o r m a n c e u n d e r the  dry matter r e g r e s s i o n s  in this  S170  p r o d u c t i o n was because l i g h t  i n t e r a c t i o n was least,  i n t e r a c t i o n s by  only significant  i t is unrealistic  would not  be  to. e x p e c t  l i m i t i n g except  management r e g r e s s i o n s were not  tall  _  fescue From  i n f o r m a t i o n would  the be  showing changes i n  the r e g r e s s i o n as a c o n s e q u e n c e o f t h e g r a z i n g management However, as t h e  product-  ( 9 l ) management.  hoped t h a t  as •  i n 1964,  treatments. when  such d i f f e r e n c e s  f o r very short p e r i o d s i  significantly different.  >  These  61 In  summary t h e r e s u l t s f r o m  LAI - l i g h t  t h e experiment  i n t e r c e p t i o n and p a s t u r e r e - g r o w t h  agrees w i t h the r e s u l t s  o f Anslow ( 1 9 6 5 )  p e r e n n i a l r y e g r a s s p a s t u r e s and t h u s  do n o t s u p p o r t t h e rate hypothesis.  f o r midsummer r e - g r o w t h  of  the hypothesis f o r L A I - l i g h t  i n t e r c e p t i o n may n e e d m o d i f i c a t i o n f o r g r a s s - c l o v e r p a s t u r e s environments.  This  i n some  62  4.  THE  PRODUCTIVITY ANTJ HILT PHOTOSYNTHESIS OF  AND  WHITE CLOVER UNDER CONTROLLED ENVIRONMENTS  The series of  experiment r e p o r t e d  o f t h r e e , mentioned  the s p e c i e s used  The  section.  growth was relative  In t h e f i r s t  studied  and  the e f f e c t  two  growth  ( o r c h a r d g r a s s ) and parts i n  o f t e m p e r a t u r e on above  h u m i d i t y on n e t p h o t o s y n t h e s i s was  General  experiment  environment  T h e r e a r e two  i n the second the i n f l u e n c e  4.1.1  ground  of temperature  and  examined.  b o t h t h e above e x p e r i m e n t s t h e s o u r c e o f t h e e x p e r i m e n t a l p l a n t s  t h e same.  The  o r c h a r d g r a s s was  of  tillers  at  t h e Canada Department  obtained by v e g e t a t i v e p r o p a g a t i o n  taken from a p r o d u c t i v e c l o n a l  B r i t i s h Columbia. of  under c o n t r o l l e d  (Ladino white c l o v e r ) .  M a t e r i a l s ^ and Methods  was  In t h i s  i n the  o x p o r i n e n t wore c h o s e n raid t h e i r  examined  4.1  In  i s the second,  s p e c i e s were Dac_tyjLis glome r a t a L.  T r i f o l _ i u m £ejoen_s. L . this  section  i n the introduction..  i n the f i r s t  and n e t p h o t o s y n t h e s i s was conditions.  in this  ORCHARDGRASS  s e l e c t i o n made b y R.H.  Turley  o f A g r i c u l t u r e R e s e a r c h Station;, S a a n i c h t o n ,  The w h i t e c l o v e r o b t a i n e d b y v e g e t a t i v e p r o p a g a t i o n  s t o l o n s e l e c t i o n s w h i c h were t a k e n from a h i g h p r o d u c i n g c l o n e  of  63  commercial Ladino white clover growing on the campus plots at the University of B r i t i s h Columbia. Propagules of these species were collected from the f i e l d i n October 1 9 6 5"-d established i n pots of f e r t i l e glasshouse s o i l . a  Pour  propagules were planted in each pot together with the moist s o i l equivalent to 1 2 5 g  of a i r dry s o i l .  Five hundred pots of each spec-  ies were prepared and placed i n a heated glasshouse. watered and allowed free drainage three times a week.  The pots were When the plants  were growing actively, groups of pots were selected for uniformity and used in the experiments.  As a l l experiments were not run simultaneously  the remaining groups of pots were held i n the glasshouse.  When the  herbage height i n these pots exceeded 2 0 cm., the herbage was trimmed to 1 cm. above the s o i l surface, and the pots f e r t i l i z e d with a 2 0 s 2 0 s 2 0 complete  liquid f e r t i l i z e r .  On completion of each experiment the herbage was removed from each pot by cutting just above the s o i l surface. The leaf area of each sample was then determined.  Two measurements were taken on orchardgrass  leaves, and the area determined by the method of Kemp ( i 9 6 0 ) ? Leaf Area = 0 . 9 0 5 x Length x Breadth mid-way along the length.  For white  clover, the leaf area was determined by the use of the scoring standards of Williams  et_ a l . ( 1 9 6 4 ) .  A computer programme was written to  compute area and leaf count number directly from leaf scores or measurements.  After measurement of the leaf area the herbage samples  were dried i n a forced a i r oven at 60°C. f o r 7 2 hours. was then determined.  The dry weight  On the day following the harvest the pots were  examined and the number of t i l l e r s i n each orchardgrass pot recorded, while in the white clover pots the number of rooted stolon nodes was  64 recorded.  4.1.2  Growth Measurement a t F o u r T e m p e r a t u r e s To p r o v i d e d i f f e r e n t  growth chambers;,  Regimes  t e m p e r a t u r e r e g i m e s two P e r c i v a l  d e s i g n a t e d A and B, were u s e d .  As i t was d e s i r e d t o  u s e f o u r t e m p e r a t u r e r e g i m e s t h e e x p e r i m e n t was s p l i t w i t h t h e l o w e s t regime others.  included  i n e a c h r u n t o g e t h e r w i t h one o f t h e  Temperatures  i n °C o f e x p e r i m e n t a l r u n s .  Growth Chamber  A  Number  Day  22 22 22  I II III  .B Night  Each c e l l  Bay  12 12 12  The p l a n t b e n c h o f e a c h chamber was d i v i d e d columns.  into three runs  The t e m p e r a t u r e r e g i m e s f o r e a c h r u n a r e shown i n T a b l e . 4.1.  T a b l e 4°_I  Run  P.G.C.No.78  24 29 34  Night  14 19 24  i n t o f o u r rows and f o u r  was t w i c e as wide i n t h e row d i r e c t i o n as i n t h e  column d i r e c t i o n and, w i t h i n each o f t h e s e 16 c e l l s , o r c h a r d g r a s s and one o f w h i t e c l o v e r was p l a c e d .  one p o t o f  The f i r s t p o t i n e a c h  row was a l t e r n a t e d between t h e two s p e c i e s - so t h a t a u n i f o r m a r r a n g e ment o f e a c h s p e c i e s e x i s t e d w i t h i n t h e chambers.  Each r u n s t a r t e d  w i t h p o t s from w h i c h t h e herbage h a d been removed t o 1 cm. above t h e soil  surface.  A f t e r 30 days t h e r u n was t e r m i n a t e d b y c u t t i n g t h e  plants c l o s e t o the s o i l  surface.  t h e n t h e herbage was d r i e d The  L e a f a r e a were t h e n measured  and weighed.  f o l l o w i n g e n v i r o n m e n t a l and c u l t u r a l c o n d i t i o n s were m a i n t a i n e d  d u r i n g each r u n . a)  and .  Day and n i g h t  , • temperatures maintained f o r 12 hours  each.  65 b) light, all  Day l e n g t h o f 16 h o u r s 14 h o u r s w i t h h a l f  the f l u o r e s c e n t  remaining fluorescent  o f incandescent  on and 12 h o u r s  on, 6 AM  on and change t o d a y t e m p e r a t u r e ,  o f f and change t o n i g h t t e m p e r a t u r e ,  lights  Before the s t a r t  with  The d a i l y s w i t c h i n g c y c l e wass  on, 5 -AM h a l f f l u o r e s c e n t l i g h t s  lights  lights  remaining fluorescent c)  the f l u o r e s c e n t l i g h t  l i g h t s on.  4 AM i n c a n d e s c e n t l i g h t s  half fluorescent  composed o f 16 h o u r s  6 PM  7 PM  o f f , 8 PM i n c a n d e s c e n t lamps o f f .  o f each r u n the l i g h t  intensity  at the-plant  b e n c h w i t h a l l t h e l i g h t s on was measured w i t h a "Weston  756"  illuminometer.  between a l l  runs  The average  light  intensity varied l i t t l e  and t h e chambers as aged f l u o r e s c e n t  mean l i g h t  i n t e n s i t y f o r a l l runs,  w i t h i n e a c h chamber was 1800 d)  t u b e s were u s e d .  The g r a n d  a l l chambers and a l l p o s i t i o n s  f t : candles.  A l l p o t s were w a t e r e d  t h r e e t i m e s p e r week w i t h  distilled  w a t e r and t h e n a l l o w e d f r e e d r a i n a g e .  The w a t e r i n g f o r t h e 10th  o f each r u n was r e p l a c e d b y a n u t r i e n t  s o l u t i o n which s u p p l i e d each pot  with f e r t i l i z e r phosphorus  The and  e q u i v a l e n t t o 100  pentoxide  l b . / a c r e n i t r o g e n , 80  and 60 l b . / a c r e p o t a s s i u m  lb./acre  oxide.  d a t a f r o m t h e t h r e e r u n s were p o o l e d f o r s t a t i s t i c a l  as t h e growth  a t the lowest temperature  day  analysis,  regime d i f f e r e d between  t h e runs i t was n e c e s s a r y t o u s e a n a l y s i s o f c o v a r i a n c e t o a d j u s t t h e growth a t t h e o t h e r t e m p e r a t u r e s  f o r t h e d i f f e r e n c e s between r u n s .  The v a r i a n c e due t o rows and columns was removed as p a r t o f t h e a n a l y s i s t o h e l p e l i m i n a t e t h e e f f e c t s o f g r a d i e n t s w i t h i n t h e growth chambers.  66 4.1.3  Net Photosynthesis Measurement Two pots of orchardgrass and two pots of white clover were  selected from the material described i n Section 4*1.1*  Within each  species the selection was made to obtain two pots with similar leaf areas, few senescent leaves and a reasonably high leaf area. Each of the pots of plants was then subject to a l l combinations of four temperatures and four relative humidities.  The four temperatures were  15 C, 20 C, 25 C and 30 C, while the four relative humidit les were 40;^ 60/o, 80$ and 90$.  These environmental conditions were achieved  by sealing the pots i n a Blue M Vapor-Temp Controlled Relative Humidity Chamber as modified by Ormrod and Woolley, in the chamber were continuously illuminated with  (1966).  2,000 f t .  The ;piants candles  of fluorescent light as measured by a"Weston"756 illuminometer. While the plants were subject to a particular combination of temperature and relative humidity a small sample of a i r was circulated through a closed system using a"Beckman"model 15A infrared gas analyser to determine the C0£ concentration i n the manner described by Decker  (1954)•  The C0 concentration of the system was adjusted to about 2  400 ppm by small additions of CO^ and then the rate of removal, by the plant, measured between 340 and 260 ppm CO^.  This process was  repeated to give three estimates of the rate of CO^ uptake by the plant before a new combination of temperature and relative humidity was established.  Readings were never taken u n t i l the chamber had main-  tained the particular environment f o r 10 minutes, and, to ensure adjustment of the plants to the new environment, only the second and third readings were used for calculation. When readings at a l l combinations of the above temperatures and  67 r e l a t i v e h u m i d i t i e s were c o m p l e t e the  t h e p o t o f p l a n t s was removed f r o m  chamber and t h e p l a n t s h a r v e s t e d b y c u t t i n g c l o s e t o t h e s o i l  surface,  and l e a f  a r e a and d r y w e i g h t  determined.  The p o t o f s o i l  r o o t s was r e - s e a l e d  i n t o t h e chamber and t h e r a t e o f COg  between 260  ppm  and 340  determined  at each o f the f o u r  w i t h t h e r e l a t i v e h u m i d i t y m a i n t a i n e d a t 60$. the  COg  l e v e l was f i r s t  l o w e r e d t o 250  the  s y s t e m t h r o u g h an " A s c a r i t e ' b y p a s s .  the  soil  evolution  temperatures  F o r these  r  measurements  ppm COg by s c r u b b i n g t h e a i r i n  1  C a r e was t a k e n t o e n s u r e  mass had come i n t o t h e r m a l e q u l i b r i u m a t e a c h  b e f o r e any measurements were t a k e n .  and  Two  that  temperature  measurements o f t h e COg  e v o l u t i o n r a t e were made a t e a c h t e m p e r a t u r e  and t h e mean c a l c u l a t e d .  The measured COg u p t a k e  r a t e s b y t h e p l a n t were t h e n c o r r e c t e d f o r t h e  a p p r o p r i a t e r a t e o f COg  e v o l u t i o n from the s o i l .  The r a t e  of net  p h o t o s y n t h e s i s b y t h e p l a n t s was r e l a t e d t o l e a f a r e a and l e a f As t h e measurements t o o k o v e r 16  h o u r s p e r p o t , the f o u r p o t s were  measured on f o u r c o n s e c u t i v e d a y s . p l a n t s were k e p t  4.2  Results  4.2.1  The  weight.  During t h i s  p e r i o d unmeasured  i n the glasshouse.  Influence  o f Temperature  on t h e p r o d u c t i o n o f a V e g e t a t i v e  Unit Mitchell  (1956) has  shown t h a t d a t a f r o m c o n t r o l l e d  environment  s t u d i e s o f f o r a g e p l a n t s , when e x p r e s s e d as g r o w t h o f a v e g e t a t i v e g i v e a good i n d i c a t i o n o f t h e p l a n t ' s f i e l d this the  s e c t i o n are t h e r e f o r e vegetative unit  r o o t e d node.  was  performance.  e x p r e s s e d i n t h i s manner.  a tiller  unit,  Results i n  For orchardgrass  while f o r white c l o v e r the u n i t  was  a  68 The at  a d j u s t e d mean w e i g h t s o f a v e g e t a t i v e u n i t  22, 24, 29  and  f o r p l a n t s grown  34°C d i f f e r e d c o n s i d e r a b l y ( P i g 4.2.I. ). 1  s p e c i e s t h e w e i g h t o f a v e g e t a t i v e u n i t was maximal a t t h e magnitude o f r e s p o n s e a t than white c l o v e r . (Fig. and  4.2.2)  with  t h e maximum a r e a o b t a i n e d i n orchardgrass  t o l e a f weight  only s i g n i f i c a n t  However,  29°C was much g r e a t e r i n o r c h a r d g r a s s  29°C i n b o t h s p e c i e s  at  than  i n white c l o v e r .  a d j u s t e d mean number o f l e a v e s on a v e g e t a t i v e u n i t  s i m i l a r trends  24°C (P =  29°C.  A r e a r e s p o n d e d i n t h e same manner as w e i g h t  the response g r e a t e r The  In b o t h  (Fig.  4.2.3).  followed  With orchardgrass, the  change i n l e a f number was an i n c r e a s e b e t w e e n 22. and  0.06).  Above  24°C t h e l e a f number o f a t i l l e r was u n a f f e c t e d .  In c o n t r a s t w h i t e c l o v e r showed a d i s t i n c t maximum number o f l e a v e s on a r o o t e d node.  optimum t e m p e r a t u r e f o r t h e As w i t h a r e a and w e i g h t  t h e optimum t e m p e r a t u r e f o r t h e maximum number o f l e a v e s on a r o o t e d node was 29°C<.  The a d j u s t e d mean number o f l e a v e s on a r o o t e d node a t  24°C was n o t s i g n i f i c a n t l y d i f f e r e n t from t h e number a t 22°C. The per l e a f (Fig. at  i n f l u e n c e o f t e m p e r a t u r e on t h e a d j u s t e d mean w e i g h t and a r e a i n orchardgrass  4.2.4  29°C.  and  4.2.5).  was d i f f e r e n t f r o m t h a t Weight p e r l e a f  i n white c l o v e r  i n orchardgrass  was maximal  However, i n w h i t e c l o v e r t h e w e i g h t p e r l e a f was n o t  significantly different  between  22°C and 29°C w h i l e above 29°C t h e  w e i g h t was s i g n i f i c a n t l y d e p r e s s e d  (P<0.05).  In each s p e c i e s t h e  r e s p o n s e s t o t e m p e r a t u r e o f a r e a p e r l e a f were s i m i l a r t o t h o s e of:  1  A l l f i g u r e s i n t h i s s e c t i o n use the f o l l o w i n g a b b r e v i a t i o n s s D.N.S. D i f f e r e n c e s n e c e s s a r y f o r s i g n i f i c a n c e between means, w i t h i n e a c h s p e c i e s a d j u s t e d means w i t h t h e same l e t t e r do n o t differ significantly. O.G. Orchardgrass. W.C. White C l o v e r .  22  24  29 DAY  Fig.  k.2.1  34  TEM PE R ATUItE ( ° C )  A d j u s t e d mean w e i g h t o f o r c h a r d g r a s s t i l l e r s a n d w h i t e c l o v e r nodes as i n f l u e n c e d b y t e m p e r a t u r e . O.N.S. a t P < 0 . 0 1 a r e f o r o r c h a r d g r a s s , 7 3 . 1 , a n d f o r w h i t e c l o v e r , 12.6  O.G. X  H  W.C  SO  u 40  5 3 0  20  10  • • S E.clover  22  24 OAY  Fig.  34  29 TEMPERATURE  (°c)  ^ . 2 . 2 A d j u s t e d mean a r e a o f o r c h a r d g r a s s t i l l e r s and w h i t e c l o v e r nodes as i n f l u e n c e d by t e m p e r a t u r e . D.N.S. at P < 0 . 0 1 are f o r o r c h a r d g r a s s , 1 0 . 8 5 , and f o r w h i t e c l o v e r , 2 . 1 1  70 4 r  OAY TEMPERATURE (°c ) Fig.  h.2.3  A d j u s t e d mean number o f l e a v e s on o r c h a r d g r a s s t i l l e r s and w h i t e c l o v e r r o o t e d nodes as i n f l u e n c e d by t e m p e r a t u r e . D.N.S. are  for  0.i»9  K  orchardgrass,  (P<  0.6A,  (P<  0.01)  and f o r w h i t e  clover,  0 . 0 5 ) .  80r  DAY TEMPERATURE ( °C ) Fig.  b.l.k  A d j u s t e d mean w e i g h t o f o r c h a r d g r a s s and w h i t e c l o v e r l e a v e s as i n f l u e n c e d by t e m p e r a t u r e . D.N.S. a r e f o r o r c h a r d g r a s s , 1 3 . 2 6 , (P < 0 . 0 1 ) , and f o r w h i t e c l o v e r , 7 . 1 8 , (P < 0 . 0 5 ) .  71  I n o r c h a r d g r a s s a r e a p e r l e a f was o p t i m a l a t 29°C, and i n  weight.  w h i t e c l o v e r t h e a r e a was n o t s i g n i f i c a n t l y d i f f e r e n t b e t w e e n 22°C and 29°C h u t d e p r e s s e d  29°C.  above  The w e i g h t s a r e a r a t i o o f t h e o r c h a r d g r a s s and w h i t e c l o v e r l e a v e s responded to temperature  (Fig. 4-2.6).  The maximum w e i g h t  per u n i t  a r e a i n o r c h a r d g r a s s o c c u r r e d b e t w e e n 24°C and 29°C whereas i n w h i t e c l o v e r the weight  p e r u n i t a r e a i n c r e a s e d o v e r t h e r a n g e 2 4 t o 34°C.  T h i s i n c r e a s e i n t h e w h i t e c l o v e r w e i g h t s a r e a r a t i o b e t w e e n 24°C and 34°C was a c o n s e q u e n c e o f i n c r e a s i n g numbers o f f l o w e r s b e t w e e n 24°C and 34°C.  The f l o w e r stems were i n c l u d e d as p a r t o f t h e l e a f  weight,  a t 34°C  b u t as t h e i r a r e a was n o t measured t h e x r e i g h t s a r e a r a t i o  was  a r t i f i c i a l l y h i g h where f l o w e r i n g was g r e a t e s t .  4.2.2  The  I n f l u e n c e o f T e m p e r a t u r e and R e l a t i v e H u m i d i t y  on  net P h o t o s y n t h e s i s Wet  p h o t o s y n t h e s i s i n b o t h o r c h a r d g r a s s and w h i t e c l o v e r  i n f l u e n c e d by temperatures Table 4 < H ) '  b e t w e e n 15 and 30°C ( F i g . 4 . 2 . 7 ,  The r e g r e s s i o n e q u a t i o n s d e s c r i b i n g t h e s e 2  a r e p o l y n o m i a l s o f t h e forms Y = a + bZ +:cX p h o t o s y n t h e s i s and X = t e m p e r a t u r e  °C.  was  and  associations  3  + dX"^ where Y = n e t  ISE.cIc  22  24  29 DAY  Fig.  TEMPERATURE  34  (°C)  k.2.5 A d j u s t e d mean a r e a o f o r c h a r d g r a s s and w h i t e c l o v e r l e a v e s as i n f l u e n c e d by t e m p e r a t u r e . D . N . S . a t P<0.01 a r e f o r o r c h a r d g r a s s , 1.83, a n d f o r w h i t e c l o v e r , 1.32  i  7  o  I  o  yy77  74  79 DAY  Fig.  TEMPERATURE  34  ( C) P  4.2.6 A d j u s t e d mean w e i g h t : a r e a r a t i o o f o r c h a r d g r a s s and w h i t e c l o v e r l e a v e s as i n f l u e n c e d by t e m p e r a t u r e . D.N.S. a t P<0.01 a r e f o r o r c h a r d g r a s s , 0.49, a n d f o r w h i t e c l o v e r , 1.08  73 T a b l e 4.II  R e g r e s s i o n s and c o r r e l a t i o n c o e f f i c i e n t s (R) r e l a t i n g t e m p e r a t u r e and n e t p h o t o s y n t h e s i s f o r P i g . 4.2.7«  Species  Units  a  mgcg dm *" hr-1  2  mgC0 g" hr-1  2  Regression c o e f f i c i e n t s b e d  w, c l o v e r • o. g r a s s  47-531  -5«505  9.539  -0.120  w. c l o v e r  -2.253  3.923  o  34.747  -0.442  0.280 US  -O.O98  R  -0.755  0.755  MS  0.703  ITS  0.662  NS  0.751  1  grass  NS  NS - c o e f f i c i e n t n o t s i g n i f i c a n t ; i . e s " t " f o r t h i s c o e f f i c i e n t i s l e s s t h a n 2.5. The r e s p o n s e t o t e m p e r a t u r e  of net photosynthesis i n orchardgrass  was s i g n i f i c a n t l y l i n e a r ( P < 0 . 0 0 l ) on b o t h a l e a f a r e a and l e a f basis.  The s l o p e s o f t h e r e g r e s s i o n s (b v a l u e s ) i n d i c a t e  weight  that the  r e d u c t i o n i n net p h o t o s y n t h e s i s , f o r each degree c e n t i g r a d e r i s e i n t e m p e r a t u r e , was s l i g h t l y more on a l e a f w e i g h t t h a n on a l e a f basis.  F o r w h i t e c l o v e r t h e optimum t e m p e r a t u r e  was w i t h i n t h e r a n g e o f t h e e x p e r i m e n t ,  area  f o r net photosynthesis  ( F i g . 4.2.2).  N e t photo-;  s y n t h e s i s on a l e a f a r e a b a s i s showed a s i g n i f i c a n t c u b i c r e s p o n s e (P<0.001) t o t e m p e r a t u r e 23°0,  and t h e optimum t e m p e r a t u r e was a p p r o x i m a t e l y  However, d i f f e r e n c e s i n n e t p h o t o s y n t h e s i s b e t w e e n 15 and j25°.C  were s l i g h t and t h e t r e a t m e n t means d i d n o t d i f f e r s i g n i f i c a n t l y . - B y c o n t r a s t , a 30°C t e m p e r a t u r e  resulted i n a highly significant  d e p r e s s i o n i n n e t p h o t o s y n t h e s i s (P<0.00l)„  Between 15 and 30°C,  n e t p h o t o s y n t h e s i s , on a l e a f w e i g h t b a s i s , showed a q u a d r a t i c r e s p o n s e 5 t h e optimum t e m p e r a t u r e was 20°C.  The r e s p o n s e o f n e t p h o t o s y n t h e s i s  TEMPERATURE ( ° C )  TEMPERATURE ( ° C )  4 . 2 . 7 The r e l a t i o n s h i p b e t w e e n t e m p e r a t u r e and n e t p h o t o s w t h e s i s i n o r c h a r d g r a s s (•———•) and i n w h i t e c l o v e r ( x - - - x) . P o i n t s - a r e t r e a t m e n t means. A) U n i t l e a f a r e a S . E . 0.22mg CO /dm / h r . B) U n i t l e a f w e i g h t S . E . 0.78mg CO / g / h ' r .  75 to  temperature  on a l e a f  on a l e a f w e i g h t b a s i s was i n r e a l i t y s i m i l a r  a r e a b a s i s and i n b o t h c a s e s h i g h l y s i g n i f i c a n t  i n n e t p h o t o s y n t h e s i s were r e c o r d e d The  at  to that  depressions  30°C.  mean r a t e s o f n e t p h o t o s y n t h e s i s  i n orchardgrass  and w h i t e  c l o v e r showed s l i g h t changes w i t h changes i n t h e l e v e l o f r e l a t i v e humidity  (Table  4*III)'  Net p h o t o s y n t h e s i s  i n white  differ  s i g n i f i c a n t l y a t any o f t h e f o u r l e v e l s  a leaf  a r e a o r l e a f weight b a s i s .  synthesis  a t 40$  (P<0.05)  b o t h on a l e a f  significant  c l o v e r d i d not  o f r e l a t i v e h u m i d i t y on  With o r c h a r d g r a s s , n e t photo-;  RH. was s i g n i f i c a n t l y g r e a t e r t h a n a t 90$ a r e a and l e a f weight b a s i s .  i n t e r a c t i o n s o f temperature  Since r e l a t i v e humidity  T h e r e were no  with r e l a t i v e  i s not i n i t s e l f  RH.  humidity.  a s i m p l e measure o f t h e  d r y i n g power o f t h e a i r , t h i s was examined b y d e t e r m i n i n g t h e v a p o u r pressure d e f i c i t  Table  4.Ill  f o r each temperature  The n e t p h o t o s y n t h e s i s o f o r c h a r d g r a s s and w h i t e as i n f l u e n c e d b y r e l a t i v e h u m i d i t y .  Units  Species  mgC02 dm-2 hr-1  w.clover  mgC 02 hr  J  - 1  and r e l a t i v e h u m i d i t y  40$  Relative humidity 80% 60%  90$  11.96  7.25  6.97  6.78  6.40  w.clover  34.18  33.97  34.12  32.08  o.grass  26.33  25-27  24.56  23.08  o.grass  clover  S.E.+  11.25  11.91  12.02  combination  0.22  0.78  76 u s i n g t h e c h a r t o f Hughes  (1961).  The d a t a were examined b y m u l t i p l e  r e g r e s s i o n a n a l y s e s b e c a u s e the' i n t e r v a l s between t h e v a p o u r d e f i c i t s were n o t e q u a l a t e a c h t e m p e r a t u r e . orchardgrass  and w h i t e  In a n a l y s i n g t h e d a t a ,  c l o v e r were t r e a t e d s e p a r a t e l y t o a l l o w f o r t h e  p r e v i o u s l y d e s c r i b e d c u r v i l i n e a r response In  pressure  t o temperature  o f white  a l l analyses c o e f f i c i e n t s f o r the i n t e r a c t i o n o f temperature  vapour pressure d e f i c i t The  were  and  calculated.  r e s u l t s o f t h e m u l t i p l e r e g r e s s i o n a n a l y s i s showed  responses  o f net photosynthesis to vapour pressure d e f i c i t  treatment  temperature  ( T a b l e 4.IV, P i g s .  4.2.8  and  significant at each  4.2.9).  In a l l  analyses the c o e f f i c i e n t s f o r the i n t e r a c t i o n o f temperature pressure d e f i c i t  clover.  were n o t s i g n i f i c a n t ;  and v a p o u r  they are t h e r e f o r e omitted  from  the r e g r e s s i o n s .  T a b l e '4- IV  M u l t i p l e r e g r e s s i o n s o f t e m p e r a t u r e (X^) and v a p o u r p r e s s u r e d e f i c i t ( X ) on n e t p h o t o s y n t h e s i s (Y) i n o r c h a r d g r a s s and i n w h i t e c l o v e r . R e g r e s s i o n forms Y = a + bX^ + c X ] _ + d X ] ^ + e X 2  2  2  Units  Species a  mgC0  2  Regression c o e f f i c i e n t s b c d  R e  w.clover  47.797 -5-554  0.281  -O.OO46  0.082  O.784  o.grass  9-644 -0.143  NA  NA  0.062  0.755  NA  0.062  0.682  NA  0.232  0.809  dm-2 hr-  1  mgC0 hr"  1  2  w.clover  -1.759  o.grass  35-142  NA c o e f f i c i e n t  not a p p l i c a b l e .  3.827 -0.097 -0.531  NA  25 15,20 °C 13  1*2  11  E -o  15  8  C  \  E t o  O u  tn  10  E  «/>  to  •u X rZ > «/>  30 °C  «/> •1  X  z  O  >  o t-  o WHITE  5  10  15  VAPOUR PRESSURE DEFICIT  Fig.  A.2.8  xUl  CLOVER  ORCHARDGRASS  z 20  (mm H g )  5  10  VAPOUR PRESSURE  15  DEFICIT ( m m H g )  The i n f l u e n c e o f v a p o u r p r e s s u r e d e f i c i t and t e m p e r a t u r e on n e t p h o t o s y n t h e s i s u n i t leaf area in^white c l o v e r , - a n d in o r c h a r d g r a s s . ( • ) 15°C, (o) 20°C,  ( x ) 25°C and ( » ) 30°C.  20  of  Fiq.  **.2.9"The i n f l u e n c e o f vapour p r e s s u r e d e f i c i t and temperature on net p h o t o s y n t h e s i s u n i t l e a f weight in w h i t e c l o v e r , and in o r c h a r d g r a s s . {•} 15 C, (o) 2 0 ° C , (x) 25°C and •(•) 30°C.  of  79 In white deficit  were  on a l e a f  clover  the i n c l u s i o n  P<0.05  significant  of vapour  at  pressure  temperature.  inclusion  of  the  terms  for  area basis  and P < . 0 . 0 0 1 .  4-II  deficit  and 4.IV in  the  all  for  and  cases  vapour  By  pressure  P.<0.1  it  the  c a n be seen  changes  of in  correlation  pressure  in  comparing  regression  caused o n l y s l i g h t In  the vapour  regression coefficients  F<0.01  i n Tables  and t e m p e r a t u r e  for  on a l e a f  The c o m p a r a b l e  coefficients  photosynthesis  by the  at  basis.  were  the regression  coefficients  regression coefficients  significant  weight  orchardgrass  the  net the-;  was  deficit.  that  improved  On t h e  •  2 average, vapour  the  pressure  indicate  that  synthesis clover  deficit  the  of  lines  was n o t  a n d 25°C b u t for  at  15 t o  different  net  each o f  the  net  r o s e b y 5«0 u n i t s  coefficient.  slopes  on b o t h a l e a f  deficit  at  value  response  the response  those  100 R  of  were  weight  30°C t h e 20°C.  4.3.1  Dry Matter  the  experiment  inclusion  The r e g r e s s i o n c o e f f i c i e n t s  "e"  deficit,  In  i n vapour  vapour  were  pressure (Fig.  lower:than  significantly  deficit  4-2.8  whitejclover  betireen;15  was s i g n i f i c a n t l y  there  white  pressure,-  i n f l u e n c e d by temperatures line  photo-,  and i n  area basis.  changes  net  regression  and 4 » 2 . 9 ) »  lines <  Production  for  regime  growth of  optimum t e m p e r a t u r e s  night).  to  temperatures  The o p t i m u m t e m p e r a t u r e this  the  pressure  the  in orchardgrass  In orchardgrass  treatment  of  and on a l e a f  regression  photosynthesis,  Biscussion  similar  photosynthesis  significantly  4.3  in  the vapour  with  In both studies  of  29°C d a y a n d 19°C n i g h t  orchardgrass,  agrees q u i t e  f o u n d b y Sprague- (1943) the  (27°C  d a y l e n g t h was 16 h o u r s ?  found'  well  with  d a y a n d 13°C  Rosenquist  and.  80  Gates  ( 1 9 6 1 ) u s i n g a 13 h o u r day l e n g t h f o u n d a s i m i l a r optimum o f  30°C d a y and 21°C because  night.  However, e x a c t c o m p a r i s o n s  o f the d i f f e r e n c e  are not  i n l i g h t e n e r g y l e v e l s between t h e  possible experiment  O t h e r w o r k e r s have u s e d c o n s t a n t t e m p e r a t u r e s and i n t h e s e i n s t a n c e s t h e optimum was Eagles (1967)  lower.  F o r example,  and B a k e r and J u n g ( 1 9 6 8 )  Mitchell  (1956),  have f o u n d t h e optimum temp-  e r a t u r e f o r o r c h a r d g r a s s g r o w t h t o be a p p r o x i m a t e l y 20°C. s t u d y t h e d a y l e n g t h was and J u n g 1 5 h o u r s .  ;  In M i t c h e l l '  1 2 h o u r s w h i l e E a g l e s u s e d 16 h o u r s and (1967)  D a v i d s on and M i l t h o r p e  Baker  also used 1 6 hours  b u t f o u n d t h a t t h e c o n s t a n t t e m p e r a t u r e g i v i n g optimum g r o w t h o f S 37 o r c h a r d g r a s s was  25°C.  A l t h o u g h genotype  experiments the evidence o f Cooper ( 1 9 6 4 ) d i f f e r e n c e i n optimum t e m p e r a t u r e was these genotype  differences,,  differed  i n e a c h o f these-,  suggests t h a t the  5°C  l i k e l y t o be a consequence .of  C o o p e r examined  three races of orchard-  g r a s s f r o m c l i m a t i c a l l y d i f f e r e n t r e g i o n s ( I s r a e l , A n g l e s e y and Norway) and f o u n d t h a t , a l t h o u g h t h e r a c e s d i f f e r e d  i n t h e i r dry matter  p r o d u c t i o n s a t t e m p e r a t u r e e x t r e m e s , a l l r a c e s showed an optimum temp-' e r a t u r e r e g i m e o f 25°C day and 12°C Eagles (1967)  a l s o examined  n i g h t under  a 1 6 hour d a y l e n g t h .  geographic r a c e s o f o r c h a r d g r a s s from  P o r t u g a l and Norway, and f o u n d t h a t t h e optimum was the geographic o r i g i n . t e m p e r a t u r e may  ;  not i n f l u e n c e d  by  From t h e s e s t u d i e s i t a p p e a r s t h a t optimum  i n t e r a c t w i t h a number o f e n v i r o n m e n t a l f a c t o r s . • None  o f the p r e s e n t s t u d i e s are s u f f i c i e n t l y comprehensive i n t e r a c t i o n s , although Alberda (1966)  to c l a r i f y  these  has r e p o r t e d an i n o r e a s e i n t h e  optimum t e m p e r a t u r e as d a y l e n g t h i s e x t e n d e d . The  optimum t e m p e r a t u r e f o r g r o w t h o f w h i t e c l o v e r i n t h e p r e s e n t  s t u d y v a r i e d w i t h t h e p a r a m e t e r examined.  L e a f a r e a and w e i g h t were  81  24  maximal between  optimum t e m p e r a t u r e  (1956), under  Weight,  a regime  However, t h i s  and 29°C  day w h i c h was  f o r growth  o f white c l o v e r r e p o r t e d by  under  of  29°C  day and  experiment  19°C  and t h a t  night  (1962, 1963, 1964)  a number o f day and n i g h t  14.5  23°C  to  gave optimum growth.  i n the p r e s e n t  of M i t c h e l l  indicated  examined t h e growth temperature  Takeda  i n light  and A g a t a  the growth  i n t e n s i t y from  (1966) have  of white c l o v e r  experiments.  and  600  to  clover  intensity  not i n f l u e n c e d  2000 f t . c a n d l e s .  concluded that  ( c v . L a d i n o ) was  By  regimes. range was  by  contrast,  the optimum t e m p e r a t u r e f o r  20°C.  day l e n g t h p e r Be. has  h o u r s compared w i t h  30°C.  14*5  o f white hours  little  clover;  i n Beinhart's.  However, i n t h e p r e s e n t e x p e r i m e n t , where t h e optimum  t e m p e r a t u r e was  h i g h e r t h a n t h o s e p u b l i s h e d t h e optimum t e m p e r a t u r e  have b e e n i n f l u e n c e d b y the d a y l e n g t h o f 16  4.3.2  20  c o m b i n a t i o n w i t h i n the  i n f l u e n c e on the optimum t e m p e r a t u r e f o r growth  (1956) u s e d 12  a general  In t h e s e e x p e r i m e n t s t h e d a y l e n g t h  P u b l i s h e d evidence suggests that  Mitchell  experiment.  of white  and l i g h t  h o u r s and the optimum t e m p e r a t u r e range was  increase  Mitchell  t o t e m p e r a t u r e s between  In g e n e r a l any day and n i g h t t e m p e r a t u r e  17  24°C  a r e a and l e a f number o f a r o o t e d node were a l l maximal  i n s e n s i t i v i t y o f white c l o v e r growth Beinhart  s i m i l a r to the constant  may  hours.  Net P h o t o s y n t h e s i s Murata  synthesis  V  Iyama  (1963) i n  a comprehensive  study of net  photo-  i n f o r a g e c r o p s f o u n d t h a t , f o r b o t h o r c h a r d g r a s s and  clover  20°C"  and  p h o t o s y n t h e s i s was  s u b s t a n t i a l l y c o n s t a n t between 5 and  However, e x a m i n a t i o n o f t h e i r d a t a i n d i c a t e s t h a t  temperature  white  f o r n e t p h o t o s y n t h e s i s i n o r c h a r d g r a s s was  t h e optimum  approximately  82 10°C and i n white c l o v e r approximately 15°C, The 10°C optimum f o r orchardgrass agrees with the data of the present experiment which i n d i c a t e d that the optimum temperature was 15°C or l e s s .  For white c l o v e r , however, the present experiment shows  a higher optimum temperature f o r net photosynthesis i n the region of  20-25°C, that i s 10°C higher than Murata and Iyama  (1962) reported  (1963).  Beinhart  that COg uptake i n white c l o v e r leaves was greater at  30°C than at 20°C, supporting the contention of the present experiment that the optimum i s above 20°C.  Murata and Iyama  (1963) have  reported  that the growing environment may influence the optimum observed, so that winter grown plants may have a lower optimum temperature than • summer grown p l a n t s .  This may account f o r the d i f f e r e n c e s between the  experiments i n the optimum temperature observed f o r net photosynthesis. D e f i n i t i o n of the growing environment p r i o r to observations of the optimum temperature are undoubtably necessary f o r such experiments. However, even such d e f i n i t i o n of the growing environment  i s u n l i k e l y to  provide an exact optimum temperature f o r net photosynthesis as both orchardgrass and white c l o v e r appear to have wide optimum temperature ranges. The s t i m u l a t i n g e f f e c t on net photosynthesis of increased vapour pressure d e f i c i t s between 0 and 20 mm of Hg has not been reported previously.  B i e r h u i z e n and S l a t y e r  (1964) and Baker (1965) have  examined the influence of some vapour pressure d e f i c i t s on COg uptake of cotton (Gossypium hirsutum L.) and found that above 10-20 there was a decline i n COg uptake.  mm of Hg,  The r e s u l t s o f the two experiments  considered together with the present one suggest that there may be an optimum vapour pressure d e f i c i t f o r maximum net photosynthesis. ,  83  5.  F I E L D DEFOLIATION MANAGEMENT EXPERIMENT WITH CONCURRENT MEASUREMENT OF NET The  experiment r e p o r t e d i n t h i s  series of three. orchardgrass  The  two  response  field was  and  their  seasonal  measurements, t h e  determined  M a t e r i a l s and This  s p e c i e s used i n the  T h e y were s u b j e c t e d t o the  ments o f s e c t i o n one, with these  forage  f o r the  net  s e c t i o n i s d i v i d e d i n t o two  techniques  used to determine the net  The  o f the f i e l d  o f the forage  Field  technique  experimental  research f i e l d s B.C.  The  moist  i n winter  and  experiments  together  as  same d e f o l i a t i o n manage-  y i e l d s measured.  Concurrent  photosynthesis- light  energy  of regrowth.  Methods  techniques  5.1.1  i n the  previous  swards a t v a r i o u s s t a g e s  d e s i g n and  responses  s e c t i o n i s the t h i r d ,  and w h i t e c l o v e r , were e s t a b l i s h e d a l o n e  f o r a g e swards.  5•1  PHOTOSYNTHESIS  partsj  the f i r s t  experiment  and  contains  the s e c o n d  photosynthesis,  light  the  the  energy  swards.  and  experimental  a r e a was  design  w i t h i n the D i v i s i o n o f P l a n t  Science  on t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a campus, V a n c o u v e r ,  g e n e r a l c l i m a t e of the and m i l d and  dry  a r e a has b e e n d e s c r i b e d as c o o l i n summer b y Kendrew and K e r r ,  and  (1955)•  84 A c t u a l w e a t h e r d a t a was o b t a i n e d f r o m an i n t e n s i v e m e t e r o l o g i c a l s t a t i o n l o c a t e d some 500 f t . s o u t h o f t h e e x p e r i m e n t a l a r e a . o f t h e e x p e r i m e n t a l a r e a was d e r i v e d f r o m g l a c i a l t i l l  The  soil  and outwash  p a r e n t m a t e r i a l and i s c l a s s i f i e d as a N i c h o l s o n a c i d brown-wooded sandy loam s o i l . D u r i n g t h e p e r i o d f r o m F e b r u a r y 1966 t o May 1966 t h e e x p e r i m e n t a l a r e a was p l o u g h e d  and d i s c h a r r o w e d , t r e a t e d w i t h f a r m y a r d manure a t  1,000 l b . p e r a c r e and f e r t i l i z e d 500 1°. p e r a c r e .  at  w i t h 8 : 10 : 6  N:P:K  fertilizer  I n May 1966 t h e a r e a was d i v i d e d i n t o f o u r  b l o c k s w i t h e a c h b l o c k d i v i d e d i n t o 24 p l o t s .  E a c h p l o t measured- -  5 f t . x 8 f t . and e a c h o f t h e 24 t r e a t m e n t s , were r a n d o m l y a l l o c a t e d t o The 24 t r e a t m e n t s were made;up *of  one o f t h e p l o t s w i t h i n e a c h b l o c k .  e i g h t f o r a g e swards x t h r e e d e f o l i a t i o n managements.  The  c o m p o s i t i o n o f t h e f o r a g e swards i s g i v e n i n T a b l e 5«I«  seeding The  orchardgrass  ( D a c t y l i s g l o m e r a t a L.) was S143 s t r a i n w h i l e t h e w h i t e c l o v e r ( T r i f o l i u m repens  L.) was t h e c u l t i v a r L a d i n o .  Table  5.1.  The w h i t e c l o v e r s e e d  was  S e e d i n g d e n s i t i e s o f g r a s s and c l o v e r f o r t h e e i g h t f o r a g e swards i n l b . / a c r e o f 100$ v i a b l e s e e d . t  Sward  Orchardgrass.  A B C D E F G H  5 25 0 0  5 25 5 25  White C l o v e r 0 0  3 9 3 9 9 3  ,  ' .  ' .  85 inoculated  w i t h R h i z o b i u m AB p e a t c u l t u r e  A 1 0 $ w/v m a l t o s e gum a r a b i c  rate,  a t d o u b l e t h e recommended  s o l u t i o n was u s e d as a c a r r i e r . A l l  s e e d s were b r o a d c a s t e a r l y i n May 1966 and t h e a r e a l i g h t l y  r a k e d and  rolled. The d e f o l i a t i o n managements (1)  The h e r b a g e grew t o 9-10 o f 1 i n . v i z . management  (3)  followss  The h e r b a g e grew t o 3-4 i n . and t h e n was d e f o l i a t e d o f 1 i n . v i z . management  (2)  were as  at a height  (3-1). i„  and t h e n was d e f o l i a t e d a t a h e i g h t  n  (9-l).  The h e r b a g e grew t o 9-10  i n . and t h e n was d e f o l i a t e d  o f 3 i n . v i z . management  (9-3).  at a height  A l t h o u g h t h e d e f o l i a t i o n management t r e a t m e n t s were a l l o c a t e d t o the  p l o t s a t t h e t i m e o f s o w i n g t h e y were n o t a p p l i e d  later.  6 months -  D u r i n g t h e e s t a b l i s h m e n t phase when t h e a v e r a g e h e i g h t o f  h e r b a g e i n a l l p l o t s r e a c h e d 14 i n . t h e p l a n t s height of 4 i n . and  until  were d e f o l i a t e d t o a  A l l d e f o l i a t i o n s were made w i t h an 18 i n . r o t a r y mower  a f t e r e a c h d e f o l i a t i o n t h e p l o t s were r a k e d c l e a n o f mown h e r b a g e . By J u l y 1 9 6 6 t h e swards were w e l l e s t a b l i s h e d  further  500 l b . p e r a c r e o f 8 s 10  s 6  IT:PJK  and r e c e i v e d  fertilizer.  The  a final  d e f o l i a t i o n o f t h e f i r s t g r o w i n g s e a s o n , i n O c t o b e r 1 9 6 6 , was a t t h e treatment height. the  This minimised bias  I967 s e a s o n when t h e r e g u l a r  growing season the d e c i s i o n  i n material  c a r r i e d over  d e f o l i a t i o n s commenced.  In the  to d e f o l i a t e a p a r t i c u l a r treatment  made on t h e a v e r a g e h e i g h t o f t h e s w a r d s .  into I967  was  The h e i g h t s o f t h e s w a r d s  were d e t e r m i n e d w e e k l y w i t h a g r a d u a t e d s t a f f on 1 0 s i t e s p e r p l o t . I n May I 9 6 7 t h e p l o t s r e c e i v e d fertilizer  55O l b . p e r a c r e o f 4 : 1 0 : 10 ' IT:P;K  and on J u l y 9 t h e p u r e g r a s s p l o t s  ( T r e a t m e n t s A and B)  86 as s u l p h a t e o f ammonia a t a r a t e o f 200  received further f e r t i l i z e r  To m a i n t a i n a d e q u a t e s o i l m o i s t u r e d u r i n g t h e 1967  per acre. season  t h e e x p e r i m e n t a l s i t e was  a p p r o x i m a t e l y e v e r y 3 weeks. the next  i r r i g a t i o n was  lb.  growing  g i v e n water by s p r i n k l e r i r r i g a t i o n  When r a i n o c c u r r e d a f t e r an i r r i g a t i o n  d e l a y e d i n p r o p o r t i o n t o t h e amount o f r a i n  received.  i  Y i e l d s were t a k e n f r o m q u a d r a t s d u r i n g September 1967 t i m e as t h e n o r m a l d e f o l i a t i o n t r e a t m e n t s .  The  ;  a t t h e same  quadrats c o n s i s t e d of  a mown s t r i p t a k e n f r o m t h e c e n t r e o f e a c h p l o t w i t h t h e same mower used f o r d e f o l i a t i o n treatments. by 8 f t . l o n g .  The  The  quadrat  s t r i p s were 18  s p e c i e s c o m p o s i t i o n of e a c h p l o t was  t w i c e ( J u l y and O c t o b e r ) u s i n g t h e G r a s s l a n d s R e s e a r c h H u r l e y , (1961) p r o c e d u r e . estimates.  On e a c h o c c a s i o n two  estimated  Institute,-  o p e r a t o r s made t h e  S e a s o n a l y i e l d s o f t h e s p e c i e s were d e t e r m i n e d  y i e l d s , number o f d e f o l i a t i o n s ,  i n . wide  and t h e mean p e r c e n t a g e  from  quadrat  f o r each •  component. 11  5.1.2  Pasture Sampling  and t h e Measurement o f P h o t o s y n t h e s i s 'i  The  r e s p o n s e , o f n e t p h o t o s y n t h e s i s i n t h e f o r a g e swards t o c h a n g e s  i n the l i g h t energy from the p l o t s .  l e v e l were e x a m i n e d by r e m o v i n g h e r b a g e - s o i l c o r e s  F o r s a m p l i n g , t h e f o r a g e swards were g r o u p e d  t h r e e t y p e s v i z . p u r e g r a s s (A + B) c l o v e r m i x t u r e s -(E + F + G + H ) .  , p u r e : c l o v e r (C + D)  into  and  grass  These t h r e e f o r a g e s w a r d t y p e s were  sampled f r o m e a c h o f t h e t h r e e d e f o l i a t i o n managements m a k i n g 9 ments i n a l l . E a c h o f t h e s e 9 t r e a t m e n t s were s a m p l e d on 10  1  L e t t e r s r e f e r t o t h e sward t y p e s as d e s c r i b e d i n T a b l e  treat-  occasions  5«I-  87 spread over the regrowth  periods.  sampled u n i f o r m l y o v e r t h e g r o w i n g randomly. treatment  To ensure  that  a l l treatments  s e a s o n t h e t r e a t m e n t s were  was r e p r e s e n t e d b y as wide as p o s s i b l e  a range  of LAIs.  The s o i l b l o c k s were  trimmed t o 16 x 16 cm b y 10 cm deep and p l a c e d i n a p l e x i g l a s s A l l h e r b a g e - s o i l c o r e s were c o l l e c t e d  The t i m e t a k e n f r o m t h e removal  t h e l a b o r a t o r y was l e s s t h a n 15 the l a b o r a t o r y the s o i l  taken to the  minutes. c o r e s were w a t e r e d  The p l a n t s were a l l o w e d 2 hours  c o n d i t i o n s b e f o r e COg  box  at the p l o t s to a r r i v a l i n  and t h e n s e a l e d i n a p l e x i g l a s s chamber f o r COg ment ( B ) .  then  i n t h e e a r l y m o r n i n g when  t h e r a d i a t i o n on t h e p l a n t s was low and i m m e d i a t e l y laboratory.  each  a p p r o x i m a t e l y 20 x 20 x 15 cm  were removed f r o m t h e p l o t s w i t h a spade.  In  sampled  However, t h e s a m p l i n g o c c a s i o n s were a d j u s t e d so t h a t  The h e r b a g e - s o i l c o r e s m e a s u r i n g  (A)V  were  to f i e l d  capacity  a s s i m i l a t i o n measure-  t o a d j u s t t o t h e chamber  a s s i m i l a t i o n measurements commenced.  The COg  a s s i m i l a t i o n chamber measured 22 b y 22 cm b y 41 cm h i g h w i t h t h e s i d e s s u r r o u n d e d b y a 6 . 5 cm t h i c k w a t e r j a c k e t . chamber was mounted on was  i n . plywood.  The u p p e r  i n t h e f o r m o f a h i n g e d l i d (C) t o admit  The l i d was  joint.  Two  T h e s e p o r t s were a l s o  f l u s h t h e chamber w i t h l a b o r a t o r y a i r , when t h e sample was  placed  1  t h e h e r b a g e - s o i l sample.  gas s a m p l i n g p o r t s (D) a l l o w e d gas t o be  drawn o f f t o measure t h e COg c o n c e n t r a t i o n .  not  p o r t i o n o f t h e chamber  s e a l e d t o t h e chamber w i t h a s i l i c o n e r u b b e r g a s k e t t o  make a gas t i g h t  to  The p l e x i g l a s s b a s e o f t h e  i n t h e chamber and whenever COg  used  first  a s s i m i l a t i o n measurements were  i n progress.  L e t t e r s r e f e r t o items marked i n F i g u r e  5.1.1,  H  LSI  RGA.  E  muni ion i  1.1  Chamber and a p p a r a t u s rates of herbage-soil text.  t o measure C 0 u p t a k e and r e l e a s e cores. For e x p l a n a t i o n o f l e t t e r s 2  89 C a r b o n d i o x i d e a s s i m i l a t i o n o f t h e herbage c o r e was  measured  gas a n a l y s e r  b y c o n n e c t i n g the gas s a m p l i n g p o r t s t o an  (BeckmaANo,215) (E) t o form a c l o s e d c i r c u i t  ( D e c k e r , 1954)was  The  gas l i n e s were "Tygon"  drawn f r o m t h e b o t t o m  At t h e s t a r t  per hour w i t h a t o t a l  a d j u s t e d t o about 400  260  ppm  (I).  t u b i n g ( P ) , and t h e gas  pump ( H ) .  into  The gas f l o w r a t e  system volume o f 17-1  ppm  COg  litres.  c o n c e n t r a t i o n change  and t h e n t h e t i m e f o r a 340 t o ; was  r e c o r d e d on t h e c h a r t  The volume o f COg u s e d by t h e p l a n t was  D u r i n g t h e measurements o f COg  recorder  corrected f o r  t e m p e r a t u r e p r i o r t o c a l c u l a t i o n o f t h e COg u p t a k e  samples  (o)>,  o f e a c h measurement o f CO,-, a s s i m i l a t i o n t h e c o n c e n t r a t i o n  was  COg  system  o f t h e a n a l y s e r and t h e n pumped b a c k  the t o p o f t h e chamber w i t h a T h u n b e r g 28c3 l i t r e s  soil  infrared  o f the chamber t h r o u g h a f l o w meter  through the measuring c e l l  was  g r o w i n g on t h e  rate.  a s s i m i l a t i o n the p a s t u r e sward  were i l l u m i n a t e d w i t h l i g h t  from a 300 watt narrow  s p o t ''cool  beam" i n c a n d e s c e n t lamp ( j ) as d e s c r i b e d by B e e s l e y , et a l . (1963). The lamp was jacket  suspended  o v e r t h e chamber and shone  o f the chamber l i d .  the lamp, t a p water was s u s p e n s i o n system  To e l i m i n a t e the r a d i a n t h e a t i n g e f f e c t  circulated  (K) was  through the j a c k e t .  a r r a n g e d so t h a t  lamp and t h e chamber c o u l d be v a r i e d intensities.  The  a i r temperature  chamber were measured p o t e n t i o m e t e r (M).  b y wet  These  commencement o f e a c h COg  through the water  The  lamp •  the d i s t a n c e between the  and t o p r o v i d e a r a n g e o f l i g h t  and v a p o u r p r e s s u r e d e f i c i t  and d r y t h e r m o c o u p l e s  w i t h i n the  (L) c o n n e c t e d t o a  t e m p e r a t u r e measurements were t a k e n a t t h e a s s i m i l a t i o n measurement.  E a c h h e r b a g e - s o i l c o r e was  of  s u b j e c t t o 10 l i g h t  intensities,  u s u a l l y v a l u e s o f a p p r o x i m a t e l y 10,000, 8,000, 6,000, 4,000, 3,000,  90  2,000, .1,000, 800 , 400. measured.at. e a c h .  and  100 f t .  candles,  D u p l i c a t e .measurements were made a t e a c h l i g h t ••• '  i n t e n s i t y with the order  o f a l l measurements b e i n g random.  i n t e n s i t i e s r e f e r to l i g h t  i n c i d e n t on t h e u p p e r l e a v e s  as measured wj.th a Weston 756 i l l u m i n o m e t e r ;  o f t h e chamber.  and t h e COg a s s i m i l a t i o n  Subsequently the l i g h t  placed  upper leaves incident  o f t h e herbage  j u s t below the l i d  i n t e n s i t i e s were e v a l u a t e d ,  e n e r g y between 400 a n d 7 0 0 nm, b y d e t e r m i n i n g d i s t r i b u t i o n with  The l i g h t  as  t h e i r s p e c t r a l energy  an "ISCO" s p e c t r a l r a d i o m e t e r .  To e n s u r e t h a t -the  o f e a c h sample o f herbage r e c e i v e d c o m p a r a b l e d o s e s o f  light  a small  laboratory jack  (N) was u s e d t o p o s i t i o n t h e  ' s o i l b o x so t h a t t h e U p p e r l e a v e s  o f t h e herbage were w i t h i n a few mm  o f t h e chamber] l i d .  i n t e n s i t i e s , b e c a u s e o f t h e low COg  At low l i g h t  a s s i m i l a t i o n , the s o i l COg  concentration  r e s p i r a t i o n o f t e n caused a net increase  o f t h e system.  In t h i s  i n the  instance the p r e v i o u s l y  d e s c r i b e d measurement p r o c e d u r e was r e v e r s e d  and t h e COg  concentration  was  l o w e r e d t o 250 ppm COg w i t h  and  pump ( 0 ) , and t h e n t h e time f o r t h e c o n c e n t r a t i o n t o i n c r e a s e , f r o m  260 t o 340 ppm COg On was  the completion  o f measurements o f COg a s s i m i l a t i o n t h e chamber  opened and t h e sample removed. at the s o i l  chamber.  Duplicate  s o i l block.  herbage-soil from the s o i l  surface  The herbage was t h e n s e v e r e d  and t h e s o i l b l o c k  replaced  core  from  i n the  measurements were made o f t h e COg e v o l u t i o n f r o m  A l l measurements o f COg exchange i n t h e l i g h t  core  tower •  recorded.  the r o o t s  the  a bypass " a s c a r i t e " scrubbing  by- t h e  were c o r r e c t e d f o r t h e mean r a t e o f COg e v o l u t i o n t o g i v e a measurement o f COg a s s i m i l a t i o n b y t h e  plant. The  herbage removed f r o m t h e c o r e  was s e p a r a t e d  into grass,  clover  91 leaves, clover petioles and dead material (grass and c l o v e r ) i  The area  of the grass leaves was calculated with the equation of Kemp (1961) for length and mid point breadth measurements, while the area of the clover leaves was estimated with the aid of the scoring standards of Williams  et_ a l . (1964).  The area of the clover petioles was  estimated by considering the structure as a cylinder and taking length and width measurements.  To maintain comparability with the other leaf  areas which were for one side only, half the c y l i n d e r ' s surface area was used.  After the area measurements, a l l herbage fractions were dried  in a forced draft oven at 90°C for 48 hours and the dry weight of each determined. 5.2  Results  5.2.1  Results of F i e l d Investigations  .* The above ground production for each pasture forage sward in the 1967 growing season differed significantly (Table 5.II). The total production between swards differed significantly ( P < 0 . 0 l ) with swards G and H having significantly greater production than the others.  In  the two major components, grass and clover, the swards differed s i g n i f icantly at the P< 0.001  level.  The major difference in grass and  clover production was a consequence of seedings in the swards where no clover was sown in treatments A and B and no grass in treatments C and D.  However, as can be seen from Table 5 . I I there was some grass i n -  vasion into treatments C and D and some clover invasion into treatments A and B.  T a b l e 5^II The 1967 above g r o u n d p r o d u c t i o n o f o r c h a r d g r a s s and w h i t e c l o v e r swards w i t h d i f f e r i n g s e e d i n g c o m p o s i t i o n s i n l b . / a c r e d r y m a t t e r . L e t t e r s r e f e r t o sward as i n T a b l e 5.1.  Sward Composition  A  C  B  D  E  P  G  H  1 SE  Total  6470 6080 6030 5901 6948 7095 8402 9056 656  Grass  5736 5956  1  359  Clover  1  X  SE  814  927 4753 5102 5369 6933 549 .:  ••  73 4520 4294 I856 1920 2654 2000 327  S t a n d a r d e r r o r o f t r e a t m e n t means, +_.  The t h r e e d e f o l i a t i o n managements d i f f e r e d g r o u n d p r o d u c t i o n ( P < 0.01) lower than the r e s t  s i g n i f i c a n t l y i n above  w i t h t h e (9-3) t r e a t m e n t s i g n i f i c a n t l y  (Table 5»Hl).  The d e f o l i a t i o n managements however,  d i d n o t s i g n i f i c a n t l y i n f l u e n c e t h e g r a s s p r o d u c t i o n and hence t h e e f f e c t was a l m o s t e n t i r e l y a consequence o f t h e s i g n i f i c a n t l y l o w e r c l o v e r p r o d u c t i o n under the  (9-3)  management  (P<0.00l).  •;.  I t c a n be s e e n ( T a b l e 5-III) t h a t t h e s i m i l a r s e a s o n a l t o t a l ; f o r t h e (3-1) a n d ( 9 l ) d e f o l i a t i o n s -  were a consequence o f c o m p e n s a t o r y  changes i n d e f o l i a t i o n f r e q u e n c y and i n amount o f m a t e r i a l removed w i t h each d e f o l i a t i o n . and (9 3) -  The d r y m a t t e r y i e l d p e r d e f o l i a t i o n f r o m t h e (3-l)  treatments d i d not d i f f e r s i g n i f i c a n t l y .  Under t h e ( 9 l ) -  t h e amount removed was s i g n i f i c a n t l y g r e a t e r t h a n t h a t removed u n d e r the  (3-1)  and  (9 3) _  managements  (9-1) p l o t s were d e f o l i a t e d  (P<0.00l).  T h u s , even t h o u g h t h e  l e a s t f r e q u e n t l y , t h e i r g r e a t e r yield;'  compensated f o r t h e i r i n f r e q u e n t d e f o l i a t i o n .  The (9 3) -  defoliation  93 management p e r m i t t e d o n l y 8 d e f o l i a t i o n s , lowest  y i e l d per d e f o l i a t i o n ,  caused  and t h i s t o g e t h e r w i t h t h e  t h e (9-3) management t o be l e a s t  productive.  Table 5 , I l l  The i n f l u e n c e o f d e f o l i a t i o n management on t h e 1967 above ground p r o d u c t i o n o f o r c h a r d g r a s s and w h i t e c l o v e r p a s t u r e s and on t h e i r components i n l b . / a c r e d r y m a t t e r .  Management  3 - 1,  9-1  9-3  S.E.+  Total  7687  7475  5831  401  Grass  4198  4629  4519  336  Clover  2902  2627  1081  201  783  1289  740  57  10  6  8  -  Y i e l d per defoliation  Mean number o f defoliations  The  d e f o l i a t i o n managements i n t e r a c t e d  . .  w i t h t h e f o r a g e swards ( T a b l e  5. I V ) , i n t o t a l p r o d u c t i o n ( P < 0 . 0 0 1 ) , g r a s s p r o d u c t i o n ( P < 0 . 0 5 ) " and c l o v e r  production (P<0.05).  interactions  The most s t r i k i n g f e a t u r e o f t h e s e  was t h e s i g n i f i c a n t l y low c l o v e r  content  (P<0.05) o f  swards E , F, G and H u n d e r t h e (9-3) d e f o l i a t i o n management w i t h t h e (3-1) and ( 9 ~ l ) t r e a t m e n t s .  compared  F u r t h e r i t c a n be s e e n t h a t t h e  most p r o d u c t i v e management f o r t h e swards sown o n l y w i t h w h i t e (C and D) was t h e (3-1) o r ( 9 ~ l ) t r e a t m e n t  clover  whereas t h e (9-1) t r e a t m e n t  was most p r o d u c t i v e i n t h e swards sown o n l y w i t h o r c h a r d g r a s s Taken t o g e t h e r , t h e s e r e s u l t s , w i t h t h e r e s u l t s  (A a n d , B ) .  o f t h e managements  94 per s e . i n d i c a t e t h a t t h e poor re-growth  r a t e o f t h e swards s u b j e c t t o  t h e ( 9 - 3 ) management was l a r g e l y a c o n s e q u e n c e o f slox* r e - g r o w t h o f c l o v e r i n swards w i t h t h e ( 9 - 3 ) management.  Table  5 . IV  Management  The i n t e r a c t i o n o f d e f o l i a t i o n management w i t h t h e p a s t u r e swards i n t o t a l p r o d u c t i o n , g r a s s p r o d u c t i o n , and c l o v e r p r o d u c t i o n i n l b . / a c r e d r y m a t t e r . Standard e r r o r o f t r e a t m e n t means + 1 1 3 6 f o r T o t a l p r o d u c t i o n , + 9 5 1 f°r G r a s s p r o d u c t i o n and + 567 f o r C l o v e r p r o d u c t i o n .  A  B  C  P a s t u r e Sward E D Total  P  G  H  production  .1  3 - 1  5742  4225  8457  6623  7905  7367  10,010  9-1  7201  8954  6472  5794  6163  9943  6691  8582  9 - 3  6465  5061  3160  5285  6776  3975  85O6  7416  Grass  5 '  11,171 .  production  3 - 1  4759  4047  1400  575  4508  5350  5011  7937  9 - 1  6487  8764  446  424  4458  • 6443  3918  6092 1  9 - 3  5960  5057  601  1784  5290  3513  7177  67,69  Clover production 3 - 1  222  94  9 - 1  543  310  9 -  3  5950  5268  2753  1840  4240  2848  119  5455  4899  1572  3500  2532  2393  5  2155  2714  1243  423  H89  608  . The m e t e o r o l o g i c a l d a t a f o r t h e g r o w i n g s e a s o n ( T a b l e 5.V), examined w i t h r e f e r e n c e t o t h e d a t a f r o m t h e c o n t r o l l e d s t u d i e s ( S e c t i o n 4 - 2 . 1 ) provided evidence  when  environment  o f n e a r optimum g r o w i n g  95 c o n d i t i o n s d u r i n g the growing season. have l i m i t e d p a s t u r e precipitation  growth i n A p r i l  i n a l l months e x c e p t  However, low t e m p e r a t u r e s and May.  Evaporation  A p r i l , but t h i s d e f i c i t  may  exceeded was, t o a  reasonable  e x t e n t , made good b y i r r i g a t i o n .  Table  M e t e o r o l o g i c a l d a t a d u r i n g t h e 1967 g r o w i n g s e a s o n . F i g u r e s a r e m o n t h l y means f o r s c r e e n t e m p e r a t u r e s and r a d i a t i o n and t o t a l s f o r r a i n f a l l , c l a s s A pan e v a p o r a t i o n and i r r i g a t i o n .  5»V  >  v  April  Month  2.62  Rainfall i n .  May  1.64  June  0.54  Aug.  July  1.18  Sept.  0.18  2.73  10.'2  14-9  20.1  21.1  24.2  19.4-  3.9  8.8  12.8  13.3  14.3  12.2"  Radiation l a n g l e y s / day  356  430  565  583  524  341''  Evaporation i n .  2.32  4.15  6.64  6.76  7.01  3.33  Irrigation i n .  0.0  0.0  2.0  3.5  5-0  0.0  Max. Temp. °C Min.  Temp. °C  D a t a on t h e s o i l i n A p p e n d i x 8.1.  environment o f the experimental  a r e a may be  found  This also indicated favourable conditions f o rplant  growth,  5.2.2  L i g h t E n e r g y Response o f Wet P h o t o s y n t h e s i s Rabinowitch  photosynthesis be  (1951) s u g g e s t e d  (Pn) o f i n d i v i d u a l  i n the Forage  t h a t t h e r e l a t i o n between n e t l e a v e s and l i g h t  d e s c r i b e d by a r e c t a n g u l a r hyperbola  i n t e n s i t y .(I) c o u l d  so t h a t !  Pn = A ( I - Io) 1 + B ( I - Io) where Io i s t h e l i g h t  Stands  c o m p e n s a t i o n p o i n t and where A and B a r e  .,(5.1)  )  96 parameters whioh characterize the shape and position of the response curve.  The v a l i d i t y of this equation for maize (Zea mays) and tobacco  (Nicotinia tqhacum L.) leaves was confirmed "by Hesketh and Moss ( 1 9 6 3 ) . Pearce  et a l . (1967) neglected the light compensation value and found  that the equation: P  n  =  (5.2)  A I  1 + B I gave a good description of the net photosynthesis/light intensity response, of barley (Hordeum yuigare L.) leaves,  van den Driessche and  Waring (1966) have further shown that the monomolecular growth equation (Richards, 1959) gave a good f i t f o r net photosynthesis/light intensity data from seedlings of Pinus spp. Thus: Pn = A - Bexp""  1  (5.3)  For the decay phase of the monomolecular growth equation Brody (1945) gives the following: Pn = A - Bexp"  01  ..(5.4)  which i s more convenient than equation ( 5 « 3 ) as the parameter C characterizes the shape of the response curve. Equations (5«1) and (5*2) have been used by a number of workers i n models of photosynthesis i n plant comnrunitiess de Wit (1965) > Duncan; et a l . (1967) and Idso and Baker (1967).  A l l these models calculate  net photosynthesis of individual leaves and then extrapolate to a layer of leaves with the same illumination and f i n a l l y to the community as a whole.  However, very l i t t l e attention has been given to a general  light response equation f o r .a plant community as a whole, although Baker and Musgrave (1964) state that equation (5*1) gave a good f i t with a maize community.  97  In d e c i d i n g w h i c h o f t h e above f o u r e q u a t i o n s w o u l d be s u i t e d f o r t h e p r e s e n t community d a t a e q u a t i o n (5»l)  w a  s  most  e l i m i n a t e d as  t h e l i g h t c o m p e n s a t i o n v a l u e s were n o t known i n a d v a n c e , w h i l e (5-3)  was  (5-4)«  a l s o e l i m i n a t e d as i t was  The  Equation  c h o i c e t h e r e f o r e , was  (5*2)  will  and Paauw ( 1 9 5 2 ) plot of l o g The  g  a l e s s convenient  between e q u a t i o n s  will  g i v e a good f i t i f a  c o r r e l a t i o n c o e f f i c i e n t s f o r l i n e a r r e g r e s s i o n s of the  gave non  d a t a sets.. (5«4)  t h e most s u i t a b l e e q u a t i o n .  w h i l e the t r a n s f o r m a t i o n f o r e q u a t i o n  T h e r e f o r e b e c a u s e o f t h e good f i t w i t h t h e  i t was  (5»4)  o f maximum n e t c a r b o n  for'i  equation  a l l d a t a s e t s and t h e g e n e r a l l y h i g h e r d e c i d e d t o use  t h i s equation to  compare t h e n e t p h o t o s y n t h e s i s / l i g h t e n e r g y r e s p o n s e equation  1  (5*2)  t r a n s f o r m a t i o n exceeded those f o r e q u a t i o n  t r a n s f o r m a t i o n i n almost  In  level, with  With 80 d a t a s e t s the c o r r e l a t i o n c o e f f i c i e n t s  correlation coefficients  In a l l  gave a h i g h l y s i g n i f -  s i g n i f i c a n t c o r r e l a t i o n s , at the P< 0 . 0 0 1  transformation.  data  t r a n s f o r m a t i o n s were c a l c u l a t e d and compared f o r t h e  icant c o r r e l a t i o n ( P < 0 . 0 0 l )  (5.4)  (5*4)•  and  t  d a t a s e t s the t r a n s f o r m a t i o n f o r equation ( 5 . 4 )  equation  equation  Pn v s l / l i s l i n e a r .  90 s e t s o f d a t a t o d e c i d e w h i c h was  15  (5'2)  of  g i v e a good f i t i f a p l o t o f l / P n v s l / l i s l i n e a r  s t a t e s t h a t e q u a t i o n (5»4)  w i t h t h e above two  (5.2)  form  equation  curves.  t h e p a r a m e t e r A ( t h e a s y m p t o t e ) i s an dioxide fixed  at s a t u r a t i n g l i g h t .  estimate  The  p a r a m e t e r B i s an e s t i m a t e o f t r u e p h o t o s y n t h e s i s a s s u m i n g t h a t t h e net carbon  d i o x i d e e v o l v e d a t z e r o l i g h t e n e r g y i s an e s t i m a t e  p h o t o r e s p i r a t i o n ; t h e d i f f e r e n c e b e t w e e n A and B i s an e s t i m a t e photorespiration.  The  C p a r a m e t e r i s a measure o f t h e  c u r v i l i n e a r i t y of the response  line  of of  relative  and t h e l a r g e r t h e C v a l u e t h e more  98 a c u t e l y c u r v e d w i l l "be t h e r e s p o n s e  line.  The t h r e e p a r a m e t e r s A, B  and C were e s t i m a t e d u s i n g an i t e r a t i v e l e a s t s q u a r e s that suggested  b y G l a s s (1967).  method s i m i l a r t o  From t h e s e p a r a m e t e r s two o t h e r I  v a l u e s were c a l c u l a t e d ? t h e l i g h t e n e r g y a t w h i c h n e t p h o t o s y n t h e s i s i s z e r o i . e . t h e " l i g h t c o m p e n s a t i o n " p o i n t and t h e l i g h t e n e r g y \ t o . g i v e 50$ o f t h e A v a l u e .  The i n i t i a l  s l o p e o f t h e r e g r e s s i o n was  measured b y c a l c u l a t i n g t h e c a r b o n d i o x i d e u p t a k e a t t h e l i g h t c o m p e n s a t i o n p o i n t p l u s 2.0 c a l . / c m / h r .  The e q u a t i o n p a r a m e t e r s and  r e g r e s s i o n l i n e s f o r two t y p i c a l d a t a s e t s t r e a t e d i n t h e above manner a r e p r e s e n t e d The  i n F i g . 5-2.1.  ;  r e l a t i o n s h i p between t h e e q u a t i o n  (5*4)  parameters, the  c a l c u l a t e d v a l u e s and t h e sward c h a r a c t e r i s t i c s were t h e n examined b y multiple r e g r e s s i o n analyses f o r a l l data. regrowth  d a y s , L A I and w e i g h t  g e n e r a l l y regarded  The sward c h a r a c t e r s were  p e r u n i t a r e a o f l a n d and t h e s e were  as t h e i n d e p e n d a n t v a r i a b l e s .  As i t h a d b e e n  f o u n d p r e v i o u s l y , ( S e c t i o n 4.2.2), t h a t o r c h a r d g r a s s had d i f f e r i n g optimum t e m p e r a t u r e s  and w h i t e c l o v e r  f o r net photosynthesis, the data •  were t r e a t e d i n t h r e e s e p a r a t e g r o u p s .  These g r o u p s were p u r e o r c h a r d -  g r a s s s w a r d s , p u r e w h i t e c l o v e r and swards w h i c h c o n t a i n e d b o t h s p e c i e s . W i t h i n each o f t h e sward groups homogeneity o f t h e r e g r e s s i o n s f o r e a c h o f t h e t h r e e managements was t e s t e d .  5.2.2.1 P u r e G r a s s Swards The  r e l a t i o n s h i p b e t w e e n L A I and days o f r e g r o w t h w i t h t h e  orchardgrass  swards was l i n e a r  highly significant  (P<0.00l)  ( F i g . 5-2.2).  T h i s r e l a t i o n s h i p was  and t h e r e were no s i g n i f i c a n t d i f f e r e n c e s  b e t w e e n t h e s l o p e s o f t h e r e g r e s s i o n s f o r t h e i n d i v i d u a l managements.  LIGHT  ENERGY  [ cal. cm^Kr."  1  )  1 The i n f l u e n c e o f l i q h t e n e r g y o n ^ e t p h o t o s y n t h e s i s p e r m e t r e land. Lines f i t t e d to e q u a t i o n s i n t h e f o r m Y= • A - B e x p - • D a t a (X)--a m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d 25 days a f t e r d e f o l i a t i o n , and (o) an o r c h a r d g r a s s s w a r d 9 days a f t e r d e f o l i a t i o n .  100 As c a n be  about t h e r e g r e s s i o n . the non-uniform sample t a k e n same L A I  5»2.3  seen f r o m F i g u r e  T h i s v a r i a b i l i t y was  just  prior  LAI o f the  (Figure  l a n d a r e a , was  5.2.3).  The  mm  22.0  + 1.1°C  s l o p e s of the r e g r e s s i o n s f o r the significantly.  and  temperature  t h e mean v a p o u r p r e s s u r e  i  swards, a t s a t u r a t i n g l i g h t  ments showed d i f f e r i n g r e s p o n s e s swards w i t h the positive (P<  ments d i d n o t to the  (3-l)  (9-3)  (P<0.0l)  and  The  (9~l) but  d a t a from  show e v i d e n c e  (Figs.  i n the  e n e r g i e s , the t h r e e manage-  5«2.4  5.2.5)5  A and B and  managements showed s i g n i f i c a n t  the  slopes d i f f e r e d  the swards w i t h t h e o f an optimum L A I .  significantly  (9-1)  (3-l) and The  swards  management showed a s i g n i f i c a n t l y q u a d r a t i c an optimum L A I o f about  Examination that  and  l i n e a r responses  0.001).  deficit  Hg.  In t h e r e l a t i o n s h i p s between L A I and n e t p h o t o s y n t h e s i s orchardgrass  and,  highly significant  u p t a k e c o m p a r i s o n s w h i c h f o l l o w , the mean  o f measurement was  of the d a t a from  and :  manage-  subjeqted  response  12.  the o r c h a r d g r a s s  swards  indicated'  t h e d i f f e r e n c e s between managements were p a r t l y a c o n s e q u e n c e  the p a r t i c u l a r LAIs  sampled w i t h e a c h management.  The  t h e optimum L A I was  q u a d r a t i c r e g r e s s i o n was 14.5*  The  of  T h e r e f o r e the -data  were combined t o g i v e a g e n e r a l r e g r e s s i o n f o r t h e g r a s s ( T a b l e 5.VI),  the  sward.  i n d i v i d u a l managements d i d not d i f f e r In t h e COg  of  so t h a t a sward  swards t h e r e l a t i o n s h i p between L A I  weight o f herbage from u n i t  2.9 +. 1.12  l a r g e l y a consequence  t o a d e f o l i a t i o n would not n e c e s s a r i l y have  Ih t h e o r c h a r d g r a s s  was  considerable v a r i a b i l i t y  d i s t r i b u t i o n of p l a n t s i n the f i e l d  as t h e average  (P<0.001),  t h e r e was  significant  r e g r e s s i o n s between n e t  swards  (P<0.00l)  and  photosynthesis  Hi RAAOI  Fig.  5.2.3  The r e l a t i o n s h i p area o f land f o r  W | KVt<  b e t w e e n LAI and w e i g h t o f h e r b a g e pure o r c h a r d g r a s s swards.  from  unit  Fig.  2 5 . 2 . * » The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s per metre l a n d , and LAI i n o r c h a r d g r a s s s w a r d s a t s a t u r a t i n g l i g h t e n e r g y l e v e l s ; A) s u b j e c t t o ( 3 - 1 ) d e f o l i a t i o n management and B) s u b j e c t t o (9-1) d e f o l i a t i o n management.  o  io.:  o  to «/» Ui  X  •—  z  >t/> O  Y= - 1 0 1 3 + 0 7 1 1 X - 0 0 2 7 X  8  10  11.1?  13  14  15  R=0845  2  16  17  A  Fig.  5.2.5  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e land and LAI I n o r c h a r d g r a s s s w a r d s a t s a t u r a t i n g l i g h t e n e r g y l e v e l s s u b j e c t t o a (9~3) d e f o l i a t i o n management.  104 and L A I were a l s o c a l c u l a t e d a t f o u r l i g h t e n e r g y l e v e l s , and T a b l e 5 . V I ) .  ( F i g . 5.2.6  I t c a n be s e e n t h a t t h e optimum L A I i n c r e a s e s w i t h  i n c r e a s i n g l i g h t energy l e v e l s .  A t 3 . 0 c a l . / o m / h r , t h e optimum L A I  2 was 1 1 a n d a t 1 2 . 0 c a l , / c m / h r . t h e optimum was 1 4 . L A I 1 4 was r e a c h e d t h e r e was v e r y l i t t l e  change  However, once  i n t h e optimum  2 between  1 2 . 0 c a l , / c m / h r . and s a t u r a t i n g l i g h t l e v e l s .  Saturating  l i g h t e n e r g i e s were u s u a l l y r e a c h e d a t a' maximum o f 3 0 c a l . / c m / h r . T a b l e 5 - V I The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g n e t p h o t o s y n t h e s i s (Y) t o L A I (X) i n o r c h a r d g r a s s swards a t s a t u r a t i n g a n d , f o u r other l i g h t energy l e v e l s . Regressions i n the form Y = a + bX + c X n = 3 0 , c o e f f i c i e n t s i n g C0 /m /hr,. 2  2  9  L i g h t energy cal/cm / h r .  a  Coefficients b  R c  Saturating  0.746  0.437  -0.0163  O.696  24.0  0.663  O.4O6  -0.0146  O.695  12.0  0.453  0.338  -0.0122  0.671  6.0  0.220  0.242  -0.0092  °-591  3.0  0.057  0.154  -0.0068  0.433  From t h e c u r v e s i n F i g .5 . 2 . 6  i t was p o s s i b l e t o c a l c u l a t e t h e  e f f i c i e n c y o f l i g h t use, i n the uptake o f C 0 , 2  swards.  ,  by the orchardgrass  F o r t h e s e c a l c u l a t i o n s t h e CO,, u p t a k e a t t h e optimum L A I f o r  e a c h l i g h t e n e r g y l e v e l was t a k e n and c o n v e r t e d t o c a l o r i e s w i t h t h e calorific (1964).  e q u i v a l e n t f o r CO^ ( 2 , 7 0 0 c a l . / g CO^) u s e d b y Yocum, e t I ' a l . When t h e e f f i c i e n c i e s o f l i g h t e n e r g y u s e t o g e t h e r w i t h i t h e  optimum L A I and CO^ u p t a k e f o r t h e f o u r l i g h t  e n e r g y l e v e l s a r e compared  ( T a b l e 5 . V I I ) i t c a n be s e e n t h a t maximum e f f i c i e n c y o f l i g h t e n e r g y u s e  energy cal. crri^hrH  Light  1  ?  3  4  5  7  6  I  5.2.6  8  9  A  10  11  1?  1 3 1 4  15  16  I  2 The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e land a n d LAI i n o r c h a r d g r a s s s w a r d s a t f o u r l e v e l s o f i n c i d e n t light energy. The r e g r e s s i o n e q u a t i o n s f o r t h e f o u r l i n e s a r e g i v e n i n T a b l e 5.VI  106  Table 5 . V I I  The e f f i c i e n c y o f l i g h t e n e r g y u s e i n COg u p t a k e i n o r c h a r d g r a s s swards a t f o u r l i g h t e n e r g y l e v e l s t o g e t h e r w i t h t h e i r COg u p t a k e and optimum L A I a t t h e s e e n e r g y levels.  L i g h t energy cal/cm^/hr.  CO2  Uptake  Efficiency  Opt imum LAI  g C0 /m2/hr. 2  3.0  0.93  11.0  8.35  6.0  1.81  13.0  8.15  -  12.0  2.79  14.0  6.29  .J  24.0  3.49  14.2  3.93  occurred  i n the r e g i o n of 3 t o 6cal/cm / h r .  T h i s , compared w i t h , t h e  mean  e n e r g y r e q u i r e d t o p r o d u c e 5 0 $ o f t h e maximum COg u p t a k e r a t e  light  .g  (6.8 + 1.9cal/cm / h r . )  s  i n d i c a t e d a r e l a t i v e l y e f f i c i e n t l i g h t energy  use b y t h e swards when a t t h e optimum L A I .  Calculation of efficiencies  2  b e l o w 3 . 0 c a l . / c m / h r , were n o t made as t h e l i g h t c o m p e n s a t i o n f o r some g  o f t h e swards o c c u r r e d the h i g h e s t  within this region.  A value  2 » 3 c a l . / c m / l o r . was  l i g h t compensation p o i n t recorded.  The i n i t i a l  • -?  slope o f the l i g h t response curves, c a l c u l a t e d from  the net p h o t o s y n t h e s i s  at the l i g h t compensation point  plus  2 2 . 0 c a l . / c m / h r . , was r e l a t e d t o L A I ( F i g . 5 « 2 . 7 ) ° the  In t h i s  i n t e r c e p t s o f r e g r e s s i o n s f o r t h e t h r e e managements  significantly (P<0.00l)i  c o e f f i c i e n t s f o r these three regressions 5.2.7)  differed  however, t h e s l o p e s o f t h e t h r e e  r e g r e s s i o n s were n o t s i g n i f i c a n t l y d i f f e r e n t .  show t h a t t h e ( 9 ~ l ) and ( 9 - 3 )  (Table  instance  management  The r e g r e s s i o n 5.VIII)  and ( P i g . ,  swards b o t h showed an optimum L A I  w h i c h i n b o t h i n s t a n c e s was c l o s e t o 1 4 .  The i n t e r c e p t o f t h e ( 9 - 3 ) ,  Fig.  5.2.7  The r e l a t i o n s h i p b e t w e e n t h e i n i t i a l s l o p e , o f t h e l i g h t e n e r g y r e s p o n s e c u r v e s o f o r c h a r d g r a s s s w a r d s , and LAI o f t h e s w a r d s under t h r e e d e f o l i a t i o n managements. The r e g r e s s i o n e q u a t i o n s f o r t h e t h r e e managements a r e g i v e n i n T a b l e 5 . V I I I  Table 5.V1U  TlSe" r e g r e s s i o n  c o e f f i c i e n t s r e l a t i n g the i n i t i a l slope (Y) of the l i g h t response curves i n orchardgrass swards to t h e i r LAI (X). Regressions i n the form ^ Y = a + hX + c X u n i t s of c o e f f i c i e n t s g CO /m / h r . 2  Management  Coefficients b  a  n  R  e  (3-1)  0.0059  0.150  -0.0040  (9-1)  -0.0656  0.153  -O.OO56  (9-3)  -0.3442  0.160  -0.0057  11  0.818  9  .0.911  10  0.847  management was s i g n i f i c a n t l y d i f f e r e n t from the other two managements, and i t r e f l e c t e d the 3 inch c u t t i n g height i n that the m a t e r i a l l e f t r e s u l t e d i n a lower rate, o f C 0 for  the (3-1)  and (9-1)  2  f i x a t i o n f o r the (9-3)  managements.  management than  T h i s s t r o n g l y suggests t h a t ,  under f i e l d c o n d i t i o n s of moderate l i g h t energy l e v e l s , the 3 inches of herbage l e f t was i n f a c t a d r a i n on the swards CO^ f i x i n g  ability  of the stand compared with those stands with managements where the | c u t t i n g height was 1 inch.  j  Weight of dry t i s s u e i n f l u e n c e d the net photosynthesis of the : orchardgrass swards with no s i g n i f i c a n t d i f f e r e n c e s between the ': managements. LAI the  100 R 2  When weight was combined i n a m u l t i p l e r e g r e s s i o n with increased from  39J$ to 60$ (P<0.00l).  This regression  equation wass Y =  1.645 + 0 . 4 2 7 X 1 - 0.642X2  R = O.778  where Y = net photosynthesis i n g CO^/m /hr, , and Xg = herbage weight g/dm  = LAI and ..,  land.  T h i s r e f l e c t e d i n c r e a s i n g p h o t o r e s p i r a t i o n with increased herbage weight and t h i s agreed with the r e l a t i o n found between estimated  109 p h o t o r e s p i r a t i o n , p a r a m e t e r s A - B, and L A I (P<0.00l).  This  relation  was n o t s i g n i f i c a n t l y d i f f e r e n t f o r t h e t h r e e managements and wass  Y = O.O89 - 0.101X where Y = p h o t o r e s p i r a t i o n  r = -0.671  ( A - B) i n g COg/m / h r .  and X = h e r b a g e  2  w e i g h t g/dm The  land.  j. >  decline i n l e a f e f f i c i e n c y with increasing LAI at saturating  l i g h t energies  i s shown b y a l i n e a r d e c r e a s e i n n e t a s s i m i l a t i o n r a t e  (WAR) w i t h i n c r e a s i n g L A I ,  (P<0.00l).  The managements showed no  significant differences i n this relationship.  On a v e r a g e t h e r e  was  o n l y a s l i g h t p o s i t i v e NAR a t an L A I o f 17 w h i c h s u g g e s t s i n a l l * p r o b a b i l i t y some o f t h e l o w e r l e a v e s This  had a n e g a t i v e  FAR ( P i g . 5«2.8);.  i s c o n s i s t e n t w i t h t h e c o n c e p t o f an optimum L A I and t h e l i n e a r  d e c r e a s e i n NAR t o L A I 17 i s a l s o c o n s i s t e n t w i t h t h e optimum c o n c e p t . As s t a t e d e a r l i e r t h e C v a l u e regressions  gives  a measure o f t h e r e l a t i v e c u r v a t u r e  Thus swards w i t h a l a r g e C v a l u e become l i g h t  of the l i g h t response  exponential  of this  line.  have more p r o s t r a t e l e a v e s , and (they  s a t u r a t e d more r a p i d l y ( p e r u n i t o f l i g h t e n e r g y ) t h a n :•  swards w i t h a l o w e r C v a l u e . were t r a n s m i t t e d  These d i f f e r e n c e s i n l e a f  i n t o d i f f e r e n c e s i n t h e amount o f l i g h t  inclination energy  r e q u i r e d t o p r o d u c e 50$ o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s ^  -In  t h i s r e l a t i o n t h e r e were no s i g n i f i c a n t d i f f e r e n c e s b e t w e e n managements ( P i g . 5-2.9)• negative  The r e l a t i o n was s i g n i f i c a n t l y q u a d r a t i c  (P<0.00l) i n d i c a t i n g t h a t beyond, a p a r t i c u l a r l e a f  a t i o n (C = 0.18) with leaves  t h e r e was l i t t l e  andi inclin-  gain i n the e f f i c i e n c y of l i g h t , u s e  c l o s e r to the h o r i z o n t a l .  10 r-  r  ?74 -0  325 X  r . o 702  2 ^  i  10  _i  12  14  16  I Fig.  5.2.8  The r e l a t i o n s h i p s w a r d s and L A I .  between net a s s i m i l a t i o n  rate  (NAR) of  leaves  in  orchardgrass " y  Y :19.093 - 1 5 2 0 1 3 X + 3 8 7 . 9 9 4 x  ' • 06  • .08  1  '  1  .10  .12  C  2  R=0889  • .14  .16  .18  .20  VALUE  The r e l a t i o n s h i p b e t w e e n t h e l i g h t e n e r g y r e q u i r e d t o p r o d u c e 50% o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s i n o r c h a r d g r a s s s w a r d s and C v a l u e o f t h e e x p o n e n t i a l e q u a t i o n s f i t t e d t o t h e l i g h t energy response c u r v e s .  112  5.2.2,2  Pure C l o v e r  Swards  F i g u r e 5 = 2 . 1 0 shows t h e r e l a t i o n s h i p b e t w e e n L A I and regrowth w i t h the white (P<0.00l)  significant  c l o v e r swards. and  T h i s r e l a t i o n s h i p was  t h e r e were no  highly  significant differences  between the s l o p e s o f the r e g r e s s i o n s f o r the With these  days o f  swards as w i t h t h e o r c h a r d g r a s s  i n d i v i d u a l managements.  swards, the  about t h i s r e g r e s s i o n r e f l e c t e d t h e sampled v a r i a b i l i t y  variability i n sward c l o v e r  plant density. I n t h e w h i t e c l o v e r swards t h e r e l a t i o n s h i p b e t w e e n L A I and w e i g h t o f h e r b a g e f r o m a u n i t l a n d a r e a , was (P< 0 . 0 0 1 ) , ( F i g , 5 . 2 . 1 1 ) .  The  i n d i v i d u a l managements d i d n o t I n t h e COg  3.1  +0-9  mm  highly significant  s l o p e s o f t h e r e g r e s s i o n s f o r the, differ  significantly.  u p t a k e c o m p a r i s o n s w h i c h f o l l o w t h e mean t e m p e r a t u r e  o f measurement was was  of  22.1  + 1.2°C  and  t h e mean v a p o u r p r e s s u r e  c l o v e r swards, at s a t u r a t i n g l i g h t e n e r g i e s ,  t h e r e were no  significantly linear  c o m p a r a b i l i t y w i t h the orchardgrass  ( F i g . 5-2.12 A), .  (P<0.00l).  For  d a t a , t h e r e g r e s s i o n s a t t h e >four X  e n e r g y l e v e l s were c a l c u l a t e d as q u a d r a t i c s e v e n t h o u g h t h e  c o e f f i c i e n t s were b a r e l y s i g n i f i c a n t , Table  i n the  s i g n i f i c a n t d i f f e r e n c e s b e t w e e n t h e managements and. t h e  o v e r a l l r e g r e s s i o n was  light  deficit  Hg.  Ln t h e r e l a t i o n s h i p b e t w e e n L A I and n e t p h o t o s y n t h e s i s white  ;  5°IX).  (P<0.2),  (Fig. 5-2.12 B  By c o m p a r i n g F i g u r e s 5 - 2 . 1 2 A and B i t c a n be  t h e optimum L A I s were a l l o i i t s i d e t h e r a n g e o f e x p e r i m e n t a l  2  and  seen t h a t data  except  2  those  d e t e r m i n e d at 3 . 0 c a l . / c m / h r .  I t i s of note that the  optimum L A I s behave i n t h e same manner as t h o s e  f o r the pure  s w a r d s , i . e . , t h e optimum i n c r e a s e d w i t h i n c r e a s i n g l i g h t  estimated grass  energy  levels  J —  1  I  I  1  I  4  8  12  16  20  24  DAYS  Fig.  5.2.10  FROM  LAST  J_ 28  I  32  MOWING  The r e l a t i o n s h i p b e t w e e n L A I and days o f r e g r o w t h l a s t mowing f o r p u r e w h i t e c l o v e r s w a r d s .  .  since the  3-1  « 9-1 - 'J - 3  r=0 9 6 3 1 •  J  1  1  i  1  2  3  4  HERBAGE  Fig.  5.2.11  WEIGHT  g/drr.2  land  The r e l a t i o n s h i p b e t w e e n LAI and w e i g h t o f h e r b a g e area o f land f o r pure w h i t e c l o v e r swards.  from unit  • Fig--5.2.12  The r e l a t i o n s h i p b e t w e e n - n e t p h o t o s y n t e h s i s p e r m e t r e l a n d and LAI i n w h i t e c l o v e r swards, A) a t s a t u r a t i n g l i g h t e n e r g y l e v e l s and B) a t t h e l i g h t e n e r g y l e v e l s as shown. The r e g r e s s i o n e q u a t i o n s f o r B a r e g i v e n i n T a b l e 5 . I X  115 t o 1 2 c a l . / c m / h r . a f t e r w h i c h i t showed no f u r t h e r i n c r e a s e . quadratic X at P < 0 . 1  2  2 c o e f f i c i e n t f o r t h e 3 . 0 c a l . / c m / h r , l e v e l was  and as t h e optimum L A I o f 5  c o n f i d e n c e c a n he p l a c e d i n t h i s  Table 5 . I X  w  a  The  significant  w i t h i n the d a t a , r e a s o n a b l e  s  figure.  The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g n e t p h o t o s y n t h e s i s (Y) t o L A I (X) i n w h i t e c l o v e r s w a r d s a t f o u r l i g h t ^ energy l e v e l s . R e g r e s s i o n s i n tjje f o r m Y = a + bX + c X n = 30, c o e f f i c i e n t s i n g C0 /u / h r . r  L i g h t energy cal./cm^/hr.  a  Coefficients b  R c  24.0  0.863  0.630  -0.0443  0.817  12.0  0.645  O.589  -0.0442  0.783.  6.0  0.253  0.504  -0.0438  O.691  3.0  -0.142  0.419  -0.0445  0.533  From t h e c u r v e s i n F i g . 5 « 2 . 1 2 B i t has b e e n p o s s i b l e t o c a l c u l a t e t h e e f f i c i e n c y o f l i g h t e n e r g y u s e , i n CO^ u p t a k e , b y t h e w h i t e c l o v e r swards.  These c a l c u l a t i o n s u s e d t h e same method and c a l o r i f i c v a l u e as  in Section 5-2.2.1,  and when t h e r e s u l t s are. compared w i t h t h e  r i a t e l i g h t e n e r g y l e v e l s , CO^  approp-  u p t a k e and optimum L A I ( T a b l e 5.X) i t  c a n be s e e n t h e maximum e f f i c i e n c y o f l i g h t e n e r g y u s e o c c u r r e d w i t h 2 approximately 6.0cal./cm /hr.  T h i s l e v e l of l i g h t energy i s c l o s e to  t h a t r e q u i r e d t o p r o d u c e 5 0 $ o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s 2 ( 5 ° 3 +. 0 . 9 c a l . / c m / h r . ) and i n d i c a t e d a r e l a t i v e l y e f f i c i e n t e n e r g y u s e b y t h e swards when a t t h e i r optimum L A I .  light  As w i t h the-; g r a s s  s w a r d s , c a l c u l a t i o n s o f t h e e f f i c i e n c y o f l i g h t u s e were n o t made b e l o w 2 ^ 3 . 0 c a l . / c m / h r . as l i g h t c o m p e n s a t i o n v a l u e s as h i g h as 2 . l e a l . / c m ' " / h r .  116  had been recorded with these clover swards.  The maximum efficiency of  light use was within the range of the data and occurred at about 6 . 0 c a l . / c m /hr.  (Table  5.X).  The i n i t i a l slope of the light response curve calculated from the 2 net photosynthesis at the light compensation point plus 2 . 0 c a l . / c m /hr., was related l i n e a r l y to LAI ( P < 0 . 0 0 l )  (Fig. 5 . 2 . 1 3 ) .  There were no  significant differences between the regressions for the three manage- . ments.  Considerable v a r i a b i l i t y was evident and indicated v a r i a b i l i t y  in the light compensation.,  There was no evidence of an optimum LAI,, here. c  Table 5-X  '  The efficience of light energy use in COg uptake in white clover swards at four light energy levels together with : their CO2 uptake and optimum LAI at these energy levels.  Light energy cal./cm2/hr.  CO2 Uptake g C02/m2/hr.  Opt imum LAI  Efficiency i  :  ;  "L  3.0  O.84I  5-0  7.57  6.0  1.700  6.0  7.65  12.0  2.598  6.8  5.85  ,  24.0  3.102  7.0  3.45  :  <  The weight of dry tissue influenced net photosynthesis in that the estimated photorespiration, parameters A - B, was l i n e a r l y related to tissue weight ( P < 0 . 0 2 ) .  The relation was similar over a l l the  managements as there were no significant differences between the " regressions for them; this was3. Y = - 0 . 0 4 3 - 0 . 2 3 6 X,  ;,  r = -0.442  where Y = photorespiration (A - B) in g C0 /m2/hr. and X = herbage ?  1I •  1.2  r  7  4  L Fig.  5.2.13  A  6  S  I  The r e l a t i o n s h i p b e t w e e n t h e i n i t i a l s l o p e o f t h e l i g h t r e s p o n s e c u r v e s o f w h i t e c l o v e r s w a r d s and t h e i r L A I .  energy  118 w e i g h t g/dm  land.  Wet a s s i m i l a t i o n -rate d e c l i n e d c u r v i l i n e a r l y w i t h i n c r e a s i n g L A I ( P < 0 . 0 0 l ) , ( F i g . 5-2.14).  T h e r e were no s i g n i f i c a n t  between t h e r e g r e s s i o n s f o r t h e t h r e e managements.  differences  A l l t h e swards  showed a p o s i t i v e a v e r a g e NAR e v e n a t L A I s o f a l m o s t 6 i n d i c a t i n g t h a t t h e r e were f e w l e a v e s  i n the lower s t r a t a with negative  JTARs.  The shape  o f t h e r e s p o n s e c u r v e s u g g e s t e d f o r t h e s e swards t h a t minimum a s s i m i l a t i o n p e r u n i t l e a f a r e a o r maximum p e r u n i t a r e a o f l a n d ; occurred  a t about L A I 6.  However, t h e r e was no e v i d e n c e t o s u g g e s t <.  t h a t t h e r e w o u l d n e c e s s a r i l y be a d e c l i n e i n CO^ a s s i m i l a t i o n above -LAI 6.  T h e r e f o r e t h i s d a t a c o u l d n o t c l e a r l y d e f i n e an optimum L A I . As t h e l e a f a n g l e o f t h e c l o v e r swards was more h o r i z o n t a l t h a n  the grass  swards (mean C 0.17 c l o v e r and 0.12 g r a s s ) t h e amount o f  l i g h t e n e r g y r e q u i r e d t o p r o d u c e 50$ o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s was l e s s ( F i g . 5-2.15).  T h e r e was no s i g n i f i c a n t  difference  between t h e r e g r e s s i o n s f o r t h e t h r e e managements and a l i n e a r r e g r e s s i o n f i t t e d the data best 5.2.2.3 The  Mixed Grass - C l o v e r  :  (P<0.001).  Swards  r e l a t i o n s h i p b e t w e e n L A I and days o f r e g r o w t h w i t h t h e g r a s s -  c l o v e r swards was l i n e a r significant  ( F i g . 5-2.16).  The r e g r e s s i o n was h i g h l y  ( P C O . O O l ) and t h e r e were no s i g n i f i c a n t d i f f e r e n c e s between  t h e r e g r e s s i o n s f o r t h e i n d i v i d u a l managements.  With these mixed  s w a r d s , as w i t h t h e p u r e s w a r d s , t h e v a r i a b i l i t y about t h i s  regression  r e f l e c t e d sampled v a r i a b i l i t y i n t h e p l a n t d e n s i t y . I n t h e swards c o n t a i n i n g b o t h o r c h a r d g r a s s  ;.  and w h i t e c l o v e r t h e .  r e l a t i o n s h i p b e t w e e n L A I and w e i g h t o f h e r b a g e f r o m a u n i t l a n d  area  Fig.  5 . 2 . 1**  The r e l a t i o n s h i p b e t w e e n n e t a s s i m i l a t i o n r a t e (NAR) l e a v e s and p e t i o l e s i n w h i t e c l o v e r s w a r d s and L A I .  of  9  Y -8  587  -19.262 X  r = 0-812-  4)  -C-  C O  o o  o  \j « LU  a +  v c  •  0  .08  .10  .12  .14  .16  .18  • 20  •2 2  .24  .26  VALUE  Fig.  5.2.15  The r e l a t i o n s h i p b e t w e e n 1 i g h t e n e r g y r e q u i r e d t o p r o d u c e 50-? o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s i n w h i t e c l o v e r s w a r d s and t h e C v a l u e of t h e e x p o n e n t i a l e q u a t i o n s f i t t e d to the l i g h t energy response c u r v e s .  • 28  121 was  (P<0.00l)j  highly significant  r e g r e s s i o n s f o r the i n d i v i d u a l  (Fig. 5.2.17).  The  s l o p e s o f the  managements d i d n o t d i f f e r  significantly.  Ln t h e COg u p t a k e c o m p a r i s o n s w h i c h f o l l o w t h e mean t e m p e r a t u r e o f measurement was 2.2  + 1.3mm  2 2 . 9 + 1.5°C  and t h e v a p o u r p r e s s u r e d e f i c i t  was.  o f Hg.  In t h e r e l a t i o n s h i p between L A I and n e t p h o t o s y n t h e s i s a t  j  s a t u r a t i n g l i g h t e n e r g i e s , the mixed o r c h a r d g r a s s - w h i t e c l o v e r showed no s i g n i f i c a n t 5.2.18)5 T h e r e was  the o v e r a l l  the  d i f f e r e n c e s between t h e managements ( F i g . r e g r e s s i o n was  s i g n i f i c a n t l y quadratic  an optimum L A I i n t h e s e swards  be s e e n , t h e r e was  swards  o f about 105  some v a r i a b i l i t y about t h i s .  f o u r l i g h t energy l e v e l s  were c a l c u l a t e d as  (P<0.00l).  however, as c a n  The r e g r e s s i o n s f o r Section 5 . 2 . 2 . 1 $ ( F i g . o  5 . 2 . 1 9 and T a b l e 5 . X I ) .  A l l the X c o e f f i c i e n t s  and X  coefficients  2 2 were s i g n i f i c a n t ( P < 0 . 0 0 l ) e x c e p t f o r X at 3 . 0 c a l . / c m / h r . w h i c h was .* * s i g n i f i c a n t at P < 0 . 0 5 . The optimum L A I s b e h a v e d i n the same manner as p r e v i o u s l y and i n c r e a s e d w i t h i n c r e a s i n g l i g h t e n e r g y f r o m 8 . 0 a t 2 3 . 0 c a l . / c m / h r t o $.0 pure swards,  above  2 at 12cal./cm / h r .  However, b y c o n t r a s t  2 1 2 c a l . / c m / h r . t h e r e was  a significant  to t h e  increase i n  2 the  optimum L A I t o 1 0 . 0 From t h e above  at 24cal./cm / h r .  d a t a i t has been p o s s i b l e t o c a l c u l a t e  i e n c y of l i g h t energy use  i n COg  the  effic-  u p t a k e b y t h e s e mixed f o r a g e swards.  These c a l c u l a t i o n s u s e d t h e same method and c a l o r i f i c v a l u e as i n Section 5 . 2 . 2 . 1  and t h e r e s u l t s a r e p r e s e n t e d , t o g e t h e r w i t h t h e  a p p r o p r i a t e l i g h t e n e r g y l e v e l s , COg As c a n be s e e n t h e  maximal 2  approximately 6.0cal./cm /hr.  uptake and optimum L A I ( T a b l e 5 » X I l ) .  l i g h t energy use o c c u r r e d w i t h  T h i s l e v e l o f l i g h t e n e r g y was  ;  close to  t h a t r e q u i r e d t o p r o d u c e 5 0 $ o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s  12  10  -  8•  8  12 D«<)  Fig.  5 . 2 . 1 6  16  fOOM  IAS!  2 0  2 4  28  3 2  3 6  MOWING  The r e l a t i o n s h i p b e t w e e n LAI and days o f re g r o w t h s i net" t h e mowing f o r m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s .  14  12  io  2  3  HERBAGE WftGHT  Fig.  5 - 2 . 1 7  dm?  lonrt  The r e l a t i o n s h i p b e t w e e n LAI and w e i g h t o f h e r b a g e f r o m u n i t a r e a o f land f o r mixed o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s .  Inst  Fig.  5.2.18  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e and LAI i n m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s a t s a t u r a t i n g l i g h t energy l e v e l s .  land  4  Fig.  5.2.19  The r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s p e r m e t r e land and LAI i n m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s a t f o u r " levels of incident l i g h t energy. The r e g r e s s i o n e q u a t i o n s f o r the f o u r l i n e s are given in Table 5 . X I  125 T a b l e 5»XI  The r e g r e s s i o n c o e f f i c i e n t s r e l a t i n g n e t p h o t o s y n t h e s i s (Y) t o L A I (X) i n m i x e d o r c h a r d g r a s s - w h i t e c l o v e r swards at f o u r l i g h t energy l e v e l s . Regressions i n the form Y = a + bX + c X n = 3 0 , c o e f f i c i e n t s i n g COp/m^/hr. 2  Light  energy  cal./cm2/h  r>  Coefficients b  a  R c  24.0  0.728  O.518  -0.0272  0.918  12.0  0.503  0.477  -0.0263  0.880.  6.0  0.281  0.345  -0.0203  0.767  3.0  0.188  O.I64  -0.0107  O.5I5  T a b l e ^>.XII  L i g h t energy cal./cm2/hr.  The e f f i c i e n c y o f l i g h t e n e r g y u s e i n CO2 u p t a k e i n m i x e d f o r a g e s t a n d s c o n t a i n i n g o r c h a r d g r a s s and w h i t e c l o v e r , at f o u r l i g h t energy l e v e l s t o g e t h e r w i t h t h e i r C02 uptake and optimum L A I a t t h e s e l i g h t e n e r g y l e v e l s .  CO2 U p t a k e g C02/m2/hr.  Optimum L A I  3.0  O.815  8.0  7-34  6.0  1.742  8.5  7.84  12.0  2.666  9.0  6.00  24.0  3.188  10.0  3.59  (5.9 light  + 1 . 7 c a l . / c m / h r . ) and  Efficiency fo  i n d i c a t e d a r e l a t i v e l y e f f i c i e n t use of .  e n e r g y b y t h e s e swards when a t t h e i r - o p t i m u m L A I .  As w i t h the-  g r a s s , and c l o v e r swards c a l c u l a t i o n o f t h e e f f i c i e n c y o f l i g h t u s e were n o t made b e l o w 3 . 0 c a l . / c m / h r . , as l i g h t c o m p e n s a t i o n v a l u e s u p t o 2 2 . 2 c a l . / c m / h r . had been r e c o r d e d .  126  The i n i t i a l slope of the light response curve, calculated from the net photosynthesis 2 . 0 c a l . / c m /hr.,  Pig. 5 ' 2 . 2 0 .  at the light compensation point plus  was c u r v i l i n e a r l y related to LAI (P< 0 . 0 0 1 )  see  There were no significant differences between the  regressions for the three managements and there was considerable v a r i a b i l i t y about the general regression.  However, the response line  indicated that the optimum LAI was about 9 which agrees with the optimum calculated from the response curve at 12.0cal./cm Weight of dry tissue influenced net photosynthesis.  /hr. When  included in a multiple regression with LAI the coefficient for tissue weight was negative indicating increased photorespiration with increased tissue weight.  The 1 0 0 R  with LAI only, was 68.2/S and with the  inclusion of tissue weight this increased to 7 3 . 4 $ . was significant  (P<0.00l)  and was;  Y = 1.638 + 0.384 X  where Y = net photosynthesis Xg = herbage weight g/dm  The regression •  x  - 0 . 4 4 9 Xg  R = O.857  in g COg/m /hr. and X^^ = LAI and  land.  The effect of tissue weight on photorespiration was confirmed by the regression of estimated photorespiration, parameters A - B, on tissue weight.  In this regression photorespiration increased with the  higher tissue weights in a curvilinear manner, and there was no significant difference between the regressions for the three managements.  The regression was significant ( P < 0 . 0 0 l ) ' , and wass Y = 0.272 - 0.439 X + 0.040  X  2  R = 0.754  where Y = photorespiration (A - B) in g COg/m /hr. and X = herbage weight g/drn  land.  Fig.  5.2.20  The r e l a t i o n s h i p b e t w e e n i n i t i a l s l o p e o f t h e l i g h t e n e r g y r e s p o n s e c u r v e s o f m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s and their LAI.  128 The'curvilinear nature of the above regression indicated that past a particular leaf weight, there was very l i t t l e increase in photorespiration, presumably a point where the canopy had reached an equilibrium and where further increases in leaf weight represented only an increased quantity of senescent material.  .,  Net assimilation rate declined curvilinearly with increasing LAI (P<0.001), (Fig. 5-2.21).  There were no significant differences between  the regressions for the three managements. positive average NAR,  A l l of the swards showed a  however, at LAIs around 12 i t was so low that in  a l l probability some leaves in the lower strata had negative NARs. The shape of the response curve suggested for these swards that minimum assimilation per unit leaf area, or maximum per unit area of land occurred at about LAI 12.  However, there was no evidence to suggest  that there would necessarily be a decline in COg assimilation above LAI 12.  Therefore this data does not establish an optimum LAI.  The leaf angle influenced the amount of light energy required to produce 5O/0 of the maximum rate of net photosynthesis (Fig. 5.2.22). There was no significant difference between the regressions for the three managements on these mixed orchardgrass-white  clover swards.  As  with the pure grass there was a curvilinear decrease in the amount of light energy required to produce 50$ synthesis (P< O.OOl).  of the maximum rate of net photo-  The response line showed a rise above a C value  of O.245 however, as this covers a region with only one observation i t is of l i t t l e significance.  Thus, in general, as the leaves became more  horizontal there was a decrease in the amount of light energy required to produce 50;o of the maximum rate of net photosynthesis.  12  r  • I  • 2  •  • 3  • A  • 5 -  •  .  6 l_  Fig.  5.2.2.1.  8'  7 A  .  . 9  10  11  , 12  . 13  M  I  The r e l a t i o n s h i p b e t w e e n n e t a s s i m i l a t i o n r a t e (NAR) o m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s and L A I .  f  leaves  and o e t i o l e s  •' ^  9 r  Y = 19 5 9 6 - 1 2 1 . 1 2 4 X + 2 6 6 761 X  8  c >.  R = 0.949  o o  -c a  c o  s> o  -E  CM  i  I  t X o i  ~  5  o  o  BC  UJ Z  .08  .10  .12  .14  .18  .16  C  .20  .22  .24  .26  .28  VALUE  Ffgv 5 v 2 . 2 2 - T h e - r e - l a t i O T t o p r o d u c e 50% o f t h e maximum r a t e o f n e t p h o t o s y n t h e s i s i n m i x e d o r c h a r d g r a s s - w h i t e c l o v e r s w a r d s and t h e C v a l u e s o f the e x p o n e n t i a l e q u a t i o n s f i t t e d t o the l i g h t energy response c u r v e s . k  o  131 5.3  Discussion Although t h i s i s p r i m a r i l y a d i s c u s s i o n o f the r e s u l t s from  Section 5 i t i s useful to include r e s u l t s from S e c t i o n s  i n t h e d i s c u s s i o n , some o f t h e  3 and 4 as t h e e x p e r i m e n t s i n t h e s e  sections  were s t u d i e s w h i c h l e a d t o t h e S e c t i o n 5 e x p e r i m e n t s .  5.3.1  The  Influence  o f t h e G r o w i n g Season E n v i r o n m e n t  on Net  P h o t o s y n t h e s i s and P r o d u c t i o n The m o i s t u r e r e g i m e s o f t h e A u s t r a l i a n and Canadian"^ ( T a b l e s 3«. I I and 5*V) the r a i n f a l l  show t h a t f o r t h e spring-summer  and i r r i g a t i o n i n t h e C a n a d i a n t r i a l  the A u s t r a l i a n t r i a l by 5 to 8 i n c h e s .  trials  growing season  exceeded t h a t i n  However, when t h e d i f f e r e n c e s  i n s o i l p h y s i c a l c o n d i t i o n s b e t w e e n t h e two t r i a l s a r e t a k e n i n t o a c c o u n t , t h e A u s t r a l i a n t r i a l w i t h a heavy, c r a c k i n g c l a y probably,; h a d considerably  l e s s a v a i l a b l e moisture than the Canadian t r i a l with; a  sandy l o a m .  T h i s d i f f e r e n c e i n a v a i l a b l e m o i s t u r e b e t w e e n t h e two  t r i a l s was  a c c e n t u a t e d b y t h e i r r e g u l a r and, a t t i m e s , h e a v y r a i n i n  the A u s t r a l i a n t r i a l the r o o t i n g zone.  which o f t e n r e s u l t e d i n poor i n f i l t r a t i o n  The  into •  lower l e v e l of a v a i l a b l e moisture i n the  .,  A u s t r a l i a n t r i a l was u n d o u b t e d l y a m a j o r c a u s e o f t h e l o w e r y i e l d s . The 26°C.  summer t e m p e r a t u r e s i n t h e A u s t r a l i a n t r i a l r a n g e d f r o m 12 t o  I t seems t h e r e f o r e  t h a t d i f f e r e n c e s i n temperature would not  have b e e n a m a j o r f a c t o r i n t h e y i e l d d i f f e r e n c e s b e t w e e n t h e trials.  Viewed  i n the l i g h t of the c o n t r o l l e d environment  o r c h a r d g r a s s and w h i t e c l o v e r t h e f i e l d d i u r n a l r a n g e was  1  two  studies f o r s i m i l a r ] to-;  F o r p u r p o s e s o f c o n v e n i e n c e t h e d e f o l i a t i o n e x p e r i m e n t s w i l l be r e f e r r e d t o b y t h e c o u n t r y i n w h i c h t h e y were r u n v i z . S e c t i o n 3> A u s t r a l i a , S e c t i o n 5 Canada. . :  132 that used i n the growth c a b i n e t s .  The optimum t e m p e r a t u r e s  found f o r  t h e s e two s p e c i e s i n d i c a t e d t h a t w i t h t h e p r e v a i l i n g summer g r o w t h was p r o b a b l y n e a r o p t i m a l i n b o t h t r i a l s . d u r i n g A p r i l and May i n t h e C a n a d i a n t r i a l while w i n t e r temperatures l i m i t e d growth.  However,  temperatures  probably l i m i t e d  (0-12°C) i n t h e A u s t r a l i a n t r i a l  temperatures  growth most  likely  The n e t p h o t o s y n t h e s i s and g r o w t h s t u d i e s ( S e c t i o n ,4)  i n d i c a t e t h a t CO^ f i x a t i o n o c c u r r e d o v e r a g r e a t e r l e n g t h o f t i m e for  t o p growth i n t h e swards.  than  ;  U s i n g t h e t a b l e s p u b l i s h e d b y de W i t (1965) and t h e a c t u a l s o l a r r a d i a t i o n l e v e l s ( T a b l e 5 ") t h e p o t e n t i a l p r o d u c t i o n f o r t h e g r o w i n g <v  season o f t h e Canadian t r i a l  was c a l c u l a t e d . To u s e t h e s e t a b l e s , t o  e s t i m a t e p r o d u c t i o n , i t was assumed t h a t s t a n d g e o m e t r y was o p t i m a l and t h a t r a d i a t i o n b e t w e e n 400 and 700nm was 44$ o f t h e t o t a l s o l a r : r a d i a t i o n and f u r t h e r t h a t , g r o s s r e s p i r a t i o n was 3 3 $ f o f p h o t o s y n t h e s i s , t h e l a t t e r two f i g u r e s b e i n g t h o s e o f Loomis and W i l l i a m s (1963). > Under the environmental  c o n d i t i o n s o f the Canadian t r i a l  potential  photo-  s y n t h e s i s w o u l d be 42,650 l b . ( C H 0 ) / a c r e and b y a l l o w i n g f o r r e s p i r a t i o n 2  p o t e n t i a l p r o d u c t i o n c o u l d be 28,580 l b . ( C H 0 ) / a c r e . 2  This  estimate,  compared w i t h t h e h i g h e s t a c t u a l p r o d u c t i o n ( T a b l e 5 « H ) o f about 11,200 l b . d r y m a t t e r / a c r e p r o d u c t i o n was a c h i e v e d . cribed by S t a n h i l l  i n d i c a t e s o n l y 38$ o f t h e p o t e n t i a l The s i t u a t i o n a p p e a r s s i m i l a r t o t h a t d e s -  (1962) who c o n c l u d e d  l i g h t energy wastage. 9 inches d i d not permit l i g h t energy a v a i l a b l e .  In the present  t h a t t h e d i s c r e p a n c y was .,due t o  t r i a l , the d e f o l i a t i o n height of  a h i g h enough a v e r a g e L A I t o u t i l i z e  a l l the  T h i s , however, i s n o t c o m p l e t e l y c o n s i s t e n t  w i t h t h e r e s u l t s , a s t h e h i g h e s t p r o d u c t i o n was a c h i e v e d f r o m t h e (3-1) management.  I n t h e g r a s s swards i t was shown t h a t t h e (9-1) managed  133 p a s t u r e s had a h i g h e r r a t e o f n e t p h o t o s y n t h e s i s (Pig. for  5-2.4  A) t h a n t h o s e c u t a t 9 i n c h e s .  at c u t t i n g  This probably  height  compensated  any e x t r a l i g h t w a s t a g e b y t h e (3-1) managed swards and f r o m  field  o b s e r v a t i o n s a p p e a r e d t o be a c o n s e q u e n c e o f more h o r i z o n t a l l e a v e s and less light  5-3.2  wastage,  D r y M a t t e r P r o d u c t i o n and t h e I n f l u e n c e o f F r e q u e n c y and Intensity of D e f o l i a t i o n The d r y m a t t e r  p r o d u c t i o n o f t h e two d e f o l i a t i o n  experiments d i f f e r e d g r e a t l y . and  5 - I I the y i e l d  in Australia.  management  As c a n be s e e n b y c o m p a r i n g T a b l e  i n the t r i a l  3.IV  i n Canada was 3 t i m e s t h a t o f t h e t r i a l  However, d e s p i t e t h i s g r e a t d i f f e r e n c e i n y i e l d t h e  o r d e r o f y i e l d s f r o m t h e t h r e e managements was e s s e n t i a l l y t h e same i n both t r i a l s .  D i f f e r e n c e s b e t w e e n t h e (3-1) and ( 9 ~ l ) managements  were s l i g h t w h i l e t h e (9-3) management showed a s i g n i f i c a n t l y t o t a l production. (1959) and B r y a n t  lower  Thus t h e s e r e s u l t s agree w e l l w i t h t h o s e o f Brougham and B l a s e r (1968) i n t h a t l e a v i n g 3 i n . o f h e r b a g e  g e n e r a l l y r e s u l t s i n lower y i e l d s than a c l o s e r d e f o l i a t i o n . c o n f i r m t h e t r e n d s shown b y B r y a n t  They a l s o  and B l a s e r (1968), t h a t w i t h  managements o f t h i s t y p e b o t h c l i p p i n g and g r a z i n g g i v e t h e same order.  The r e s u l t s a l s o s u p p o r t  yield  t h e d a t a of M a t c h e s (1968) t h a t i n  s u c h t r i a l s t h e y i e l d r a n k w i l l be t h e same w i t h c l i p p i n g , o r g r a z i n g by sheep o r c a t t l e . In t h e t r i a l s  i n A u s t r a l i a t h e (9-3) management d e p r e s s e d  both  g r a s s as w e l l as c l o v e r p r o d u c t i o n w h i l e i n t h e C a n a d i a n t r i a l  only  c l o v e r p r o d u c t i o n was d e p r e s s e d .  In the A u s t r a l i a n t r i a l  p r o d u c t i o n c o u l d have a r i s e n f r o m t h e l o w c l o v e r c o n t e n t  t h e poor  grass  o f t h e sward  134and hence l o w l e v e l s o f f i x e d n i t r o g e n as d e m o n s t r a t e d b y H e r r i o t t H e l l s (i960) and C o w l i n g (1961  and 1962).  A l t e r n a t i v e l y t h e use  and  of  a n i m a l s as d e f o l i a t o r s c o u l d have c a u s e d s e l e c t i v e d e f o l i a t i o n o f t h e g r a s s component. adequate  In the Canadian  as t h e (9-3)  trial  s o i l n i t r o g e n was  apparently  managed p l o t s had t h e same y i e l d o f g r a s s h e r b a g e  as t h e p l o t s w i t h t h e ( 3 - l ) o r (9-1) managements d e s p i t e t h e l o w e r c l o v e r c o n t e n t i n the (9~3). y i e l d , u n d e r t h e (9-3)  In t h i s t r i a l  then, the lower c l o v e r  management, a p p e a r s t o have b e e n c a u s e d  c o m p e t i t i o n f o r l i g h t as d e s c r i b e d b y D o n a l d W i l k i n s o n and G r o s s  (196I).  The  by  evidence  (1964) has shown t h a t r e d u c t i o n i n l i g h t  compet-  i t i o n i s a necessary p r e r e q u i s i t e to r e - e s t a b l i s h i n g Ladino white c l o v e r i n t o o r c h a r d g r a s s swards*  of  ;  Thus i n t h e C a n a d i a n e x p e r i m e n t  the  3 i n c h mowing l e v e l a l l o w e d t h e o r c h a r d g r a s s t o shade t h e w h i t e c l o v e r and d e p r e s s i t s y i e l d . of  T h i s however, does n o t agree w i t h t h e e v i d e n c e  T r a u t n e r and G i b s o n (1966) who  f o u n d g r e a t e s t b r a n c h i n g and  i n w h i t e c l o v e r u n d e r moderate s h a d e .  flowering  Other w o r k e r s , f o r example,  e r t a l . (1966) have f o u n d w h i t e c l o v e r shade i n t o l e r a n t .  They,  a l s o f o u n d d i s e a s e and i n s e c t s r e d u c e d t h e p e r s i s t a n c e o f t h i s  plant.  Blake  The  t o t a l seasonal y i e l d s i n the Canadian  t r i a l were about  2,000 l b . / a c r e l o w e r t h a n t h o s e r e p o r t e d b y G a r d n e r  ;  e t a l . (i960) f o r  o r c h a r d g r a s s - p e r e n n i a l r y e g r a s s - w h i t e c l o v e r p a s t u r e s grown on Is.  ,  some 35 m i l e s d i s t a n t f r o m t h e p r e s e n t e x p e r i m e n t .  These  Vancouver workers  f o u n d t h a t w i t h o u t p o t a s s i u m f e r t i l i z a t i o n t h e i r p a s t u r e s showed a lower c l o v e r content than those r e c e i v i n g potassium. present experiment  However, i n . t h e 1  t h i s i s u n l i k e l y as a l l t h e managements r e c e i v e d t h e  same p o t a s s i u m f e r t i l i z e r a p p l i c a t i o n s w h i l e t h e l o w e r c l o v e r p e r c e n t age o n l y o c c u r r e d u n d e r t h e (9-3)  management.  The  ( 3 - l ) management  135 had  38$ c l o v e r , t h e (9-1) 31$ c l o v e r w h i l e t h e (9-3) had o n l y 14$  clover. In t h e p u r e o r c h a r d g r a s s was t h e (9-1) w h i l e (9~l)  swards t h e h i g h e s t y i e l d i n g  i n t h e pure w h i t e  management  c l o v e r swards b o t h t h e ( 3 - l ) and  managements gave t h e b e s t p r o d u c t i o n ( T a b l e 5 . I I I ) .  The  o b s e r v a t i o n t h a t a 3 i n c h c u t t i n g h e i g h t gave a l o w e r y i e l d 1 i n c h c u t i n the pure white Gervais  ( i 9 6 0 ) who u s e d  g r a s s swards Drake at  c l o v e r swards a g r e e s w i t h t h e r e s u l t s o f  similar cutting heights.  e t a l . (1963) f o u n d  3 inches than from  than a  W i t h pure  orchard-  g r e a t e r y i e l d s from  those c u t at 1 i n c h while Blake  f o u n d c u t t i n g h e i g h t , between -5- and 2 i n c h e s had l i t t l e  swards c u t  e t a l . (1966) i n f l u e n c e on  the o r c h a r d g r a s s y i e l d s .  Thus t h e s u p e r i o r i t y o f y i e l d s f r o m  inch c u t , i n the Canadian  experiment,  was b y no means u n i q u e .  swards c u t t o 1 i n c h t h o s e w i t h c u t t i n g i n i t i a t e d y i e l d e d l e s s than those with c u t t i n g i n i t i a t e d T h i s a g r e e s w e l l with, t h e f i n d i n g s o f Dewey  yield  at 9 inches  orchardgrass-white In  who f o u n d  that seasonal  Wolf and S m i t h (1964)  have a l s o f o u n d h i g h c u t t i n g f r e q u e n c i e s (5 t i m e s ) in  :  (9~l).  gave a l o w e r  t h a n p l a n t s c l i p p e d when 10 i n c h e s t a l l .  Of the  at 3 inches ( 3 - l )  (196I)  o r c h a r d g r a s s p l a n t s c l i p p e d when 3 i n c h e s t a l l  the 1  gave l o w e r  yields  c l o v e r swards t h a n o n l y 3 c u t s p e r s e a s o n .  t h e mixed swards i n t h e A u s t r a l i a n t r i a l  t h e (3-1) management  f a v o u r e d o r c h a r d g r a s s w h i l e t h e (9-1) f a v o u r e d c l o v e r p r o d u c t i o n . In  the Canadian  trial  t h e (3-1) management f a v o u r e d c l o v e r p r o d u c t i o n  w h i l e swards c u t when $ i n c h e s h i g h were t h e b e t t e r g r a s s  producers.  As t h e d i f f e r e n c e s between t h e managements i n g r a s s and c l o v e r y i e l d s were c o m p a r a t i v e l y s m a l l , t h e r e i s l i t t l e  significance  management showing s u p e r i o r g r a s s o r c l o v e r p r o d u c t i o n .  i n a particular However,.with  136 the Canadian t r i a l Ward  et a l .  t h e t r e n d s were s i m i l a r t o t h o s e  (1966) and b y  Reid  (1968) f o r  r e p o r t e d by-  orchardgrass-white  clover  swards. Apart  from these  c o n s i d e r a t i o n s o f component y i e l d , t h e v a l u e o f  l e a v i n g some L A I a f t e r d e f o l i a t i o n b y t h e u s e o f t h e (9-3) must be c o n s i d e r e d trial,  w i t h response t o t o t a l y i e l d .  management  In the A u s t r a l i a n  t h e y i e l d s were l o w , t h e r e f o r e when c o n s i d e r i n g L A I a n d l i g h t  i n t e r c e p t i o n i t seems u n l i k e l y t h a t t h e m a t e r i a l l e f t w i t h t h e 3 i n c h g r a z i n g h e i g h t c o n f e r r e d any advantage i n t h e form o f l i g h t i n t e r c e p t i o n . In t h e Canadian t r i a l ,  5'2.2, 5.2.10  Figs. the  (9-3)  t h e y i e l d s and L A I s were h i g h e r  and  5.2.16).  As t h e f r e q u e n c y  management was l o w e r t h a n w i t h t h e (3-1)  o f m a t e r i a l removed p e r d e f o l i a t i o n was s i m i l a r  ( T a b l e 5.II and  of defoliation  and t h e q u a n t i t y  (Table 5«H) t h e 3  i n c h s t u b b l e c l e a r l y depressed t h e growth r a t e o f the c l o v e r . result  i s c o n t r a r y t o t h a t f o u n d b y Brougham  (1956) who  This  found t h e  r e g r o w t h r a t e o f swards d e f o l i a t e d t o 3 i n c h e s was g r e a t e r t h a n defoliated to 1 inch.  (1956)  occurred  I t was o f v e r y s h o r t d u r a t i o n and h a d no i n f l u e n c e on In o t h e r words, t h e l a g phase b e f o r e  h i g h l e v e l o f l i g h t e n e r g y i n t e r c e p t i o n was n o t s h o r t e n e d swards d e f o l i a t e d a t 3 i n c h e s compared w i t h t h o s e  M i t t a r a and W r i g h t  The  (1966) who  i n the  (1964)  and  found that the basal l e a f blades,  s w a r d , were l e s s e f f i c i e n t  a  d e f o l i a t e d at 1 inch.  T h i s a g r e e s w e l l w i t h t h e f i n d i n g s o f B e g g and W r i g h t  the upper l e a f  those  I f an e f f e c t s u c h as t h a t d e s c r i b e d b y Brougham  the average r e g r o w t h r a t e .  reed canarygrass  with  i na  i n supporting,regrowth  than  blades.  p o o r r e g r o w t h f r o m t h e (9-3)  management t r e a t m e n t s  could also  have b e e n a c o n s e q u e n c e o f h i g h r a t e s o f t i s s u e s e n e s c e n c e as  137 suggested by Brougham (1962), Hunt (1965) and Hunt and Brougham (1966), They found that during active growth the accumulation of dry matter i n grass or clover swards was dependant on the rate of tissue senescence. Hunt (1965) suggested therefore that defoliation managements with close cutting initiated at maximal light interception would minimise tissue senescence.  In the present t r i a l s therefore, the (9-3)  management would be expected to have greater senescence than the (9-1)  and hence a lower y i e l d . Prom Figures 5-2.3, 11 and 17 i t i s evident that both the grass  and grass-clover swards under the ( 9 l ) and (9-3) defoliation manage_  ments attained LAIs between 10 and 17 together with up to 8,000 l b . / acre dry matter.  Thus sward LAIs were often above the optimum LAI for  t a l l fescue swards estimated by Murtagh and Gross (1966).  However i t  is quite l i k e l y that these high LAIs had l i t t l e effect on the pasture growth rate as a wholej  especially in view of the findings of Anslow  (1965) that midsummer growth i s not necessarily related to LAI and that high LAI does not always exert a depressing effect on growth. Discussion of the presence of an optimum LAI in these swards i s given in Sections 5»2.3 and 4 where i t i s considered with reference to ••the rate of C0^ assimilation. Apart from the factors discussed above, the outcome of the management treatments may also have been influenced by differences in the  :  reserve carbohydrate l e v e l .  Ward and Blaser (1961) have shown that  carbohydrate reserves and leaf area can also interact so that the whole situation i s complex.  In the Australian t r i a l s where year-.round  yields were taken, examination of the season x management interaction could have provided evidence of different carbohydrate reserves i n  138 response  t o the d e f o l i a t i o n treatments  environmental  ( T a b l e 3.V). However, as  c o n d i t i o n s were s u c h t h a t t h e y i e l d p o t e n t i a l was l o w i t  seems u n l i k e l y t h a t t h e r e s e r v e s w o u l d have b e e n i m p o r t a n t was  so s m a l l .  as regrowth  I n t h e C a n a d i a n t r i a l where t h e e n v i r o n m e n t d i c t a t e d a  s h o r t e r growing season any e f f e c t o f e x t r a r e s e r v e carbohydrate t h e (9~3) management w o u l d have o c c u r r e d i n t h e l a t e f a l l .  from  However,  d u r i n g t h i s p e r i o d no g r o w t h o c c u r r e d on any o f t h e t r e a t m e n t s .  Thus  i n g e n e r a l even i f a c t i v e g r o w t h and d e f o l i a t i o n a t 1 i n c h r e s u l t e d i n lowering of carbohydrate  l e v e l s i n t h e (3-1) and (9-1) managements, •  the environment o f b o t h t r i a l s p r o b a b l y prevented s h o w i n g any d i s a d v a n t a g e  5.3.3  t h e s e managements  compared w i t h t h e ( 9 - 3 ) •  >  The I n f l u e n c e o f F r e q u e n c y and I n t e n s i t y o f D e f o l i a t i o n on Wet P h o t o s y n t h e s i s  The  i n Forage Stands  d e f o l i a t i o n management t r e a t m e n t s  caused s i g n i f i c a n t  changes  i n t h e r e l a t i o n s h i p b e t w e e n n e t p h o t o s y n t h e s i s and L A I i n t h e o r c h a r d grass swards.  However, i n t h e w h i t e c l o v e r and mixed  orchardgrass-  w h i t e c l o v e r sward t h e r e were no d i f f e r e n c e s between t h e managements, i n t h e above r e l a t i o n s h i p . 5.2.4 A and IJ, 5*2.5  I n t h e pure o r c h a r d g r a s s  swards (Figs;,  5*2.7) t h e i n c r e a s e i n n e t p h o t o s y n t h e s i s p e r  u n i t L A I was much g r e a t e r i n (3-1) managed swards t h a n i n t h e (9-1) o r (9-3)  managements d e s p i t e t h e g r e a t e r f r e q u e n c y  (3-1).  of d e f o l i a t i o n with the  The ( 9 - l ) managed swards w i t h a l i n e a r r e l a t i o n b e t w e e n n e t  photosynthesis permitted the y i e l d to continue t o increase with and L A I even w i t h t h e most i n f r e q u e n t d e f o l i a t i o n . The was  o n l y sward t o show an optimum L A I a t s a t u r a t i n g l i g h t  time i levels  t h e sward w i t h t h e (9-3) d e f o l i a t i o n management w h i c h i n d i c a t e d t h a t  139 the  3 inch stubble l e f t  Donald  (I96I)  It  not  was  and  particularly  expected  (9-1)  However, t h e f a c t  management even up  f i n d i n g s o f Anslow was  not r e l a t e d  the  (9-1)  (1966) t h a t  management would  t o L A I 17  no  optimum  gives confidence to  midsummer growth r a t e s i n g r a s s At l o w e r  light  levels  T h i s wae  t h e r e was  c o n s i s t e n t w i t h the t h e r e was  no optimum L A I w h i l e  an optimum L A I where t h e l o w e r  the f i n d i n g s o f Brougham  the optimum  (9-3)  levels  (1958) t h a t  initial  slope of the l i g h t response  optimum L A I a t a l l l i g h t l e v e l s s y n t h e s i s from  l e a v e s were i n f a c t  Wright  fact  L A I except  there i s a  LAI further  diurnal.change  curve v s . L A I f o r the  gained  a b s o l u t e l y no  "parasitic".  (1964) and leaves  (9~3)  managed  e_t a l .  (1966) w h i c h  an photo-  initial  that these  to the reed canarygrass  Brown  lower  orchardgrass swards o f . were shown t o  "parasitic".  t h a t the white  a t v e r y low  These  e x t r a net  Thus the  also provides f u r t h e r evidence  swards were t h e r e f o r e s i m i l a r  The  o f an optimum  swards as w e l l as h a v i n g  the 3 i n c h e s o f s t u b b l e l e f t .  slope vs.LAI curve  have the l o w e r  ,  LAI.  managed swards shows t h a t t h e s e  Begg and  as  l e a v e s c o u l d be  i s i n agreement w i t h , and  at low l i g h t e n e r g y  effect  at lower  appearing  The  swards  "optimum L A I t h e o r y "  The  in  the  intensities  below t h e l i g h t c o m p e n s a t i o n p o i n t .  confirms  LAI  managed swards d i d show an optimum L A I f o r maximum net  saturating light levels  light  (3-1)  t h a t t h e r e was  t o , o r dependant on L A I .  photosynthesis. at  i n the l a t t e r p h a s e s o f t h e L A I i n c r e a s e .  t h a t t h e swards w i t h the  show an optimum L A I . w i t h the  a c t e d as " p a r a s i t i c " m a t e r i a l as d e s c r i b e d by-  levels  c l o v e r swards d i d n o t show an c o u p l e d w i t h the f a c t  d i f f e r e n c e s between t h e managements i s e v i d e n c e  optimum  t h a t t h e r e were  of d i f f e r e n c e s  no  i n the  140 behaviour  of t h e s e  these c l o v e r  swards compared w i t h the p u r e g r a s s swards.  swards L A I s  above 6 were not  i n d i c a t e d t h a t the c a n o p y became s t a b l e l e v e l b y new (9 3)  the  growth and  leaf  and was  senescence.  The  management c o n s i s t e d o f a l m o s t  -  senescenced  r a p i d l y as new  o b t a i n e d , and  regrowth  17  and where s e n e s c e n c e  lower  yield  maintained  occurred.  (9-3)  at  stumps w h i c h  continued to  increase to  much s l o w e r . t o be  consequence o f c o m p e t i t i o n w i t h o t h e r s p e c i e s w h i c h i n v a d e d  The no  fact  i n net  t h a t t h e mixed o r c h a r d g r a s s - w h i t e  behaviour  by  (9-1)  (9-3)  the  however, n o t  the c l o v e r  i n the  entirely (9-3)  clover  apparent.  swards showed  a consequence o f t h e  above  swards compared w i t h the  The  The  results clearly  o v e r a l l optimum L A I a t s a t u r a t i n g l i g h t the e v i d e n c e  o f " p a r a s i t i c " l e a v e s was  :  these', p l o t s  (3-1)  and  reason f o r  t h i s l a c k o f d i f f e r e n c e between t h e managements i n e s s e n t i a l l y not  a  photosynthesis-LAI  swards c o n t a i n e d t h e l e a s t c l o v e r .  dominant swards was  The •  photosynthesis.-,  d i f f e r e n c e s betxreen the managements i n t h e n e t  r e l a t i o n s h i p was,  with  This behaviour s t r o n g l y  management appeared  i n t h i s management r a t h e r t h a n d i f f e r e n c e s  this  3 inch stubble l e f t  i n t h e 3 i n c h s t u b b l e was  o f c l o v e r w i t h the  observations  entirely petiole  c o n t r a s t e d t o the g r a s s swards where the L A I s  In  grass  indicated  an  l e v e l s f o r a l l swards so t h a t not c o n f i n e d t o the  (9-3)  managed swards.  5.3.4  The  Influence of B o t a n i c a l Composition  Photosynthesis The white  o f the F o r a g e  on  Wet  Stand  net p h o t o s y n t h e s i s - L A I response  o f t h e mixed  c l o v e r swards showed t h a t the optimum L A I was  between t h e L A I s  from  orchardgrass-  intermediate  swards where the s p e c i e s were grown a l o n e .  At  141 24.0  cal./cm  14.2  and  / h r the o r c h a r d g r a s s  the white  clover  optimum f o r m i x t u r e s  swards an optimum L A I o f 7*0,  o f t h e two  s p e c i e s was  optimum might have b e e n e x p e c t e d c o n t e n t was that  o n l y 33$  on  swards showed an optimum L A I  10.0.  a dry matter b a s i s .  i n t h e mixed swards t h e w h i t e  s u r v i v e were l o n g e r t h a n t h o s e  while  the  A slightly  f o r the mixture  of  higher  as t h e mean c l o v e r  However, i t was  observed  c l o v e r p e t i o l e s w h i c h were a b l e t o  o f the c l o v e r  i n pure swards.  Hence  the  c l o v e r l e a v e s i n the mixed swards were l o c a t e d a t t h e t o p o f t h e  canopy  in. a f a v o u r a b l e p o s i t i o n f o r c a r b o n  even  though t h e c l o v e r p r o p o r t i o n was the c l o v e r l e a v e s r e s u l t e d  dioxide assimilation.  o n l y l/3  Thus,  b y w e i g h t the p o s i t i o n i n g  of  i n an optimum L A I c l o s e r t o t h a t o f the  pure  c l o v e r swards t h a n t h e p u r e g r a s s . In t h e pure o r c h a r d g r a s s  and  white  c l o v e r forage stands,  a  light  2 energy  level  o f 24.0  cal./cm  /hr.  i n swards w i t h an o p t i m a l  LAI,  " * resulted The  2 i n net p h o t o s y n t h e s i s l e v e l s  mixed sward o f t h e s e two  difference  5.2.12  species d i d not,  i n net p h o t o s y n t h e s i s from  B and  5.2.19)•  o f between 3.0  Little  literature  white  (1958)  c l o v e r had  found  3.5g  as e x p e c t e d ,  the above r a n g e ;  n e t p h o t o s y n t h e s i s o f m i x i n g a g r a s s and Brougham  and  COg/m / h r .  show  (Pigs.  any  5*2.6,  i s a v a i l a b l e on t h e e f f e c t legume s p e c i e s i n a sward.  t h a t mixed swards o f s h o r t - r o t a t i o n r y e g r a s s  intermediate l i g h t  Pearce  e_t a l .  (1967)  have  shown t h a t t h e optimum L A I o f a p u r e s t a n d 'of b a r l e y d e c r e a s e d d a t a from  The  consistent with both  o b s e r v a t i o n s i n t h a t mixed swards had  t h e y had  an  i n t e r m e d i a t e mean l e a f  the present  as  l e a v e s became h o r i z o n t a l .  i n t e r m e d i a t e optimum L A I f o r c a r b o n  and  i n t e r c e p t i o n p r o p e r t i e s compared  w i t h pure swards o f t h e same s p e c i e s .  these  on  experiment  dioxide assimilation; angle  and  light  the  was an  presumably  interception  compared  142 with the swards where the species were grown alone.  The net photo-  synthesis values for orchardgrass and white clover when grown alone were similar to those obtained by Pearce  est a l .  (19^5) **d a  Wilfong  et_ a l .  (1967) for these  5.3•<5  An Assessment of the Efficiency of Light Energy Use by  species.  the Forage Associations The highest efficiencies of light energy use recorded for orchard-  8.35$j  grass and white clover in pure and mixed stands were 7.847b  respectively; with optimum LAIs of  !•  65$  and  13.0, 6.0 and 8.0. In a l l  cases Mie light energy level which gave these efficiencies was comparatively low, v i z .  6.0  cal./gm  /hr. Brougham  (1958) has  shown  that with similar light and LAI levels, white clover swards were more efficient "interceptors of l i g h t " than short-rotation ryegrass swards. The data presented i n this thesis clearly supports Brougham's findings as the optimum LAI was lowest i n the pure white clover swards.  However,  when the orchardgrass swards reached their optimum they, i n terms, of net photosynthesis, did make more efficient use of the light energy available; the efficiency of light energy use was 0.7$ that i n the white clover swards.  greater than  The more v e r t i c a l distribution of  the leaves i n the orchardgrass swards was most l i k e l y the factor responsible f o r the increase in efficiency.  However, other factors  such as leaf age which affects the level of net photosynthesis (See Brown  et_ a l . 1966) undoubtedly influenced the overall response.  The efficiencies of light use compare favourably with those r e ported by other workers.  The efficiency of  higher than that reported by Yocum  e_t a l .  8.35$ f °  r  orchardgrass i s  (1954) for a  maize crop  143  w h i c h was 5 . 1 $ . "average"  However, t h e i r e f f i c i e n c y was e s t i m a t e d o v e r an  d a y so t h a t no doubt d u r i n g t h e l o w e r l i g h t  o f t h e d a y t h e e f f i c i e n c y w o u l d have b e e n h i g h e r . of carbon  intensity periods  Allowing for a loss  d i o x i d e b y n i g h t r e s p i r a t i o n o f 33$ t h e 8 . 3 5 $ e f f i c i e n c y  f o r n e t p h o t o s y n t h e s i s becomes 5 * 5 7 $ e f f i c i e n c y f o r n e t d a i l y  carbon  dioxide assimilation  of.5•57$  i n the orchardgrass  swards.  i s lower than that o f 7 . 9 $ r e p o r t e d by Bray p r o d u c t i o n i n a P i c e a omorika p l a n t a t i o n .  This figure  f o r d r y matter  (1961)  In t h e i r estimate of  maximum c r o p p r o d u c t i v i t y Loomis and W i l l i a m s ( 1 9 6 3 )  suggested  o v e r a l l e f f i c i e n c y f o r d r y matter production o f 1 2 . 0 $ . r e p o r t e d i n t h i s t h e s i s do n o t a p p r o a c h t h i s f i g u r e .  an  The e f f i c i e n c i e s However, t h e y a r e  i n t h e same r e g i o n as t h o s e o f o t h e r w o r k e r s u s i n g f o r a g e s w a r d s . and C o o p e r ( 1 9 6 7 ) f e s c u e swards k e p t  found  a 6.0$ e f f i c i e n c y o f n e t a s s i m i l a t i o n  at 90$ l i g h t  in-.tall  interception.  A l l o w i n g f o r a 33$ n i g h t r e s p i r a t i o n l o s s t h e f i g u r e l i g h t use f o r w h i t e c l o v e r (1964)  found  Hunt  i n t h e present experiment  w o u l d be 5 - 3 2 $ .  energy Black  a maximal s e a s o n a l n e t a s s i m i l a t i o n e f f i c i e n c y o f 4 . 2 $ •  in subterranean  clover.  K i n g and E v a n s ( 1 9 6 7 )  e f f i c i e n c i e s of l i g h t use f o r subterranean exchange d a t a , and m a k i n g an a l l o w a n c e  have  clover  calculated  from carbon  dioxide  o f 33$ f o r n i g h t r e s p i r a t i o n ,  t h e i r e f f i c i e n c y l i g h t u s e i n COg n e t a s s i m i l a t i o n was 6 . 5 $ f o r subterranean experiments  clover. indicate  C l e a r l y t h e d a t a o f t h i s and o t h e r p u b l i s h e d t h a t t h e r e i s c o n s i d e r a b l e room f o r i n c r e a s e d .  e f f i c i e n c y of l i g h t use i n forage  stands. o  Taking a figure  P  o f 4.0gC0 /m / h r , a t 4 0 c a l . / c m / h r . o f 4 0 0 - 7 0 0 n m 2  r a d i a t i o n , as t h e mean maximum m e a s u r e d n e t p h o t o s y n t h e s i s f o r t h e orchardgrass, white c l o v e r  and m i x e d swards a t t h e i r optimum L A I , a  144 c o m p a r i s o n c a n be made o f t h i s v a l u e w i t h e s t i m a t e s o f p o t e n t i a l p h o t o s y n t h e s i s f o r t h e V a n c o u v e r r e g i o n from The  t h e t a b l e s o f de Wit ( 1 9 6 5 ) .  d a i l y mean n e t p h o t o s y n t h e s i s f o r V a n c o u v e r , a t a p p r o x i m a t e l y  latitude  5 0 , r e a d o f f from  405 cal./cm  de W i t ' s  t a b l e s f o r a c l e a r day, w i t h ;  2 2 /day o f 4 0 0 - 7 0 0 n m r a d i a t i o n , would be 6.1gC0 /m / h r . 2  the mid-day r a t e s c o u l d be d o u b l e  t h e d a i l y mean, t h e measured  o f 4.0gC0 /m / h r c o n f i r m s t h e e a r l i e r 2  m a r g i n f o r improvement  i n carbon  suggestion that there  dioxide  assimilation.  As  level  i s a.wide  145  6.  SUMMARY AND CONCLUSIONS  6.1  The F i e l d E x p e r i m e n t An e x p e r i m e n t  w i t h D e f o l i a t i o n by A n i m a l s  i s r e p o r t e d i n which the e f f e c t s o f three g r a z i n g  managements, d i f f e r i n g i n f r e q u e n c y and i n t e n s i t y , on t h e p r o d u c t i v i t y of  s e v e n g r a s s - c l o v e r p a s t u r e s w a r d s were e x a m i n e d .  The g r a s s - c l o v e r  swards s t u d i e d c o n t a i n e d one o f t h e f o l l o w i n g f o r a g e g r a s s e s a s t h e i r m a j o r g r a s s components  perennial ryegrass, A r i k i ryegrass, Grasslands  C o c k s f o o t , S143 C o c k s f o o t , S26 C o c k s f o o t , A l t a T a l l f e s c u e and S 1 7 0 j T a l l fescue.  H e r b a g e , b o t a n i c a l c o m p o s i t i o n , L A I and d r y m a t t e r i .  y i e l d were measured a t g r a z i n g .  The r e l a t i o n s h i p b e t w e e n d r y m a t t e r  y i e l d and L A I was e x a m i n e d b y r e g r e s s i o n a n a l y s i s . The  F e s t u c a a r u n d i n a o e a p a s t u r e s showed t h e g r e a t e s t a n n u a l i  herbage p r o d u c t i o n . appeared  ;  I n t h e two y e a r s o f t h i s e x p e r i m e n t  moisture  t o be a p r i m e l i m i t i n g f a c t o r f o r p a s t u r e g r o w t h .  c o n d i t i o n s g r a z i n g managements w h i c h ground l e v e l those which  Under t h e s e  involved intensive grazing to  ( 3 - l ) and ( 9 - l ) showed g r e a t e r h e r b a g e p r o d u c t i o n t h a n left  some h e r b a g e ( 9 - 3 ) .  This effect applied to a l l t h e ;  g r a s s - c l o v e r m i x t u r e s examined. The of  stress  :  r e s u l t s showed t h a t , u n d e r c o n d i t i o n s o f l o w r a i n f a l l ,  t h e p a s t u r e d i d n o t exceed  4 . 0 and hence l i g h t  ;  the L A I  interception d i d  146 l i m i t p l a n t growth.  The p o s s i b l e r e a s o n s why  t h e (9-3)  g r a z i n g manage-  ment d i d n o t show s u p e r i o r p r o d u c t i o n t o t h o s e managements b a s e d t o t a l herbage consumption  on  are d i s c u s s e d .  The r e g r e s s i o n s o f L A I and d r y m a t t e r showed s i g n i f i c a n t d i f f e r e n c e s between t h e g r a s s s p e c i e s i n t h e w e i g h t s a r e a r a t i o  but  no d i f f e r e n c e s as a c o n s e q u e n c e o f t h e p a r t i c u l a r g r a z i n g managements. 6.2  C o n t r o l l e d Environment In  Studies  S e c t i o n 4 o f t h e t h e s i s t h e i n f l u e n c e o f d i f f e r i n g day temp-  e r a t u r e s was  s t u d i e d on t h e g r o w t h and n e t p h o t o s y n t h e s i s o f o r c h a r d -  g r a s s and L a d i n o w h i t e c l o v e r . w i t h t h e n i g h t t e m p e r a t u r e s 10°C  C r o w t h was  s t u d i e d f r o m 22°C t o 34°C,  l o w e r ; w h i l e n e t p h o t o s y n t h e s i s was  measured f r o m 15 t o 30°C a t a number o f v a p o u r p r e s s u r e d e f i c i t s . ! L i g h t , m o i s t u r e and n u t r i e n t s were k e p t a t a c o n s t a n t and h i g h l e v e l . Growth i n o r c h a r d g r a s s as measured b y t i l l e r w e i g h t o r a r e a , , r e a c h e d a maximum a t a 29°C day t e m p e r a t u r e ; w e i g h t p e r l e a f and a r e a p e r l e a f were a l s o m a x i m a l at- t h i s t e m p e r a t u r e . of  Weight p e r u n i t  area  l e a f was maximal b e t w e e n 24 and 29°C w i t h no c l e a r optimum  t e m p e r a t u r e , however, above o r b e l o w t h e s e t e m p e r a t u r e s t h e l e a v e s were l i g h t e r p e r u n i t a r e a .  The number o f l e a v e s p e r t i l l e r was  u n a f f e c t e d between 24 and 34°C b u t a t 22°C t h e number was  <•  k  significantly  reduced. The optimum t e m p e r a t u r e f o r n e t p h o t o s y n t h e s i s i n o r c h a r d g r a s s was o u t s i d e t h e range o f t h e e x p e r i m e n t u p t a k e g r e a t e s t a t 15°C  (15-30°C) w i t h n e t c a r b o n d i o x i d e  and d e c l i n i n g l i n e a r l y above t h i s  temperature.  I n c r e a s i n g v a p o u r p r e s s u r e d e f i c i t from. 5 t o 20 mm Hg i n c r e a s e d n e t p h o t o s y n t h e s i s i n o r c h a r d g r a s s , however, t h e i n c r e a s e was  slight  and  147 variable. In L a d i n o w h i t e c l o v e r t h e i n f l u e n c e o f d a y t e m p e r a t u r e s , 22 and 34°C, on v e g e t a t i v e g r o w t h was s l i g h t . at  However, p l a n t s grown  29°C h a d s i g n i f i c a n t l y g r e a t e r w e i g h t s and a r e a s .  l e a v e s p e r r o o t e d node was maximal a t 29°C. were g r e a t e s t w i t h d a y t e m p e r a t u r e s  Weight and a r e a p e r l e a f  F o r L a d i n o w h i t e c l o v e r optimum t e m p e r a t u r e  The w e i g h t p e r  f r o m 22 t o 29°C  u n i t l e a f a r e a was u n a f f e c t e d b y d a y t e m p e r a t u r e s  a p p r o x i m a t e l y 20°C.  The number o f  between 24 t o 29°C.  o c c u r r e d w i t h i n t h e range o f t h e experiment  between  f o r net photosynthesis  and i t was maximal a t  However, t h e r e was n o t a s h a r p optimum temp-  e r a t u r e and t h e r e were o n l y s l i g h t d i f f e r e n c e s i n n e t c a r b o n d i o x i d e a s s i m i l a t i o n b e t w e e n 15 a n d 25°C.  Increased vapour pressure  deficit  a l s o appeared t o i n c r e a s e carbon d i o x i d e a s s i m i l a t i o n i n white c l o v e r o v e r t h e r a n g e 5 t o 20 mm Hg, however, as w i t h o r c h a r d g r a s s was 6.3  variability  high. H e t P h o t o s y n t h e s i s and D r y M a t t e r P r o d u c t i o n i n t h e F i e l d The  e x p e r i m e n t a l work f o r t h i s phase o f t h e t h e s i s i s i n two  p a r t s ; f i r s t a f i e l d t r i a l w i t h measurement o f f o r a g e y i e l d u n d e r a number o f d e f o l i a t i o n t r e a t m e n t s , and s e c o n d ,  concurrent laboratory-  d e t e r m i n a t i o n o f n e t p h o t o s y n t h e s i s i n m a t e r i a l from t h e f i e l d t r i a l . In t h e f i e l d t r i a l o r c h a r d g r a s s , w h i t e c l o v e r and o r c h a r d g r a s s w h i t e c l o v e r f o r a g e s t a n d s were s u b j e c t e d t o t h r e e  defoliation  managements w h i c h d i f f e r e d b o t h i n f r e q u e n c y and i n t e n s i t y o f defoliation.  Over a c o m p l e t e g r o w i n g  ,  s e a s o n t h e managements w i t h ,  d e f o l i a t i o n t o w i t h i n 1 i n c h o f t h e g r o u n d gave t h e g r e a t e s t y i e l d o f herbage d r y matter.  The f r e q u e n c y w i t h w h i c h t h e s e f o r a g e s t a n d s were  148 d e f o l i a t e d had l i t t l e The (at  i n f l u e n c e on t h e h e r b a g e  d e f o l i a t i o n management w i t h t h e l e a s t  yield. intensive  defoliation  3 i n c h e s ) showed t h e l o w e s t y i e l d w i t h t h e e f f e c t b e i n g m a i n l y  t h e c l o v e r component. when g r o w i n g  The  on  y i e l d s o f t h e o r c h a r d g r a s s and w h i t e c l o v e r  as p u r e s t a n d s a l s o c o n f i r m e d t h e above c o n t e n t i o n t h a t  d e f o l i a t i o n a t 3 i n c h e s had  a depressive e f f e c t through the c l o v e r  component.  !.  W i t h t h e l a b o r a t o r y measurements o f n e t p h o t o s y n t h e s i s o f m a t e r i a l taken from the f i e l d  trial  i t was  p o s s i b l e t o examine t h e  relationship  between L A I and c a r b o n d i o x i d e u p t a k e b y t h e f o r a g e s t a n d s a t a number of  l i g h t energy  levels.  W i t h i n t h e o r c h a r d g r a s s p a s t u r e s t h e r e were d i f f e r e n c e s i n t h e net p h o t o s y n t h e s i s - L A I response The  due  t o t h e d e f o l i a t i o n managements.  o r c h a r d g r a s s swards d e f o l i a t e d t o w i t h i n 1 i n c h o f t h e g r o u n d  showed a l i n e a r r e l a t i o n between n e t p h o t o s y n t h e s i s and L A I when a t s a t u r a t i n g l i g h t energy l e v e l s .  T h i s l i n e a r r e l a t i o n s h i p was  as h i g h as L A I 17 w i t h no e v i d e n c e  o f an optimum L A I .  The  found  frequency  w i t h w h i c h t h e s e swards were d e f o l i a t e d i n f l u e n c e d t h e s l o p e o f t h e net p h o t o s y n t h e s i s - L A I response  so t h a t t h e most f r e q u e n t l y d e f o l i a t e d  swards a t L A I 6 f i x e d as much c a r b o n d i o x i d e as t h e l e s s f r e q u e n t l y d e f o l i a t e d swards a t L A I 17. t h e s e two managements was,  F a i l u r e t o show y i e l d d i f f e r e n c e s b e t w e e n  t h e r e f o r e , a t r u e r e f l e c t i o n o f the  p h o t o s y n t h e s i s c a p a c i t i e s of the  swards.  In t h e o r c h a r d g r a s s s w a r d s , d e f o l i a t e d a t 3 i n c h e s and a l l o w e d t o r e g r o w t o 9 i n c h e s , t h e r e was about 1 2 .  This suggested  c l e a r evidence  o f an optimum L A I a t  t h a t t h e management p r a c t i c e o f  lenient  d e f o l i a t i o n l e f t h e r b a g e w h i c h b e c a m e " p a r a s i t i c " a s f a r as t h e  overall  149 c a r b o n d i o x i d e u p t a k e s o f t h e sward was c o n c e r n e d .  Over t h e i n i t i a l .  p o r t i o n s o f t h e l i g h t r e s p o n s e c u r v e s t h e r e were d i f f e r e n c e s b e t w e e n t h e managements i n t h e n e t p h o t o s y n t h e s i s - L A I r e s p o n s e s o f t h e o r c h a r d g r a s s swards.  At t h e s e l o w e r l i g h t e n e r g y l e v e l s t h e l e a s t f r e q u e n t and  i n t e n s i v e d e f o l i a t i o n (9 i n c h e s d e f o l i a t e d t o 1 i n c h ) a l s o showed an optimum L A I c l o s e t o t h a t o f t h e management with height5  the 3 i n c h  i n t h i s c a s e t h e optimum L A I s were about 14.  3 i n c h d e f o l i a t i o n t r e a t m e n t s was  defoliation  The c u r v e f o r t h e  d i s p l a c e d t o the r i g h t  and l o w e r t h a n  t h a t f o r t h e 1 i n c h d e f o l i a t i o n g i v i n g f u r t h e r e v i d e n c e t h a t some o f the  l e a v e s i n the 3 i n c h d e f o l i a t e d p l a n t s w e r e p a r a s i t i c ' i , r  At t h e optimum L A I t h e e f f i c i e n c y o f l i g h t e n e r g y u s e b y t h e o r c h a r d g r a s s swards v a r i e d f r o m 3.9$ l e v e l s t o 8.4$ The  at near s a t u r a t i o n l i g h t energy  at low l i g h t energy l e v e l s .  >,. •>  optimum L A I f o r t h e L a d i n o w h i t e c l o v e r swards was n o t c l e a r l y  d e f i n e d b y t h e e x p e r i m e n t s b u t a p p e a r e d t o be  i n t h e r e g i o n o f 4-6.  t h i s r e g i o n t h e e f f i c i e n c y o f l i g h t e n e r g y u s e r a n g e d f r o m 3.5$ s a t u r a t i n g l i g h t e n e r g y l e v e l s t o 7.5$  In  at near  at low l i g h t energy l e v e l s .  There were n o t any d e t e c t a b l e d i f f e r e n c e s b e t w e e n t h e r e s p o n s e s o f • net  p h o t o s y n t h e s i s - L A I u n d e r t h e d i f f e r i n g d e f o l i a t i o n managements. In t h e swards c o n s i s t i n g o f a m i x t u r e o f o r c h a r d g r a s s and w h i t e  c l o v e r t h e optimum L A I was c l e a r l y d e f i n e d and was t h o s e o f t h e swards o f t h e s p e c i e s a l o n e .  i n t e r m e d i a t e between  W i t h t h e optimum L A I o f  about 10 t h e e f f i c i e n c i e s o f l i g h t e n e r g y u s e were s i m i l a r t o t h o s e f o r t h e w h i t e c l o v e r swards b e i n g 7.3$ and 3.6$  at low l e v e l s of l i g h t energy  at near s a t u r a t i n g l i g h t energy l e v e l s .  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Ecological p a r a m e t e r s o f an a l f a l f a community u n d e r f i e l d c o n d i t i o n s . Crop S c i . , 4^577-580. S p r a g u e , V. G., 1 9 4 3 . The e f f e c t s o f t e m p e r a t u r e and d a y l e n g t h on , s e e d l i n g emergence and e a r l y g r o w t h o f s e v e r a l p a s t u r e s p e c i e s . S o i l S c i . Soc. Am. P r o c , 8 : 2 8 7 - 2 9 4 . S t a n h i l l , G., 1 9 6 2 . The e f f e c t o f e n v i r o n m e n t a l f a c t o r s on t h e growth of a l f a l f a i n the f i e l d . Neth. J . Agr. S c i . , 1 9 : 2 4 7 - 2 5 3 . S t e p h e n s , C. G., I 9 6 2 . A manual o f A u s t r a l i a n A u s t r a l i a CSIRO. , M e l b o u r n e P u b l i s h e r s .  soils. ,;  S t e r n , W. R. and D o n a l d , C. M., 1 9 6 1 . R e l a t i o n s h i p of r a d i a t i o n , l e a f a r e a i n d e x and c r o p g r o w t h r a t e . N a t u r e , l 8 9 s 5 9 7 ~ 5 9 8 . S t e r n , W. R. and D o n a l d , C M . , 1962. Light relationships i n g r a s s - c l o v e r swards. A u s t r a l i a n J . Agr. Res., 1 3 s 5 9 9 ~ 6 l 4 . S u l l i v a n , J . T. and S p r a g u e , V. G., 1 9 4 3 . Composition of the r o o t s and s t v i b b l e o f p e r e n n i a l r y e g r a s s f o l l o w i n g p a r t i a l d e f o l i a t i o n to 1 . 5 i n . P l a n t P h y s i o l . , I 8 5 6 5 6 - 6 7 O . S u l l i v a n , J . T. and S p r a g u e , V. G., 1 9 5 3 . Reserve carbohydrate i n o r c h a r d g r a s s c u t f o r hay. P l a n t P h y s i o l . 28s304-313. Sweet, G. B. and W a r e i n g , P. P., . regulating photosynthesis.  1966. Role of p l a n t growth Nature, 210s77~79.  T a k a k u r a , T., 1 9 6 6 . The e f f e c t o f room v e n t i l a t i o n on n e t synthesis rate. B o t a n . Mag. ( T o k y o ) , 7 9 1 4 3 - 1 5 1 . s  in c  photo-  165  T a k e d a , T. and A g a t a , W., 1 9 6 6 . A n a l y s i s of the growth of forage c r o p s . 5 . E f f e c t o f r e p e a t e d t r e a t m e n t o f " c u t and r e g r o w t h " on t h e growth responses of Ladino c l o v e r under v a r i o u s temperatures. P r o c . C r o p S c i . Soc. J a p a n , 34s28l-286. T r a u t n e r , J . L. and G i b s o n , P. B., 1 9 6 6 . Fate of white c l o v e r a x i l l a r y buds a t f i v e i n t e n s i t i e s o f s u n l i g h t . Agron. J . , 5 8 5 5 7 — 5 5 8 . s  Tsuno, Y. and F u j i s e , K., 1 9 6 5 . S t u d i e s on t h e d r y m a t t e r p r o d u c t i o n o f t h e sweet p o t a t o . V I I I . The i n t e r n a l f a c t o r s i n f l u e n c e on p h o t o s y n t h e t i c a c t i v i t y o f t h e sweet p o t a t o l e a f . Proc. Crop S c i . Soc. J a p a n , 3 3 . 2 3 0 - 2 3 5 . Tsuno, Y. and P u j i s e , K., 1 9 6 5 S t u d i e s on t h e d r y m a t t e r p r o d u c t i o n o f sweet p o t a t o . B u l l . N a t . I n s t , Agr. S c i . ( J a p a n ) , S e r i e s D, No.13.1-131. 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V i c e n t e - C h a n d l e r , J . , S i l v a , S. and F i g a r e l l a , J . , 1 9 5 9 ° Effects o f n i t r o g e n f e r t i l i z a t i o n and f r e q u e n c y o f c u t t i n g on t h e y i e l d and c o m p o s i t i o n o f N a p i e r g r a s s i n P u e r t o R i c o . J . A g r . U n i v . Puerto Rico, 433215-227. a  V i c e n t e - C h a n d l e r , J . , S i l v a , S. and F i g a r e l l a , J . , 1 9 5 9 b . Effects o f n i t r o g e n f e r t i l i z a t i o n and f r e q u e n c y o f c u t t i n g on t h e y i e l d and c o m p o s i t i o n o f G u i n e a g r a s s i n P u e r t o R i c o . J . A g r . U n i v . Puerto R i c o , 43s228-239. V i c e n t e - C h a n d l e r , J . , S i l v a , S. and F i g a r e l l a , J . , 1 9 5 9 Effects o f n i t r o g e n f e r t i l i z a t i o n and f r e q u e n c y o f c u t t i n g on the y i e l d and c o m p o s i t i o n o f P a r a g r a s s i n P u e r t o R i c o . J . A/;r. U n i v . Puerto R i c o , 4 3 . 2 4 0 - 2 4 8 . c  W a l k l e y , A. and B l a c k , I . A., 1 9 3 4 . An e x a m i n a t i o n o f t h e D e j t j a r e f f method f o r d e t e r m i n i n g s o i l o r g a n i c m a t t e r and a p r o p o s e d m o d i f i c a t i o n o f t h e c h r o m i c a c i d t i t r a t i o n method. S o i l S c i . , 37s29-38.  •  Ward, C. Y. and B l a s e r , R. E., 1 9 6 1 . Carbohydrate l e a f area i n regrowth of orchardgrass. Crop  i  f o o d r e s e r v e s and S c i . , Is366-37Q.  166  Ward, C. Y., J h o n e s , J . N., L i l l a r d , J . H., Moody, J . E., Brown, R. and B l a s e r , R. E., 1 9 6 6 . E f f e c t o f i r r i g a t i o n and c u t t i n g management on y i e l d and b o t a n i c a l c o m p o s i t i o n o f s e l e c t e d legume-grass m i x t u r e s . Agron. J . , 5 8 * 1 8 1 - 1 8 4 .  H.  Waren W i l s o n , J . , 1 9 6 5 S t a n d s t r u c t u r e and l i g h t p e n e t r a t i o n . 1. A n a l y s i s by p o i n t quadrats. J . A p p l . E c o l o g y , 2s383—3^0• Watson, D. J . , 1 9 4 7 C o m p a r a t i v e p h y s i o l o g i c a l s t u d i e s on t h e g r o w t h of f i e l d crops. 1. V a r i a t i o n i n n e t a s s i m i l a t i o n r a t e and l e a f a r e a b e t w e e n s p e c i e s and v a r i e t i e s and w i t h i n and b e t w e e n y e a r s . Ann. B o t a n y ( L o n d o n ) , NS l l s 4 7 ~ 7 6 . Watson, D. J . , 1 9 5 8 . The dependence o f n e t a s s i m i l a t i o n r a t e on l e a f a r e a i n d e x . Ann. B o t a n y ( L o n d o n ) , NS 22337—55* •< Watson, D. J . and F r e n c h , S. A. N., 1 9 6 2 . An a t t e m p t t o i n c r e a s e y i e l d b y c o n t r o l l i n g l e a f a r e a i n d e x . Ann. A p p l . B i o l . , 5 0 s l - 1 0 . Weinmann, H., 1 9 6 1 , Total a v a i l a b l e carbohydrates legumes. Herbage A b s t r . , 3 1 * 2 5 5 2 6 l .  i n grasses  and  -  W i l f o n g , R. T., Brown, R. H. and B l a s e r , R. E., 1 9 6 7 . Relationships b e t w e e n l e a f a r e a i n d e x and a p p a r e n t p h o t o s y n t h e s i s i n a l f a l f a (Medicago s a t i r a L.) and L a d i n o c l o v e r ( T r i f o l i u m r e p e n s L.}*. Crop Sci.', 7 * 2 7 - 3 0 . " • ;  W i l k i n s o n , G. R. and G r o s s , C. F., 1 9 6 4 . Competition f o r l i g h t , [ s o i l m o i s t u r e and n u t r i e n t s d u r i n g L a d i n o c l o v e r e s t a b l i s h m e n t . Agron. J . , 5 6 s 3 8 9 ~ 3 9 2 . W i l l i a m s , R. F., E v a n s , L. T„ and L u d w i g , L. J . , 1 9 6 4 . Estimation o f l e a f a r e a f o r c l o v e r and l u c e r n e . A u s t r a l i a n J . A g r . R e s . , 15s213-215. W i l l i a m s , W. A., L o o m i s , R. S. and L e p l e y , R. C., 1 9 6 5 Vegetative g r o w t h o f c o r n as a f f e c t e d b y p o p u l a t i o n d e n s i t y . I I . Components o f g r o w t h , n e t a s s i m i l a t i o n r a t e and l e a f a r e a i n d e x . Crop . S c i . , 5s215-219.  W i l l o u g h b y , W. M., 1 9 5 4 . Some f a c t o r s a f f e c t i n g g r a s s - c l o v e r relationships. A u s t r a l i a n J . A g r . Res., 5 * 1 5 7 - 1 8 0 . de Wit, C. T., 1 9 6 5 » Photosynthesis of l e a f canopies. Chem. Res, F i e l d C r o p s A g r . R e s . Rep., No. 663.  Inst. B i o l . •  W o l f , D. B., 1 9 6 7 . A s s i m i l a t i o n and movement o f r a d i o a c t i v e c a r b o n i n a l f a l f a and r e e d c a n a r y g r a s s . C r o p . S c i . , 7 s 3 1 7 - 3 2 0 . ;  W o l f , D. D. and S m i t h , B., I 9 6 4 . Y i e l d and p e r s i s t e n c e o f s e v e r a l l e g u m e - g r a s s m i x t u r e s as a f f e c t e d b y c u t t i n g f r e q u e n c y and nitrogen f e r t i l i z a t i o n . Agron, J . , 56*130-133.  167 W r i g h t , J . L. and Lemon, E. R., 1966. P h o t o s y n t h e s i s u n d e r f i e l d conditions. IX. V e r t i c a l d i s t r i b u t i o n o f p h o t o s y n t h e s i s w i t h i n a c o r n c r o p . A g r o n . J . , 58°265-268. Yocum, C. S., A l l e n , L. H. and Lemon, E. R., I964.. P h o t o s y n t h e s i s u n d e r f i e l d c o n d i t i o n s . V I . S o l a r r a d i a t i o n b a l a n c e and photosynthetic efficiency. A g r o n . J . , 56;249-253.  168  8.  APPENDIXES  8.1  S o i l P h y s i c a l and C h e m i c a l  8.1.1  Methods  Data Canadian  A t t h e c o n c l u s i o n o f mowing i n 1967 1  i n . c o r e s f r o m 0 t o 6 i n . and 6 t o 12  Experiments  t h e s o i l was in.  The  s a m p l e d "by t a k i n g  p l o t s s a m p l e d were  p a s t u r e s A, B, C, D, E and G f o r e a c h o f t h e t h r e e d e f o l i a t i o n managements. pooled  W i t h i n e a c h management s a m p l e s f r o m a l l A and B p l o t s were (i.e.  p u r e g r a s s ) , C and D were p o o l e d  E and G ( i . e . m i x e d g r a s s c l o v e r ) .  (i.e.  p u r e c l o v e r ) and  P l o t s P and H were o m i t t e d as., t h e  h i g h s e e d i n g r a t e s o f o r c h a r d g r a s s had r e s u l t e d i n g r a s s dominance i n many o f t h e s e p l o t s . so t h a t 9 t r e a t m e n t s  F i v e c o r e s were t a k e n f r o m e a c h p l o t and b l o c k ( 3 p a s t u r e s x 3 managements) were e a c h r e p r e s e n t e d  by a 40 c o r e c o m p o s i t e m i x e d and p a s s e d t h e n made: of  at each depth.  t h r o u g h a 2 mm  a) pH w i t h a 2 : 5  sieve.  These s a m p l e s were a i r d r i e d , • The  s o i l water r a t i o  ( S o i l R e a c t i o n Committee  the I n t e r n a t i o n a l S o c i e t y of S o i l S c i e n c e , 1 9 3 0 ) ,  b y t h e method o f W a l k l e y and B l a c k ( 1 9 3 4 )  1  f o l l o w i n g a n a l y s e s were >  See  S e c t i o n 7-  1  b) o r g a n i c  carbon  and c) c a t i o n exchange;-  Literature Cited, f o r references i n this  appendix.-.  169  c a p a c i t y b y ammonium r e p l a c e m e n t  and d i s t i l l a t i o n w i t h magnesium  oxide.  A l l d e t e r m i n a t i o n s were made i n d u p l i c a t e . Two s o i l p r o f i l e s , l o c a t e d a t e i t h e r end o f t h e e x p e r i m e n t a l  site  were examined t o d e l i n e a t e t h e e x t r e m e s o f s o i l c o n d i t i o n s i n t h e experimental area.  P u l l f i e l d d e s c r i p t i o n s were made o f t h e s e p r o f -  i l e s and l a b o r a t o r y a n a l y s i s o f e a c h h o r i z o n f o r t h e f o l l o w i n g were mades  a) pH,  b) o r g a n i c m a t t e r ,  c ) c a t i o n exchange c a p a c i t y ,  moisture  t e n s i o n and e) m e c h a n i c a l  (1962).  Prom t h e p r o f i l e s i t e s 3 i n . d i a m e t e r  determine drainage.  b u l k d e n s i t y and m o i s t u r e  a n a l y s i s b y t h e method o f B o u y o u c o s  The c o r e s were 3 i n . h i g h and t h e r e f o r e t h e p r o f i l e s were  e a c h s i t e and z o n e .  0.3,  S i x c o r e s were t a k e n  D i s t u r b e d s a m p l e s were a l s o t a k e n f r o m  t h e s e were a i r d r i e d , p a s s e d  through  from  the core  a 2 mm s i e v e and d r a i n e d a t  0 . 9 > 4 . 0 and 1 5 « 0 " b a r s on p r e s s u r e p l a t e s as d e s c r i b e d b y 1  Richards  8.1.2  c o r e s were t a k e n t o  tension r e l a t i o n s by tension liable  o n l y s a m p l e d as s u r f a c e and s u b s o i l z o n e s .  zones,  d)  (1948).  Results As a s s e s s m e n t o f t h e s o i l environment-  of the experimental area i s  g i v e n b y t h e a n a l y s e s on t h e 4 0 c o r e c o m p o s i t e Each f i g u r e i s presented  as a mean o f t h e . t w o s a m p l i n g d e p t h s as t h e r e  were no s i g n i f i c a n t d i f f e r e n c e s b e t w e e n them. trends w i t h depth  samples (Table 8 . i ) .  ( T a b l e s 8 . I I and 8 . I I I ) .  The p r o f i l e s showed some  The m o i s t u r e  tension .  :  r e l a t i o n s h i p s f o r t h e two p r o f i l e s i t e s showed some g r a d i e n t over' t h e experimental  area ( P i g . 8 . 1 . 1 ) . •  Prom t h e s o i l d a t a i t a p p e a r s t h a t l i m i t s t o p l a n t g r o w t h due t o d i f f e r e n c e s between t h e s o i l s o f p a r t i c u l a r t r e a t m e n t s  were u n l i k e l y .  170 In a l l p a r a m e t e r s  except o r g a n i c carbon the d i f f e r e n c e s between p a s t u r e s  o r d e f o l i a t i o n managements were n o t s i g n i f i c a n t  ( T a b l e 8 . 1 ) . The l o w e r  l e v e l o f o r g a n i c carbon i n the g r a s s c l o v e r p a s t u r e s , though (P<0.01) r e p r e s e n t s o n l y a s m a l l d i f f e r e n c e f r o m a b i o l o g i c a l point.  The d a t a f r o m t h e p r o f i l e s i t e s  significant stand-  ( P i g . 8.1.1, T a b l e s 8 . I P and  8 . I l l ) i n d i c a t e that the s l i g h t gradient across the experimental, s i t e l a r g e l y due t o t h e l o w e r s i l t c o n t e n t o f S i t e 1 compared w i t h S i t e  T a b l e 8.1  2.  A n a l y s e s o f 40 c o r e s o i l c o m p o s i t e s ; v a l u e s a r e e x p r e s s e d on an oven d r y s o i l b a s i s and e a c h i s t h e mean o f t h e 0-6 i n . and 6-12 i n . d e t e r m i n a t i o n s .  Pasture  0.grass  W.clover  grass/clover  S.E. +  PR-  6.14  6.06  6.14  0.043  Organic carbon $  3.40  3.52  3.12  0".078 0.436  C a t i o n exchange C a p a c i t y m.e. i> Management  pH Organic C a r b o n fo c  C a t i o n exchange C a p a c i t y m.e. 'fo  11.19  11.32  11.80  3-1  9-1  9-3  "s.E. +  6.08  6.14  6.13  0.043  3.42  3.42  3.21  0.078  11.32  11.73  11.26  0.436  171 Table 8.II F i e l d d e s c r i p t i o n o f p r o f i l e s i t e s ; + = p r e s e n t , ++ = m o d e r a t e , +++ = h e a v y , ++++ = v e r y h e a v y . C o l o u r s a r e measured w i t h t h e Manse1 System. SABL = subangular blockey s t r u c t u r e . Site 1  Horizon  Depth in.  Colour  T e x t u r e and Structure  Ap  0-8  10YR 3/2  S.Loam SABL  A/B  8-10  2.5Y 6/4  C S.Lo am  10-20  2.5Y 6/4  CS.Loam  20  —  —  B Water table  Roots  Concretions  Stones  6.5  +++  -  ++ 1/8"  -  6.5  -  -  6.5  -  —  —  PH  ++++ F e ++ F e  —  '. ; -. —  Site 2  Apl  0-4  10YR 2/2  A/B  10-14  10YR  5/4  S. C 1 . Lo am SABL S.C1.Loam SABL S.Loam —  Ap2  4-10  10YR 2/2  B  14-23  10YR  6/6  S.Loam  Water table  23  -  -  —  6.5.  ++ 3/8"  +++  6.5  ++  6.5  -  6.5  -  -  -  + Fe  ++ 1/4"  -  ++  -  —  172 T a b l e 8. I l l  Laboratory analyses f o r p r o f i l e s i t e s , values e x p r e s s e d on an oven d r y s o i l b a s i s .  Site  Horizon  Organic Carbon  C a t i o n Ex. pH Capacity m. e .'/b  Ap  2.339  10.415  6.43  A/B  0.412  1.901  B  0.253  1.942  are  1  Mechanical Analysis^ Sand S i l t Clay  Bulk Density g/cc  7.066  76.15 14.83  9.02  1.239  6.35  1.535  93.01  5.08  1.27  -  6.27  1.569  96.19  2.54  1.27  1.428  • -  Available waterl  Site  2  Apl  3.649  12.893  6.05  18.003  62.90 27.99  9.11  Ap2  3.787  12.872  6.10  11.209  68.78 20.16  11.06  A/B  1.082  5.448  5-95  4-369  80.22 13.40  6.38  -.  B  0.427  2.371  6.00  2.332  88.60  3.80  1.418  1 2  8.2  7.60  1.137  M o i s t u r e c o n t e n t between 0.333 and 15.0 - b a r s . U.S. s i z e f r a c t i o n s .  G l o s s a r y o f M a j o r Terms  Defoliations  Removal o f herbage f r o m  a forage stand; c l i p p i n g  and  g r a z i n g are p a r t i c u l a r forms o f d e f o l i a t i o n r e l a t i n g to herbage removal by mechanical Dry matter  yield  (....  and  a n i m a l means r e s p e c t i v e l y .  production);  A e r i a l p l a n t biomass i n c o r p o r a t e d  i n t o the herbage of a f o r a g e s t a n d of u n i t  area i n a  specific  time p e r i o d . F o r a g e s t a n d (.... utilized  sward)s  A community of h e r b a g e p l a n t s grown o r  to p r o v i d e f o o d f o r animal  consumption.  • s i t e 1 surface x si»e? surface 50r  MOISTURE  "  8 1 1 "•"  TENSION  ( — BARS)  M o i s t u r e t e n s i o n r e l a t i o n s h i p s f o r s u r f a c e and s u b s o i l s i n t h e t w o p r o f i l e P o i n t s a r e means o f t h r e e r e p l i c a t e s , t h e l i n e s have been f i t t e d by hand.  si  Frequency of d e f o l i a t i o n s  The  number o f o c c a s i o n s on w h i c h  d e f o l i a t i o n takes place i n a s p e c i f i c Gross  photosynthesis;  Synthesis of carbohydrate  and w a t e r u s i n g l i g h t Herbages  time p e r i o d . from carbon  energy.  Above g r o u n d p l a n t m a t e r i a l p r o d u c e d  by p l a n t s w i t h  o r more stems w h i c h d i e b a c k e a c h y e a r ; as d i s t i n c t and  trees.  leaf,  The  term r e f e r s  stem, s e e d and  The  Often t h i s  (LAI)s  shrubs  t o a l l above g r o u n d p l a n t m a t e r i a l s ,  d e g r e e o f r e m o v a l o f h e r b a g e from  The  a  i s i n d i c a t i v e o f the h e i g h t above t h e  g r o u n d a t w h i c h d e f o l i a t i o n has L e a f area index  from  one  a s s o c i a t e d organs.  Intensity of defoliations forage stand.  dioxide  ratio  taken p l a c e .  o f t h e l e a f a r e a o f a community  ,to t h e a r e a o f l a n d a s s o c i a t e d w i t h t h a t community. Light Net  energys  E l e c t r o m a g n e t i c r a d i a t i o n between 400  assimilation rates i n r e l a t i o n to u n i t i n d r y weight  and 700  nm.  R a t e o f i n c r e a s e i n a e r i a l p l a n t d r y weight„ l e a f area.  In t h i s t h e s i s r a t e o f i n c r e a s e  has b e e n r e p l a c e d b y r a t e o f n e t c a r b o n  dioxide  assimilation. Net  photosynthesis5  Gross  p h o t o s y n t h e s i s l e s s the q u a n t i t y o f o r g a n i c  oompounds r e s p i r e d b y t h e p h o t o s y n t h e t i c o r g a n s d u r i n g the o f s y n t h e s i s 5 as measured b y t h e n e t c a r b o n d i o x i d e  time  assimilation  of the a e r i a l p l a n t p a r t s . Pasture  s t a n d (....  mainly  sward)s  Synonymous w i t h f o r a g e stand5  i n the A u s t r a l i a n s e c t i o n o f the t h e s i s .  used v  

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