THE EFFECT OF AGE, BREED, DAYS OPEN, STAGE OF LACTATION AND PREGNANCY UPON DAILY BODY WEIGHT AND MILK WEIGHT IN LACTATING DAIRY CATTLE by ZEGEYE YEGEZU B.Sc , (Agr), Haile Sellassie I University, 1973 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES (Department of Animal Science) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1976 (c^ Zegeye Yegezu, 1976 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced d e g r e e at 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 , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f Aftfsu*i{n£ d**S*>rfsf £ The 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 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date fr/. 3,t II 7( i ABSTRACT The effect of age, breed, days open at conception, stage of lactation and pregnancy upon daily body weight and milk weight in lactating dairy cattle were studied. The cattle used in the study were those of the University of British Columbia Research Farm at Oyster River, B.C. 76 Hoi stein and 60 Hoi stein X Ayrshire crossbred milking cows at different stages of lactation and both either open or pregnant, were used in this study. The study was carried out during the months of May and June of 1975. The procedure involved weighing the animals daily after the afternoon milking and recording the daily milk yield along with the weight of each cow. The cows were classified into three groups, namely A l l , Open and Pregnant, the f irst group being a combination of the last two. For All and Open cows, the effect of age, breed, age by breed and number of days in milk on body weight and milk weight were investigated. For the Pregnant cows, the effect of age, breed, number of days open at conception, age by breed, age by number of days open at conception, breed by number of days open at conception and number of days pregnant on body weight and milk weight were investigated. Using the number of days open at conception as a dependent variable, the effect of age, breed and age by breed were inves-tigated. The results showed that among A l l , Open and Pregnant cow groups, the older cows were significantly (P <0.05) heavier and were producing significantly (P <0.05) more milk than the younger cows. Age accounted for 0.9%, 1.5% and 0.8% of the body weight variation and for 0.1%, 0.5% and 1.3% of the milk weight variation in A l l , Open and Pregnant cow groups respectively. Among the A l l , Open and Pregnant groups, the older cows weighed more by 143.22, 133.12 and 125.07 lbs and produced 3.72, 6.98 and 12.14 lbs more milk per day respectively than the younger cows. Among A l l , Open and Pregnant groups, a significant breed effect (P <0.05) on body weight and milk weight was observed. In the three groups, breed accounted for 2.3%, 3.8% and 0.1% of the body weight variation and for 0.1%, 0.3% and 0.6% of the milk weight variation respectively. Among All and the Open cow groups the Hoi steins were heavier by 223.00 and 214.04 lbs and were respectively producing 5.20 and 5.26 lbs more milk daily than the crossbreds. Among the Pregnant cow group the Hoi steins were lighter by 49.33 lbs and produced 7.74 lbs less milk daily than the cross-breds. The number of days open at conception was a significant source of variation (P <0.05) affecting body weight but not milk weight in the Preg-nant cow group. It accounted for 1.3% of the body weight variation. Those cows who stayed open longer than 123 days were heavier by 159.21 lbs than those who became pregnant earlier or on day 123 after calving. Age by breed for the All cow group was significant (P <0.05), accoun-ting for 0.1% of the body weight variation. This interaction was not significant for body weight of the other two groups and for milk weight of all the three groups. Age by number of days open at conception was significant (P <0.05) for the Pregnant cow group milk weight and accounted for 1.2% of the variation. This interaction was not significant for body weight. Breed by number of days open at conception was not significant for the Pregnant i i i cow group body weight and milk weight. Number of days in milk for All and Open cows had a significant (P <0.05) effect on both body weight and milk weight. It accounted for 10.2% and 2.9% of the variation in body weight and 35.6% and 18.4% of the variation in milk weight in All and Open cow groups respectively. In the Pregnant cow group, the number of days pregnant resulted in significant (P <0.05) effect on both body weight and milk weight. It accoun-ted for 30.8% and 61.4% of the body weight and milk weight variation res-pectively. When number of days open at conception was used as a dependent variable, age, breed, and age by breed showed a significant (P <0.05) effect in the pregnant cow group. Age, breed, and age by breed accounted for 0.3%, 0.4% and 0.9% of the variation respectively. i v ACKNOWLEDGEMENT I sincerely wish to thank my thesis supervisor, Dr. J. Hodges, who made it possible for me to undertake this study and provided his full support and encouragement throughout, from the beginning to the end in all phases of the study. The support, interest and constructive criticisms provided by Dr. R.G. Peterson during the analysis are sincerely appreciated. My thanks are also extended to Mrs. M. Striker for her advice and support in the computer programming and analysis of the data. I also extend my sincere thanks to the other members of my M.Sc. thesis committee for their interest in the work and the encourage-ment they gave me during the study period. V TABLE OF CONTENTS Page INTRODUCTION 1 LITERATURE REVIEW 3 The effect of age on body weight 3 The effect of age on milk yield 4 The effect of breed on body weight and milk yield 5 Factors affecting body weight 5 Factors affecting milk yield 7 The effect of body weight on milk production 7 Persistency of lactation in cattle 11 The lactation curve 11 MATERIALS AND METHODS 14 Source of data 14 Performance traits studied 16 Effects studied 16 Age classification 17 Breed classification 17 Classification of the number of days open at conception (DOPC) 18 Co-variables 18 Statistical models and methods 19 RESULTS 24 Daily body weight 24 Age group 28 Breed group 30 Number of days open at conception 32 Age by breed interaction 32 Age by DOPC and breed by DOPC interaction 34 Co-variables 34 Days milked 34 Days pregnant 36 DAILY MILK WEIGHT 38 Age group 42 Breed group 44 Number of days open at conception 46 Age by breed interaction 46 Age by DOPC interaction 46 Breed by DOPC interaction 46 Co-variables 48 Days milked 48 Days pregnant 50 PREGNANT COWS 52 vi Page DISCUSSION 55 Age 56 Breed 56 Number of days open at conception 56 Delayed f irst oestrus 57 Silent heat 57 Ineffective insemination 58 Late return to oestrus 58 Days mi 1ked 60 Days pregnant 64 CONCLUSION 69 LITERATURE CITED 72 vii LIST OF FIGURES Figure Page 1 Open cows body weight 62 2 Pregnant H body weight 63 3 Pregnant CB body weight 65 4 Open cows milk weight 66 5 Pregnant H milk weight 67 6 Pregnant CB milk weight 68 LIST OF TABLES Table 1 Analysis of variance. All cow body weights. 2 Analysis of variance. Open cow body weights. 3 Analysis of variance. Pregnant cow body weights. 4 Overall daily body weight means, age group least square constants and least square means. 5 Overall daily body weight means, breed group least square constants and least,square means. 6 Age by breed interaction least square.constants and subclass means as a deviation from the overall mean. All cow daily body weights. 7 Subclass partial regression coefficients. Daily body weight on days milked. All and Open cow groups. 8 Subclass partial regression coefficients. Daily body weight on days pregnant. Pregnant cow group. 9 Analysis of variance. All cow milk weights. 10 Analysis of variance. Open cow milk weights. 11 Analysis of variance. Pregnant cow milk weights. 12 Overall daily milk weight means, age group least square constants and least square means. 13 Overall daily milk weight means, breed group least square constants and least square means. 14 Age by number of days open at conception interaction least square constants and subclass means as a deviation from the overall mean. Pregnant cow milk weight. 15 Subclass partial regression coefficients, daily milk weight on days milked. All and Open cow groups. ix Table Page 16 Subclass partial regression coefficients. Daily milk weight on days pregnant. Pregnant cow group. 51 17 Analysis of variance. Number of days open at conception, Pregnant cow group. 53 18 Age by breed interaction least square constants and subclass means. Pregnant cow group, number days open at conception. 54 1 INTRODUCTION The history of the beginning of dairying is hidden in obscurity, however, written references on the use of milk and its products are found in the earliest Biblical records, dating to 3,000 B.C. Archeological pictures of the domestication of the cow for milk have been found as early as 9,000 B.C.; and since then many civilizations have left evidence of their use of cattle for domestic purposes. The kinds of uses made of them vary greatly with different peoples and cultures. The cow has served man as a beast of burden, as a source of food (both milk and meat), as an object of worship and as a source of sacrificial offering. Presently the dairy cow is favoured as a specialized milk producing machine. As the discrepancy between the world supply of protein and the growth of the human global population continues to increase, more and more demand is made upon the dairy cow to bridge this gap by producing milk from roughage which would not otherwise enter the human food chain. The dairy cow, being a ruminant, has the advantage of digesting and utilizing plant resources that are not acceptable to humans as food, and change i t into milk and meat products for human consumption. This ability to change plant products (both natural vegetation and waste material) to palatable foods for humans will therefore be a justification for the con-tinuous existence of the dairy cow even in a heavily populated area of the world (Foley et al_., 1973). Because of the volume of its digestive system, the body weight of a ruminant animal undergoes bigger changes during the day, and from day to day, than a simple stomached animal. A negative energy balance is known 2 to be the inevitable result of the need of the high producing dairy cow to utilize body reserves for milk production after calving. This continues for as long as 50-100 days after calving (Amir and Kali , 1974 ). Later in lactation, body weight increases and milk production starts to decrease as lactation progresses beyond 100 days (Brody, 1945). This is due to the necessity for the younger animals to grow towards their mature weight (pregnant or not) and for the/older cows to gain in weight due to pregnancy. This thesis is concerned with an investigation of the relationships of body weight and milk yield on a daily basis, and how they are affected by age, breed, stage of lactation, length of open period and pregnancy in lactating cows. The cattle used for this investigation were those of the U.B.C. Research herd at Oyster River, B.C. The records from 136 adult cows were sorted into a l l , open and pregnant to facilitate this study. This thesis will try and answer, within the limitations of the animals available, the following questions about open and pregnant cow groups. 1. Does number of days open affect body weight or milk weight? 2. Are body weight and milk weight affected by stage of lactation? 3. Does pregnancy affect body weight and milk weight from conception? 4. Does body weight and milk weight affect young differently than old cows? 5. Does breed type contribute to body weight and milk weight variation? 6. Are there breed differences in the number of days a cow remains open after calving? 3 LITERATURE REVIEW The Effect of Age on Body Weight Miller and hboven (1970 ) studied variations in body weights and body weight changes of 477 ft>l stein cows"in 1,004 lactations collected over 14 years. Body weight used were lactation average weights and weight in the second, f ifth and seventh month of lactation. In this study they found that age was the most important factor, accounting for 41.4% of the vari-ation in body weight and 19% of the variance in weight change on a linear basis. They found days open to be unrelated to average body weight but negatively related to weight change during lactation. According to Miller and tooven (1970 ) the growth curve of rolstein cows presented by Ragsdale e_t al_. (1924 ) and Matthewes and Fohrman (1954 ) show that cows continue to gain in weight up to about 7 years of age. Body weight pattern studies were also made in beef cattle. Fitzhugh et al_. (1967 ) studied Angus and Hereford cows in 10 experiment station herds and observed that the animals increased in weight up to about 9 years of age. Brinks et_ aj_. (1962 ) observed weight increment in Hereford range cows nursing calves up to about 8 years of age. In this study they found the gain in body weight from 3 years to 8 years to be about 11 to 12%. They also stated that beef cows nursing calves show an inverse relationship between their own changes in weight and the daily gain in weight of their calves. In one study Miller et al_. (1969 ) studied patterns in weight during lactation and observed cows of all ages gaining in weight from 60 days post-partum to the end of lactation. They observed two year olds gaining slightly from the f i rst to the second month, while older cows lost weight. 4 This is expected to happen because cows in their f i rst lactation are in their active stage of growth. This is supported by the early work of Morgon and Davis (1936) who reported that weight gains were greatest during f i rst lactation, varying from 65 Kg in Jerseys to 88 Kg in folsteins. With the aim of developing a method of utilizing live weight as an aid to selection Farthing and Legates (1958) studied the relative influence of live weight and age on production in fblstein and Jersey cows. Based on 305-day 2X basis lactation they found the within cow linear and quad-ratic effects of age and weight to indicate that age is more effective than weight in accounting for variance in actual production. The Effect of Age on Milk Yield It is well known that milk production increases with age at an ever increasing rate until maximum production is reached, at around 6:.to 8 years. Production then declines with advancing age. Lush and Shrode (1950) des-cribed the regression of production on age as distinctly curvlinear whose nature and amount of the curvature do not appear to be deducible from any physiological principles. Jel icic (1958 ) working with Pinzgau cattle found the 8th lactation to have the highest yield with 2683 kg in 347 days or 2465 kg on a 300 day basis. Although older cows produce more milk than the younger ones they require a longer period to reach their peak produc-tion. This was demonstrated by Rakes e_t al_. (1960 ) working with folstein and Jersey cows. Tapia et_ a]_. (1968 ) comparing 3 types of production curve arrived at the conclusion that as lactation number increased from 1 to 8, initial milk yield increased but yield in the final period of lactation was similar 5 in all lactations. Efficiency of milk energy yield by dairy cows is intimately associated with their metabolic capacity for converting feed energy into milk energy. Brody (1945) called this Dairy Merit and defined i t as the biological efficiency of milk production as measured by the percentage of consumed TDN energy which is converted into milk energy. This definition of Dairy Merit is generally accepted and appears to have been f i rst proposed by Gaines (1931 ). While the definition is straight forward and simple, practical application is essentially impossible. Production level is the essential component and this is related to size and heredity of the animals, as well as to feeding practices and other environmental conditions. The Effect of Breed on Body Weight and Milk Yield Gaines (1940) showed that when diverse population of cows are used and when age and breed differences are removed, body size and milk yield are positively correlated. Though larger animals produce more total milk energy than smaller animals the yield per unit of weight favours the smaller animals. Erb and Ashworth (1961 ) showed the k)lsteins had a greater increase in yield as weight increased, independent of age, than Ayrshires or Jerseys or Guernseys. Factors Affecting Body Weight Effects of ruminal contents upon estimate of body weight change of dairy cattle were investigated by Bath et_ al_. (1966 ). They showed body weight changes estimated from single weighing after 14 hours without feed 6 or water at the beginning and end of a period to be more accurate than changes estimated either from means of three live weights taken at the beginning and end of the period, or from a regression of daily live weights on time. They concluded that estimation of body weight change from shrunk live weights could greatly reduce variations in ruminal f i l l , which is one of the largest sources of error in dairy cattle feeding experiment. Balch and Line (1957) also attributed variation in ruminal f i l l as a major source of error in estimation of body weight changes. They found that when cows were transferred from a dry ration to a;:pasture, 84% of the weight change was due to variation in reticulo content. Koch e_t aJL (1958 ) working with beef cattle, reported that shrunk weights were influenced less by fluctuation in f i l l and considered these weights to be accurate measures of weight and gain. They also found the use of three-day average weights to be effective in reducing fluctuations due to f i l l . They recommended the use of two or three weights when individuals are to be compared. For comparisons of group of animals they found increasing the number of animals to obtain the accuracy needed to be more effective. This work was quite in agreement with an earlier work by Patterson (1947) who reported that the size of the standard error.of the mean for between animals can be reduced by increasing the number of animals in the test. That is , increasing the number of animals on test is a more efficient means of increasing precision than increasing the number of weights per animal. Stoddard and Lamb (1971 ) showed that body weights of experimental cows withheld from water for four hours before weighing to be as variable as weights of cows with continuous access to water. From this work they suggested that weights of cows, with access to water, can be adjusted to 7 weights without water by multiplying by 0.964. They claim this procedure to be more accurate than subtracting a specified amount of weight. Factors Affecting Milk Yield The milk yield of a cow is influenced mainly by environment, by the number of previous lactations, her age at calving, and the length of the dry period preceding calving. Mahadevan (1952) obtained a positive correlation between milk yield and length of the preceding calving interval. He used the yield during the f i rst 180 days of the lactation period. This elminated the effect of variations in length of current calving interval on milk yield. From an economic point of view he found the optimum length of calving interval to be 400 days for the f irst lactation and one year for subsequent lactations. Lactation milk yield is largely determined by persistency which is an inherited character between breeds. Selection for persistency would be of value in improving milk yields (Gruhn and Bartels, 1959). The Effect of Body Weight on Milk Production It is commonly asserted that on the average large cows give more milk than do small cows. Harville and Henderson (1966) gave three possible explanations for the positive phenotype association between body size and production. 1. Body size serves as a measure of cow's stage of maturity. That is , both size and production are positively associated with age at calving. Gaines (1940) recommended that body weight measurements be used to replace or supplement age measurement for purpose of adjusting active production 8 records to mature equivalents. 2. Environmental condition favourable to high milk production may also lead to large body size, and, 3. There may be a positive genetic correlation between size and production. A multiple regression analysis of f i rst lactation data done by Clark et al_. (1962 ) indicated that for a constant age, 305 day milk production increased 134 lbs and fat 7.8 lbs for each 100 lbs increase in body weight, and for a constant weight each increase of one month of age was accompanied by an increase of 46 lbs of milk and 1.2 lbs of fat in 305 day production. When all lactations were combined and the number of the lactations ignored, 305 day milk production increased by 400 lbs and fat production.by 14.4 lbs for each 100 lbs increase in weight. For a constant weight each month increase in age accompanied by an increase of 28.4 lbs of milk and 0.9 lbs of fat. Wi 1 k e_t aj_. (1963 ) indicated that the genetic correlation between weight and f i rst lactation production is quite high. Tibor (1970) found a correlation of about +0.5 between milk and body weight during f i rst lactation, fe showed that the correlation between milk yield and body weight increased with increasing body weight. Busol (1974 ) working on Simmental heifers that calved f i rst at 25-27, 28-30, 31-33, 34-36 and 41 months of age found no correlation between the age at f i rst calving and subsequent milk production or calving interval. Miller et al_. (1973 ) working on the usefulness of periodic body weight to predict yield, from intake and efficiency of lactating cows found weights to be more effective for predicting efficiency and least effective 9 for feed intake. Higher starting weight was associated with increased milk yield and efficiency. Higher end weights were associated with lower milk yield and lower efficiency. It seems body size would be a useful criterion upon which to base selection of dairy heifers or cows for which no production records are available. A positive genetic correlation between body size and production means that selection for production will result in larger cows with increased growth and maintenance costs. tooven et_ al_. (1968 ) reported that weight changes during lactation are closely associated with milk and the efficiency of milk yield. They also concluded that changes in weight during this period are largely deter-mined by environment. This agrees with the fact that high producing cows tend to lose weight in the early weeks of lactation. They found the gen-etic correlation between efficiency and production, efficiency and body weight, and body weight and production to be 0.92, 0.17 and 0.28 respect-ively. They arrived at an estimate of heritability of efficiency of 0.46 as opposed to 0.62 for production. Overall they concluded that selection for yield alone would increase the genetic potential for feed efficiency. McDaniel and Legates (1965 ) from a study of the relation between the body weight and production traits of hblstein cows subjected to good feeding and management condition concluded that: a ) Larger cows give more milk; but questioned if the increase off-sets the additional maintenance costs. b ) The genetic variance in body weight is largely independent of the genetic variance in milk yield and fat percentage. That is in slight disagreement with the positive phenotypic association reported by Harville and Henderson (1966 ) and Tibor (1970 ) between body size and production. 10 c) Selection for milk will produce l i t t le i f any change in body weight; and selection for body weight would be expected to produce l i t t le change in milk yield. d) Milk yield can be increased without materially increasing body size. e) Little attention should be given to differences in weights within a breed; except to check the stamina and productivity of cows markedly above or below the breed average in weight. f ) Undue emphasis on larger cows in the short run could lead to the development of less profitable animals. Miller and McGilliard (1959) studied the relations between weight at f i rst calving and milk production during the f i rst lactation and obtained an intrahered partial regression of about 75 pounds of milk per month of age and 200 pounds of milk per 100 pounds weight at f i rst calving. When herd was ignored they observed a large positive correlation to exist between herd averages for weight and production which contributed signif i -cantly to the partial regression of milk on weight. Bereskin and Touchberry (1966) studied the relationships between body weight and age with f i rst lactation yield in a dairy herd that was composed of tolsteins, 8/16 folsteins, 5/16 tolsteins, and Guernsey. They made the following observations: a ) Age at freshening, when included with days carried calf as the only additional covariate, was significantly associated with f i r s t lac-tation yields of milk fat and fat correctedclmilk (FCM). b ) Body weight taken soon after calving, included with days carried calf as the only additional covariate was significantly associated with 11 yields of milk, milk fat and FCM. c ) When both factors were included as additional covariates, weight retained its importance, but the independent association of yield with age was small. Lactation milk increased 395 kg with each 100 kg increase in body weight. d) Neither factor was significantly associated with percent milk fat. In a separate analysis, they showed that lactation yields of milk, milk fat and FCM varies approximately as the 0.6 power of body weight at calving. In conclusion, they proposed expressing f i rst lactation yield as a function of body weight as a meausre of performance. Persistency of Lactation in Cattle Corley (1957) after studying 1552 milk and butter fat records made by 932 Fblstein Friesian, Jersey and Crossbred cattle, showed that the main factors affecting persistency were method of milking and season of calving. Fe found machine milked cows to be 5% less persistent than hand milked cows and cows with f i rst lactations to be about 8% more persistent than those with second lactations. In an earlier work, Mahadevan (1952) showed that the highest persistency was obtained by cows calving in the winter and the lowest by summer calvers. The major part of the variation was due to non-genetic factors. Persistency of lactation decreased with increase in age at freshening and under similar conditions, Ayrshires matured earlier than the hblsteins (Hickman, 1958 ). The Lactation Curve Karavceve (1957) using records from three farms at which milk 12 production averaged 4500-6500 kg per year, on 2152 lactation of 530 Kestroma cows identified four.types of lactation curves based on the rate of decline in yield. The f i rst two types applied to low producers in which milk yield dec!ined rapidly after two to three months lactation so that in the two types 46% and 42% respectively of the total yield was produced in the f i rst three months. The'third and fourtjv types applied to high producers in which the decline was gradual. In those types 37% and 34% respectively was produced in the f i rst three months and 31% and 32% in the second three months. Production was higher in the fourth than in the third group. The above result is supported by Labouche (1957) who reported that the lactation curve showed a more rapid rate of decline in the low producers than i t did in high producers. Delage et aj_. (1953 ) after analyzing production records of 146 Dutch Friesian cows for the phase of decreasing lactation arrived at the con-clusion that:. 1. Maximum production was reached on the 60th day of lactation in 96% of the cows. 2. There were two periods during which production decreased from 60 to 250 and more rapidly from 250 to 300 days. 3. Parabolic function was found to represent the decrease in the f irst period. Maymone's and Malossini's reports in 1960 supported Delage and his co-workers' 1954 findings. Maymone and Malossini after examining daily milk yields for 2080 normal lactation of Dutch Friesian, Swiss Brown, Simmental, Dutch Red Pied and lolstein Friesian concluded that: 1. Maximum production was reached in the f i rst month after calving by 69% of cows,in the second month by 94% and in the third month by 99%. • 13 2. The average duration of the ascending phase of the lactation, i .e. the number of days from calving until the maximum yield was reached was 28.3 days. Cianci (1965) studied the effect of dry period on lactation in the cow and found average milk yield to be highest after the dry period of 91-120 days, le concluded that the shape of the lactation curve is influ-enced by length of dry period. 14 MATERIALS AND METHODS Source of Data The cattle used in this study were from the University of British Columbia (UBC) Research Farm at Oyster River, B.C. During the months of May and June of 1975, milk yield and body weight were recorded daily for all lactating cows. Milk samples from each cow were analyzed for fat,> protein and lactose, only once during the period when the routine Record of Performance (ROP ) inspection visit occurred, which was on June 12, 1975. 136 milking cows at various stages of lactation were included in the study. They consisted of 76 pure hblsteins and 60 R)lstein Ayrshire Crosses, which varied from 25% to 81% tolstein. The data collection procedure was to weigh the milking animals daily immediately after the afternoon milking. This excluded the weight of milk in the udder and was consistent for the feeding time. The weight of both afternoon and morning milk were added to give the corresponding total daily milk production. Dry cows due to calve in a month or less were also weighed every morning, at the same time to reduce the variation due to diurnal weight changes. These particular data were to be used to study the magnitude of weight changes when the cow is not subjected to lactation stress, towever, the dry cows data were not used because of small sample size. At the time the study was carried out there were 15 milking cows in the herd that received special treatment due to age, production, i l l -ness and other reasons. Because of the different management practice, these were analyzed as a separate group, towever, this proved impractical with the small number of observations and the data were therefore included 15 with the lactating cows. This decision was not easy since their inclusion obviously increases the variance, but their exclusion would have deprived the analysis of some of the highest producing cows in the herd. Apart from the body weight and milk production data mentioned above, the following information was also obtained for each cow from the existing herd records: 1. Birth date 2. Calving date preceeding or during the experimental period (Cl ). During the study, no lactating cow in the herd moved from one lactation to the next. Pregnant cows were given a minimum dry period of approximately 60 days before calving and those who calved during the experimental period were therefore dry at the beginning of the study period. 3. Last mating date (LMD) following Cl . 4. Next calving date (C2 ) after the experimental period, resulting from LMD. 5. Breed type. The above five points together with the weighing date were used to calculate the following facts about each cow for each day of the experi-mental period: 1. Classification of the cow into open or pregnant group. If, for a cow, the C2 occurred after the end of the experimental period but before March 20, 1976, she was pregnant at least for a part of the experimental period. Any cow having a successful service before the last weighing date (June 28, 1975) would have calved on March 20, 1976. All the cows used in the pregnant category calved before March 20, 1976. By calculating back, the last service date was confirmed as the conception date. 16 2. The stage of lactation measured as days after calving (DAC) at each date of recording body and milk weights. 3. The number of days open at conception (DOPC ) to determine whether or not they became pregnant before or during the experimental period. 4. The number of days pregnant (DP ). 5. The age of the cow in months at Cl. Performance Traits Studied Daily body weight (BW) and milk weight (MW ) of the cows were examined separately as affected by: 1. Age 2. Breed 3. DOPC 4. Days milked (DM ) and 5. DP For this purpose the animals in the herd were classified into three different groups: Group 1 All Cows Group 2 Open Cows Group 3 Pregnant Cows Group 1 is a combination of Groups 2 and 3. Effects Studied An attempt was made to classify each group of cows into four age categories and three breed categories. This resulted in some empty cells 17 and in gross disparity in sample sizes between cells. To overcome these problems, the classification was reduced to two age categories and two breed categories. In addition there was an attempt to classify the Preg-nant Cows into four groups of DOPC and for the reasons mentioned above this grouping was also reduced to two. In this grouping those who conceived on day 123 or earlier post-partum, were taken as Group 1 and those who remained open longer as Group 2. 123 day after calving was chosen as a day dividing the two groups because i t is close to the average DOPC for the herd. Age Classification A cow of 4 years oldcor less was defined as a younger cow (Age 1 ) and a cow 5 years or more was taken as an older cow (Age 2). Age was defined as the difference between Cl and birth date. For cows calving during the experimental period, data following this calving date were used. Age was defined in months for all groups studied. Breed Classification The experimental herd consisted of various Holstein x Ayrshire Cross-bred animals in addition to the pure folsteins. Based on this, the animals were classified into two Breed Groups which were Holsteins (H) and Cross Breds'; (CB ). An attempt was made to split the CB into two levels based on the proportion of Holstein they contained. It was decided not to follow this procedure since the number in each CB level was too small. 18 Classification of DOPC The raw data showed that some of the cows in the herd at the time of the study were milking for up to 753 days without moving into the next lactation. In the light of this, specific constraints were applied to the data regarding calving interval and lactation length as follows. A maximum calving interval of 18 months (553 days ) was applied to the herd; thus, any cow remaining open for more than 270 days was removed from the study as pregnancy was taken as a constant of 283 days (Foley et a l . , 1973 ). The imposition of these constraints was important since i t appeared to be a normal practice on the UBC farm to keep a cow in the herd as long as she was producing a minimum level of milk daily, even though she does not carry a fetus. These constraints were considered justifiable by the author for the following two reasons: 1. Under commercial conditions a dairy farmer would keep an open cow in his herd provided she produces enough milk in excess of the production cost for up to one year after calving. After a year from calving, i f she is s t i l l open but producing at a reasonable level, he will likely keep her for another six months in the hope she will conceive before making his decision to dispose of her. 2. A constant of 283 days was used for length of pregnancy (Foley et_ a l . , 1973 ) for all necessary calculations. Co-Variables There were two co-variables studied and they were: 19 1. Number of days in milk. This was defined as the number of days a cow was milking from CI to each date of record. This is relevant to both open and pregnant cow groups. 2. Number of days pregnant. This was the co-variable for the preg-nant cows only and refers to the number of days from conception to each record date. The possibility of using fat %, protein % and lactose % as co-variables by taking the average of three ROP test results, was examined. The three tests considered were the last test before the experiment, the one during the experiment and the one right after the experiment. H)wever, this procedure resulted in fitting the same single average value of the milk components for each day of BW.and MW, i t was an ineffective variable to use. It was therefore dropped, and milk was assessed only on daily weight. Another possibility investigated was to use the individual yields (in pounds ) of the above three milk components as co-variables. But the individual yields being the linear combination of their respective percentage values and the daily MW,.it again resulted in fitting the same yield values for each day of BW and MW. This was also dropped in favour of examining MW only on a daily basis. DM was considered as an additional co-variable for the pregnant cows. For the pregnant group with'in cow basis, number of days in milk and number of days pregnant are confounded. Because of this, number of days pregnant was used as the only co-variate. Statistical Models and Methods Three statistical models containing the co-variables DM (DP for the pregnant group) linear, linear + quadratic and linear + quadratic + cubic 20 were analyzed for each group of cows (Open, Pregnant and All ). The relative efficiency of each model was tested. Based on these test results DM linear, quadratic and cubic were taken as co-variables for the All cow group. In the case of the Open group, DM linear and quadratic were used. For the Pregnant group, number DP linear, quadratic and cubic were used. The tests for the different statistical models were carried out with the co-variables fitted as a commonpplanecacross all age and breed groups, however, this combined form was not used in reporting the results because of slope differences within subclasses. In the analysis on which this report is based, the co-variables were fitted within each age by breed subclasses. A least squares analysis after Harvey (1960 ) was used to estimate the effects of Age, Breed, DOPC, DM, and DP on BW and MW of the cows. For the reasons given below the f i rst three were considered discreet variables and the last two co-variables. Age is a continuous variable because i t is changing in small units daily, however, i t was used as a discreet variable because, within the short time of the experimental period, i t was varying very l i t t le and would have been a direct linear combination of days since calving or days open. Age was calculated from birth to Cl for all animals. Breed type is determined at conception and this makes i t a discreet variable. DOPC was investigated as both discreet variable and as a co-variable. Because a better allocation of R was obtained when used as a discreet variable, the other alternative was eliminated. DM and DP in days are changing from day to day over the lactation period and so were taken as a co - va r iab le . The l i near mathematical model assumed for the various analysis with d i f fe ren t modif ications i s as fo l lows . Y i j k l = * + a i + b j + d k + ( a b h + U d } i k + ( b d >Jk + d D i j k l + d' ( D 1 j k l f + d» ( D l j k l )3 + p P . j k l + p- ( P . j k l f + p" < P i j k l ? + e i j k l ' W h e r e Yjjkl = The observed value of the appropriate performance t r a i t under study of the 1st cow of the i**1 age, j** 1 breed and having day open at conception, y = The population mean for the t r a i t under study when equal frequencies ex i s t in a l l subclasses and D\-k-| and P^j^-j are equal to zero. a . = The e f fec t of i**1 age. b. = The e f fec t of j * ' 1 breed. J d. = The e f fec t of k t h open day at conception. (ab ).. = The j o i n t e f f e c t of i ^ age and j** 1 breed when the ef fects of age and breed are kept constant. (ad).j k = The j o i n t e f fec t of i**1 age and k*'1 day open at conception when the ef fects of age and days open at conception are held constant. (bd )., = The j o i n t e f fec t of j * * 1 breed and k^ day open at conception J K when the e f fec t of breed and days open at conception are held constant. d = The p a r t i a l regression c o e f f i c i e n t of Y. .^ .j on D....^ 5 i j k l s t t h D . . , , = The number of DM associated with the l r - cow of the i age 22 and j breed. d' = The partial regression coefficient of Y^.^ on (Djj^i ) • ^ i j k l ^ = ^ e n m ^ e r °^ squared associated with the 1 s t cow of the itJn age and j^*1 breed. o = The partial regression coefficient of Y.^^ on ( D ^ i ) . (D.. L 1 )3 = The cubic number of DM associated with the 1 s t cow of the i*11 age of j^*1 breed. d" 5 i jkl P = The partial regression coefficient of Y^^ on P -^^ ] ' i jkl P-nL-i = T n e number of DP associated with the 1 s t cow of i*'1 age, j**1 breed and k day open at conception, p' = The partial regression coefficient of V..^ on (Pjj^i (P-jjkl f = The quadratic number of DP associated with the 1 s t cow of i t ' 1 age> j*' 1 breed and k**1 day open at conception, p" = The partial regression coefficient of Y. .^ -j on (P-JJL-I ) • ^ i j k l ^ = T h e c u t ) l c n u m b e r o f DP associated with 1 s t cow of i t h age, .' is breed and k day open at conception. e i j k l = The random effect associated with the l s ^ cow of age, j 1 ' ' 1 breed having k^ day open at conception which is assumed to be independent and normally distributed with mean equal to zero 2 and variance a . The minimum level of significance used throughout P s 0.05. All significant effects were tested by student Newman Keules test (Zar, 1974 ), or by t-test or by F test (l&rvey, 1960 ). The standard error for the overall mean was approximated by using the formula d = s where 23 d = The standard error of the mean. s = The residual variance, and N = The total number of observation. The proportion of the total sums of squares, that was accounted for by fitting the statistical model and each effect in the model, were calculated. RESULTS Daily Body Weight (BW ) A summary of the results of the analysis of variance of daily BW for the All Cow, the Open Cow and the Pregnant Cow groups are given in Tables 1, 2 and 3 respectively. In Tables 1 and 2 the d. f 's , the mean squares, the F-values and the R (the proportion of total variation in daily BW accounted for when fitted in the statistical model)), are presented for the main effects of Age, Breed and the Age by Breed interaction. The three co-variables of DM linear, quadratic and cubic are given for the All Cow and the two co-variables of DM linear and quadratic are given for the Open Cow groups. Days open and days in milk are the same for the non-pregnant cows so days in milk was used as the co-variable. In Table 3, the d . f . ' s , the mean 2 squares, the F-values and the R are presented for the main effects of Age, Breed, DOPC and for the Age X Breed, Age X DOPC and Breed X DOPC interactions. The three co-variables of DP linear, quadratic and cubic are also presented. Also presented in all the three tables are the Total S. S., the Total Reduction S. S, and the portion of the Reduction S. S. allocated to the effects fitted in the statistical model. TABLE 1 ANALYSIS OF VARIANCE Body Weights for the All Cow Group Source of Variation d.f. Mean-Square F-Value R-Square % Main Effects Age Breed Interaction Age X Breed Co-Variables within subclass Days Milked (DM+DM2+DM3) Residual 12 6259 ,5 1753326 4250140 2246717 1584313 18807 93.2* 225.9* 13.1* 84.2* 0-9 2.3 0.1 10.2 63.4 Total S.S. = 185641300 Total Reduction S.S. = 67925720 (36.6% of the Total S.S.) The portion of the Reduction S.S. allocated is 13.5% of the total S.S. * Significant at 0.05 26 TABLE 2 ANALYSIS OF VARIANCE Body Weights for the Open Cow Group Source of Variation d.f. Mean-Square F-Value R-Square % Main Effects Age 1 1909432 107.5* 1.5 Breed 1 4982284 280.5* 3.8 Interaction Age X Breed 1 2189 0.1 0.0 Co-Variables within subclass Days Milked (DM + DM2) 8 475365 26.7* 2.9 Residual 4293 17757 58.0 Total S.S. = 131262000 Total Reduction S.S. = 55030860 (41.9% of the Total S.S.) The portion of the Reduction S.S. allocated is 8.2% of the total S.S. * Significant at 0.05. TABLE 3 ANALYSIS OF VARIANCE Body Weights for the Pregnant Cow Group Source of Variation d.f. Mean-Square F-Value R-Square % Main Effects Age 1 402431 33.6* 0.8 Breed 1 73136 6.1* 0.1 DOPC 1 709246 59.3* 1.3 Interactions Age X Breed 1 41119 3.4 0.1 Age X DOPC 1 40750 3.4 0.1 Breed X DOPC 1 20290 1.7 0.0 Co-Variables within subclass Days pregnant (DP+DP2+DP3) 24 685076 57.3* 30.8 Residual 1939 11955 43.5 Total S.S. =53304980 Total Reduction S.S. = 30123860 (56.5% of the total S.S.) The portion of the Reduction S.S. allocated is 33.2% of the total S.S. * Significant at 0.05 Age Group The overall daily BW means and Age group least square constants with their standard errors and least square means are presented in Table 4 for all the groups studied. Age 1 and Age 2 cows differ significantly in BW in all the groups, towever, the effects of Age on BW in the All Cow and the Pregnant Cow groups were less than the Open Cow group. Age accounted for 0.9%, 1.5% and 0.8% of the BW variation in A l l , Open and Pregnant Cow groups res-pectively. TABLE 4 OVERALL DAILY BW MEANS, AGE GROUP LEAST SQUARE CONSTANTS AND LEAST SQUARE MEANS1 Least Square Constants ± S.E. Group Overall Means Age 1 Age 2 All Cow 1285.75 ± 1.73 -71.61 ± 7.40* (3542) 71.61 ± 7.42* (2733) 1214.133 1357.369 Open Cow 1276.30 ± 2.03 -66.56 ± 6.42* (2306) 66.56 ± 6.42* (1999) 1209.73a 1342.869 Pregnant Cow 1283.89 ± 2.46 -62.53 ± 10.78* (1236) 62.53 ± 6.42* (734) 1221.359 1346.42a 1 Units are in pounds () Number of observation in each cell * Constants differed significantly from zero by t-test a Age group least square means 30 Breed Groups The overall daily BW means and Breed group.least square constants with their standard errors and least square means are presented in Table 5 for all the groups. Analysis of variance showed that the Hand the CB differ s ignif i -cantly in daily BW in all the groups, however, the contribution of breed to BW difference in the Pregnant Cow group was very l i t t le . The effect of Breed accounted for 2.3%, 3.8% and 0.1% of the total variation in BW for A l l , Open and Pregnant Cow groups respectively. TABLE 5 OVERALL DAILY BW MEANS, BREED GROUP LEAST SQUARE CONSTANTS AND LEAST SQUARE MEANS1 Least Square Constants ± S.E. Group Overal1 Means Hoi steins Crossbreeds All Cow 1285.75 ± 1. 73 111.50 ± 7.42* (3440) -111.50 ± 7.42* (2835) 1397.25a 1174.25a Open Cow 1276.30 ± 2. 03 107.52 ± 6.42* (2640) -107.52 ± 6.42* (1665) 1383.82a 1168.77a Pregnant Cow 1283.89 ± 2. 46 -24.67 ± 9.97* (800) 24.67 ± 9.97* (1170) 1259.22a 1308.55a 1 Units are in pounds () Number of observation in each cell * Constants differed significantly from zero by t-test a Breed group least square means 32 Number of Days Open at Conception (DOPC) The analysis of variance presented in Table 3 showed that DOPC was 2 a significant source of variation affecting BW. The R or coefficient of determination associated with daily BW attributable to DOPC was 1.3%. The DOPC BW least square constants were -79.61 + 10.34 for those cows conceiving on or before 123 day after Cl and 79.61 ± 10.34 for those conceiving later than day 123. The corresponding group means calculated by adding the appropriate least square constants to the overall means were 1204.28 pounds for the early conceivers (D0PC1 ) and 1363.49 pounds for the late conceivers (D0PC2 ). Age by Breed Interaction Age by Breed interaction for the All Cow group was significant and accounted for 0.1% of the variation. This interaction was not significant for the Open Cow group and the Pregnant Cow group. The Age by Breed interaction least square constants and subclass means are given in Table 6. Subclass means as a deviation from the over-all mean are calculated by summing the appropriate least square constants for the effects of Age, Breed and Age by Breed interaction. To compare the subclass means among themselves, student Newman Keuls (SNK ) test was applied (Zar, 1974). The subclass means were found to be significantly different from each other. TABLE 6 AGE BY BREED INTERACTION LEAST SQUARE CONSTANTS AND SUBCLASS MEANS AS A DEVIATION FROM THE OVERALL MEAN ALL COW GROUP DAILY BW1 Least Squares Subclass Means Subclass Constants ± S.E.* ± S . E . a b Age 1 H -26.86 ± 7.42 -13.03 + 14.92 (1934) Age 1 CB 26.86 + 7.42 -156.25 ± 12.41 (1608) Age 2 H 26.86 + 7.42 209.97 + 13.05 (1506) Age 2 CB -26.86 + 7.42 -66.75 + 11.66 (1227) 1 The overall mean is 1285.75 ±1.73 a Subclass means are calculated as a deviation from the overall mean b Means in this column are significantly different from each other by SNK test () Number of observation in each cell * Constants in this column differed significantly from zero by t-test 34 Age by DOPC and Breed by DOPC Interaction These two interactions were not significant sources of variations for the Pregnant Cow group. Co-Variables Number of Days Milked (DM ) DM was a significant source of variation for the All Cow and the Open Cow groups and i t accounted for 10.2% and 2.9% of the variation in BW. The subclass partial regression coefficients of BW for both the groups are reported in Table 7 and graphically compared for the Open Cow group in Figure 1. The coefficients are the increase or decrease in daily BW per day milked above the intercept where days milked linear, quadratic and cubic are zero. TABLE 7 SUBCLASS PARTIAL REGRESSION COEFFICIENTS DAILY BW ON DM1 ALL COW AND OPEN COW GROUPS All Cow Group Open Cow Group Subclass DM ± S.E. DM2±S.E, DM3±S.E. DM±S.E. DM2±S.E. Age 1 H - -0.48" 2±0.18' Age 1 CB Age 2 H Age 2 CB 1.90 ± 0.46* -0.15 _ 1±0.31 0.18" 4±0.35" 5* -1.38±0.21* 0.69~ 2±0.85~ 3 0.45±0.20* 0.24" 2 ±0.72" 3 * -4 -5* 0.37 4±0.62 ° 1 Units are in pounds * Only significant regression coefficients are reported Number of Days Pregnant (DP ) DP was a significant source of variation accounting for 30.8% of the BW changes. The subclass partial regression coefficients are reported in Table 8 and graphically compared in Figures 2 and 3. The coefficients are the increase or decrease in BW per day pregnant above the intercept where DP linear, quadratic and cubic are zero. TABLE 8 SUBCLASS PARTIAL REGRESSION COEFFICIENTS DAILY BW ON DP1 PREGNANT COW GROUP Subclass DP DP2 DP3 Age 1 H Early Conceiving 5.04 + 1.08* -0.58" 1 + -1* 0.12 1 0.20" •3 + 0.35 4 Age 1 H Late Conceiving 3.62 + 1.25* -0.61" 1 + -1* 0.13 1 0.29" •3 + 0.41 H Age 1 CB Early Conceiving - - -Age 1 CB Late Conceiving -3.79 + 1.02* - -Age 2 H Early Conceiving -0.62"1 --1* 0.24 1 -Age 2 H Late Conceiving -6.67 1.53* + -Age 2 CB Early Conceiving - 0.46"1 + -1* 0.16 1 -0.13" •3 + 0.49 H Age 2 CB Late Conceiving -8.90 + 1.04* 0.88"1 + -1* 0.13 1 -0.22" •3 + 0.46 * 1 Units are in pounds * Only significant regression coefficients are reported CO 38 Daily Milk Weight (MW) A summary of the results of the analysis of variance of daily MW for the A l l , Open and Pregnant Cow groups is given in Tables 9, 10 and 11. In Tables 11 and 12 the d . f . ' s , the mean squares, the F-values and 2 the R are presented for the main effects of Age, Breed and the Age by Breed interaction. The three co-variables of DM linear, quadratic and cubic are given for All Cow group and also given are the two co-variables of DM linear and quadratic for the Open Cow group. In Table 13, the d . f . ' s , the mean squares, the F-values and the R are presented for the main effects of Age, Breed, DOPC and for the Age X Breed, Age X DOPC and Breed X DOPC interactions. The three co-variables of DP linear,, quadratic and cubic are also presented. Also presented in the three tables are the Total S.S., the Total Reduction S.S. and the portion of the Reduction S.S. allocated to the effects fitted in the statistical model. TABLE 9 ANALYSIS OF VARIANCE Milk Weights for the All Cow Group Source of Variation d.f. Mean-Square F-Value R-Square % Main Effects Age 1 1179 9.3* 0.1 Breed 1 2316 18.2* 0.1 Interaction Age X Breed 1 49 0.3 0.0 Co-Variables within subclass Days Milked (DM+DM2+DM3) 12 48707 383.9* 35.6 Residual 6259 126 48.4 Total S.S. =1640194 Total Reduction S.S. = 846178 (51.6% of the Total S.S.) The portion of the Reduction S .S. allocated is 35.8% of the total S.S. * Significant at 0.05. 40 TABLE 10 ANALYSIS OF VARIANCE Milk Weights for the Open Cow Group Source of Variation d.f. Mean-Square F-Value R-Square % Main Effects Age 1 5250 34.1* 0.5 Breed 1 2985 19.6* 0.3 Interaction Age X Breed 1 244 1.6 0.2 Co-Variabl es within subclass Days Milked (DM+DM2) 8 24073 158.2* 18.4 Residual 4293 152 62.3 Total S.S. = 1049249 Total Reduction S.S. = 396110 (37.7% of the Total S.S.) The portion of the Reduction S.S. allocated is 19.2% of the Total S.S. *Significant at 0.05. TABLE 11 ANALYSIS OF VARIANCE Milk Weights for the Pregnant Cow Group Source of Variation d.f. Mean-Square F-Value R-Square % Main Effects Age 1 3786 78.6* 1.3 Breed 1 1802 37.4* 0.6 DOPC 1 14 0.3 0.0 Interactions Age X Breed 1 11 0.2 0.0 Age X DOPC 1 3503 72.7* 1.2 Breed X DOPC 1 6 0.1 0.0 Co-Variables within subclass Days Pregnant (DP+DP2+DP3) 24 7328 152.2* 61.4 Residual 1939 48 32.6 Total S.S. = 286401 Total Reduction S.S. = 193045 (67.4% of the Total S.S.) The portion of the Reduction S.S. allocated is 64.5% of the total S.S. *Significant at 0.05. Age Group The overall daily MW means and Age group least square constants with their standard errors and least square means are presented in Table 12 for all the groups studied. Analysis of variance showed significant difference between Age 1 and Age 2 in all the groups studied, however, the fraction of the total sum of squares due to age was small in all cows. Age accounted for 0.1%, 0.5% and 1.3% of the MW variation in A l l , Open and Pregnant Cow groups respectively. TABLE 12 OVERALL DAILY MW MEANS, AGE GROUP LEAST SQUARE CONSTANTS AND LEAST SQUARE MEANS1 Group Overall Means Age 1 Least Square Constants ± S.E. Age 2 All Cow 60.08 ± 0.14 -1.86 ± 0.61* (3542) 1.86 ± 0.61* (2733) 58.22a 61.933 Open Cow 64.45 ± 0.19 -3.49 ± 0.59* (2306) 3.49 ± 0.59* (1999) 60.96a 67.94a Pregnant Cow 51.12 ± 0.16 -6.07 ± 0.68* (1236) 6.07 ± 0.68* (734) 45.05a 57.18a 1 Units are in pounds () Number of observation in each cell * Constants differed significantly from zero by t-test a Age group least squares means 44 Breed Group The overall daily MW means and the Breed group least square constants with their standard errors and least square means are presented in Table 13. Analysis of variance showed that the H and the CB differ s ignif i -cantly in all the groups. However, the fraction of the total sum of squares accounted for the variation in MW due to Breed was small in all the groups. The effect of Breed accounted for 0.1%, 0.4% and 0.6% of the total variation in MW for A l l , Open and Pregnant Cow groups. TABLE 13 OVERALL DAILY MW MEANS, BREED GROUP LEAST SQUARE CONSTANTS AND LEAST SQUARE MEANS1 Least Square Constants + S.E. Overall Group Means Holstein Cross Breeds All Cow 60.06 + 0.14 2.60 ± 0.61* (3440) -2.60 ± 0.61* (2835) 63.37a 58.17a Open Cow 64.45 + 0.19 2.63 ± 0.59* (2640) -2.63 ± 0.59* (1665) 67.089 61.82a Pregnant Cow 51.12 + 0.16 -3.87 ± 0.63* (800) -3.87 ± 0.63* (1170) 47.25a 59.99a 1 Units are in pounds () Number of observation in each cell * Constants differed significantly from zero by t-test a Breed group least square mean. 46 Number of Days Open at Conception The analysis of variance presented in Table 11 showed that DOPC was not a significant source of variation affecting daily MW. Age by Breed Interaction The Age x Breed interaction was not a significant source of variation for any of the three groups studied. Age by DOPC Interaction The Age X DOPC interaction was a significant source of variation and accounted for 1.2% of the variation. The Age by DOPC interaction least square constants and subclass means for daily MW are presented in Table 14. Subclass means as a deviation from the overall mean are calculated by pulling the appropriate least square constants for the effect of Age, DOPC and Age X DOPC interaction. To compare the subclass means among themselves, student Newman Keuls (SNK))test was applied (Zar, 1974 ). Two of the subclass means were found not to differ significantly from each other. Breed by DOPC Interaction This interaction is not a significant source of variation. TABLE 14 AGE BY DOPC INTERACTION LEAST SQUARE CONSTANTS AND SUBCLASS MEANS AS A DEVIATION FROM THE OVERALL MEAN PREGNANT COW GROUP DAILY MW1 Least Square Subclass Means Subclass Constants ± S.E. ± S .E . a Age 1 Early Conceiving -5.92 + 0.69* 12.35 + 1.00 (682) Age 1 Late Conceiving 5.92 + 0.69* 0.21 + 1.19b(554) Age 2 Early Conceiving 5.92 + 0.69* 11.63 + 1.43b(347) Age 2 Late Conceiving -5.92 + 0.69* 0.51 + 0.84 (387) 1 The overall mean 51.12 ± 0.16 lbs a Subclass means are calculated as a deviation from the overall mean b Means in the same column superscripted by the same letter are not significantly different from each other by SNK test () Number of observation in each cell * Constants differed significantly from zero by t-test 48 Co-Variables Number of Days Milked DM was a significant source of variation for the All and the Open Cow groups and i t accounted for 35.6% and 18.4% of the variation in MW. The subclass partial regression coefficients of MW on DM for both the groups are presented in Table 15 and graphically compared for the Open Cow group in Figure 4. The coefficients are the increase or decrease in daily MW per day milked above the intercept where days milked linear, quadratic and cubic are all zero. TABLE 15 SUBCLASS PARTIAL REGRESSION COEFFICIENTS DAILY MW ON DM1 ALL COW AND OPEN COW GROUPS Subclass DM±S.E. All DM2±S.E. Cow Group DM3±S.E. Open Cow Group DM+S.E. DM2±S.E. Age 1 H -0.40' 3 ±0.15" 3*' 0.76" 6±0.27" 6* O.SS-^O^O" 1* -0.22" 3 ±0.79" .4* Age 1 CB -0.38" 3 ±0.13" 3* 0.48" 6±0.19" •6* -0.49"3+0.84" .4* Age 2 H o.mo.25"1* -0.15~2±0.16~ 3* 0.18" 5±0.30" •6* O.lliO.18"1* -0.72" 3 ±0.67" .4* Age 2 CB 0.16±0.38 _ 1* -0.14" 2 ±0.26" 3* 0.20"5±0.51" • 6* -0.43' 3 ±0.96" .4* 1 Units are in pounds * Only significant regression coefficients are reported 50 Number of Days Pregnant (DP ) DP was a significant source of variation accounting for 61.4% of the daily MW changes. The subclass partial regression coefficients are reported in Table 16 and graphically compared in Figures 5 and 6. The coefficients are the increase or decrease in MW per day pregnant above the intercept where DP linear, quadratic and cubic are all zero. TABLE 16 SUBCLASS PARTIAL REGRESSION COEFFICIENTS DAILY MW ON DP1 PREGNANT COW GROUP Subclass DP 2 DP^ 3 DP Age 1 H Early Conceiving 0.45±0.68" •1* -0.46" 2±0.73" • 3* 0.10" 4±0.22" •5* Age 1 H Late Conceiving -0.27±0.79" •1* 0.23" 2±0.84" •3 -0.81" 5±0.26' •5* Age 1 CB Early Conceiving 0.17±0.51" •1* -0.33" 2±0.66' • 3* 0.11" 4±0.23" •5* Age 1 CB Late Conceiving -0.69±0.65' •1* 0.72" 2 ± 0 . 7 T -3* -0.24" 4±0.22" -5* Age 2 H Early Conceiving - -0.31" 2±0.10" • 2* 0.92" 5±0.39" -5* Age 2 H Late Conceiving - - -Age 2 CB Early Conceiving - -0.33" 2±0.10' -2* 0.15" 4±0.31' -5* Age 2 CB Late Conceiving -0.28±0.66' -1* - -1 Units are in pounds * Only significant regression coefficient are reported 52 PREGNANT COWS A separate analysis was done for the Pregnant Cows where DOPC was treated as a dependent variable instead of as a discreet variable. This analysis was done to find out i f the Breed groups studied differed in the number of days they remain open following calving to conception. In the previous analysis by taking DOPC as a discreet variable it was assumed that the two Breed groups would respond equally to breeding and thus have an equal chance of becoming pregnant in less than 123 days or staying open longer but becoming pregnant before 270 days after CI. A summary of the results of the Analysis of Variance for this analysis is reported in Table 17. Age, Breed, and Age by Breed interaction were all significant accounting for 0.3%, 0.4% and 0.9% of the variation res-pectively. The least square constants for Age groups are -2.15 days for Age 1 cows and 2.15 days for Age 2 cows. The least squares values for the Breed groups were 2.37 days for the H and -2.37 days for the CB. The overall least square mean was 128.09 days. The least square mean for the Age groups were 125.94 days for Age 1 cows and 130.24 days for Age 2 cows. The least square means for the two Breed groups were 130.46 days for H and 125.72 days for CB. The Age by Breed interaction least square constants and subclass means as a deviation from the overall mean are reported in Table 18. SNK test showed that the Age 1 CB subclass mean was significantly different from the rest. TABLE 17 ANALYSIS OF VARIANCE Number of Days Open at Conception for Pregnant Cow Group Source of R-Square Variation d.f. Mean-Square F-Value % Main Effects Age 1 8100 5.8* 0.3 Breed 1 9792 7.1* 0.4 Interaction Age X Breed 1 24406 17.7* 0.9 Residual 1374 97.8 Total S.S. = 2763242 Total Reduction S.S. = 60566 (2.2% of the total S.S.) The portion of the Reduction S.S. allocated is 1.6% of the total S.S. *Significant at 0.05. TABLE 18 AGE BY BREED INTERACTION LEAST SQUARE CONSTANTS AND SUBCLASS MEANS DOPC1 Subclass Least Square Constants Subclass Means ± S.E. ± S.E. Age 1 H 3.74 + 0.89 3.95 ± 1.51a (530) Age 1 CB -3.74 + 0.89 -8.26 + 1.83 (706) Age 2 H -3.74 + 0.89 0.78 + 1.33a (270) Age 2 CB 3.74 + 0.89 3.52 + 1.44a (464) 1. Units are in pounds a . Means in the same column superscripted by the same letter are not significantly different from each other by SNK test b Subclass means are calculated as a deviation from the overall mean () Number of observation in each cell DISCUSSION The data from the U.B.C. Research herd #2 used in these analyses have unavoidable disadvantages, from an analytical point of view, being non-orthogonal and unbalanced. This places some limitations on the inter-pretation of the results away from this herd. Nevertheless, some greater understanding of the relationships studied is possible and this is of particular value since the U.B.C. herd has unique features. 1. It is one of the highest producing herds in Canada, and, 2. It offers an opportunity for comparison of holstein and Crossbred Ayrshire hoi stein cows, not normally available elsewhere. It should be noted that the limitations of the data resulted partly in relatively small portion of the Reduction sums of squares being allocated, these are summarized below. Reduction Sums of Squares % total fitted % alloca ted BW MW BW MW All Cow group 36.6 51.6 13.6 33.8 Open Cow group 41.9 37.7 8.2 19.2 Pregnant Cow group 56.5 67.4 33.2 64.4 Out of the total fitted in BW the linear model accounted for 33.0%, 40.9% and 43.1% of the variation in A l l , Open and Pregnant Cow groups o respectively. The corresponding linear model R for MW were 48.5%, 35.2% and 62.5% percent for A l l , Open and Pregnant Cow groups. 56 Age The primary factor influencing BW of cows independent of breed were age, Gains (1940 ), and the cows produce more milk as they grow older. In all the three groups studied, the older cows were heavier and produced more milk than the younger cows. Among A l l , Open and Pregnant the Age 2 cows weighed 143.22 lbs, 133.12 lbs and 125.06 lbs more, and produced 3.72, 6.98 and 12.14 lbs more milk daily than the Age 1 cows. Breed According to Erb and Ashworth (1961 ) Hoi stein show greater increase in BW and MW independent of Age when compared with Ayrshires or Guernseys. The finding for AH and Open Cows from this study is in agreement with their work. Among All and Open Cows the H weighed more by 223.00 and 215.04 pounds and produced 5.20 and 5.26 pounds more milk respectively than the CB. Surprisingly, among the Pregnant cows the H were lighter by 49.39 pounds and produced 7.74 pounds less milk daily than the CB. This may be due to the unusually light sample of H cows who happened to be preg-nant at the time this study was made. DOPC The fact that some cows required more than 123 days after CI to become pregnant may cause someone to ask i f there was a breeding problem of some kind in the herd. This question will even be more relevant when longer days to conception are related to heavier BW. Cows are known to undergo a short period of temporary infert i l i ty 57 mainly imposed by poor nutrition during long winter feeding. This is usually corrected when the feeding condition is improved, such as when the cows are put on spring pasture (Cacardi and Magnai,. 1964 ). fafaz (1959 ) poiiints out that severe environmental stresses result in steri l ity which is a normal adaptation syndrome. According to Amir and Kali (1974) the symptoms of temporary infer-t i l i t y are: 1. Delayed oestrus 2. Silent heat 3. Ineffective insemination, and 4. Late return to oestrus. Delayed First Oestrus Under normal farm situations the f i rst oestrus appears 60-90 days post-patrum which is the normal insemination time. towever, according to Warnick (1955) cases of delayed oestrus in cattle are rare, and as cows are usually inseminated 60-90 days after paturation, the infrequent delay of oestrus does not constitute an important obstacle to fert i l i ty on the farm. But according to Wang (1964), under severe conditions delayed f i rst oestrus is common among individual cows or in the herd as a whole. This severe condition was not applicable to this herd. The animals were well fed which eliminates delayed oestrus as a cause for delayed breeding. Silent Hieat This is defined as an ovulation without external signs of heat. It 58 is known that one third to two thirds of all ovulations come under this category. According to Kidder ejt al_. (1952 ), silent heats are very common. In some cows one silent heat takes place before the f i rst apparent oestrus and the factors that determine the number of silent heats are not fully known (Burns et al_., 1954 ). Ineffective Insemination It has been reported by Amir and Kali (1974) that 30-40% of all insem-inations are ineffective and cows return to service. They said there is a definite pattern between the number of returning cows and the various lengths of the interval between ineffective inseminations and return to service. They further reported after ineffective service only half of the cows return to service, during the f i rst 30 days after insemination, one third of the cows return after 30-60 days and the rest later than 60 days. Hate'.'Return to Oestrus Reproductive failures are due to lack of ferti l ization, failure of implantation and early embryonic mortality, followed by oestrus at normal interval and late embryonic mortality causing late return to service. According to Edy (1966 ) late returns are related to early embryonic deaths which are one of the important causes of inferti l i ty in cattle. Castle (1963) gives equal economic importance to failure to conceive and to embryonic mortality. Based on the above brief background one can assume that those cows staying open longer in this study were heavier simply because they were gaining weight as all cows do after a certain period in lactation. It may 59 also be that they were repeat breeders that required more than one insem-ination to get pregnant. Low conception to f i rst service rates in dairy herds where oestrus cycles appear to be occurring normally and animals are in reasonable body condition, can be associated with a recovery of BW during early lactation. King (1968) on Jersey herds in New Zealand, observed that this syndrome was present and that the infertile cows were those which lost the most weight between calving and mating and/or were s t i l l losing weight at the time of mating. The mean BW of ferti le cows was rising at the time of mating. King (1968 ) working with Ayrshire cows showed that the percentage of cows which conceived rose as the BW rise increased. From the present work i t is not quite clear for certain whether lac-tating cows became pregnant as a result of rising BW. It was also not clear that those with DOPC greater than 123 days were usually heavier during pregnancy. They required longer time to get pregnant. This might have given them the additional advantage of more time to put on more BW than the early conceivers. Recent work by Hodges (1976 ) demonstrates that DOPC in lactating cows is affected not by the level of BW, but by the rate of BW change per day. With DOPC as the dependent variable the younger cows conceived 4.3 days earlier than the older ones. One or more factors might have given the younger cows their earlier start in pregnancy over the older cows. These are as follows: 1. Earlier conception may be because, being younger in age, they lost less weight for a shorter period. 2. They may have less reproductive problems compared to the older cows, having calved less often with fewer chances for injury. 60 3. Management system may dictate early breeding of young animals. The CB conceived 4.74 days earlier than the H . This could be a hetrotic effect, which is defined as the increased level of performance of the offspring as compared to the average of the parental types (Rice, 1967 ). Heterosis is known to improve fert i l i ty and to shorten DOPC (Mb 11 en et al/, 1969 ). As far as management goes i t is interesting to notice with the ex-ception of the so-called high producing 15 cows, all the animals in the herd were treated exactly alike. There was no individual feeding involved outside the milking parlor. This might have helped the strong and vigorous cows to obtain more food than individuals of a less forceful nature. If this was the case i t could have been the increased food consumption which gave the younger CB cows an early start in pregnancy which could be a direct result of increased BW resulting from high breed vigor in Pregnant Cow group. The statistical analysis confirms that while the BW of the cows was affected by DOPC, the daily MW remained unaffected. However, the low daily yield obtained from the younger cows may indicate that these cows are utilizing the food they consume for body development rather than milk synthesis. DM The result of this investigation indicates that the different sub-classes of open cow groups behave differently in BW and MW through their Open period. The Age 1 H cows started to drop in BW after CI which continued to about day 100 after which time they started picking up again. 61 The Age 2 K cows maintained more or less a constant weight up to day 100 and then started to show a decline. Why they did not drop in BW as cows normally do after calving, is diff icult to explain from this data (see Fig. 1 ). however, the decline in BW from day 100 onwards seems to be consistent with the Age 2 early and late conceiving Hwhich continued to drop in BW until about day 100 in pregnancy (see Fig. 2 ). In older Hvopen cow groups, one thing was apparent, growth had come to an end. This lack of growth stimuli when coupled with high milk yield puts the cows under more stress thus diverting more and more of the nutrients to milk synthesis at the expense of BW increase. This continued until pregnancy, even though the onset of pregnancy resulted in reduced milk yield in the older H (see Fig. 5). The effect of DM on BW of open cow group was more apparent when looking at MW (see Fig. 4). The Age 1. H. showed a continuous decline in milk yield from calving and they were the lowest in the group which is in agreement with the BW result. The low milk yield in this group was not unexpected knowing the fact that the udder (mammary gland ) of a young cow will not be as fully developed as that of an older cow of 5 or more years of age. The CB of both ages increased in BW together after Cl and they were intermediate in their milk yield between the younger and the older H. (see Figs. 1 and 4). For these groups both the linear and quadratic regression coefficients associated with BW and the linear regression coefficient associated with MW were not significant (see Tables 7 and 15). OPEN COWS BODY WEIGHT figure 1 a b c d Age1 H Age 1 CB Age 2 H Age.2 CB i— 50 100 150 200 Number of days in milk 63 PREGNANT H BODY WEIGHT figure 2 to sz CJ) TJ O .Q 1600 1400 S 1200 1000 1 Age1 early conceiving 2 Age1 late conceiving 3 Age2 early conceiving 4 Age 2 late conceiving 50 100 150 200 Number of days pregnant 64 DP In the pregnant cow group among the H's, the younger cows appeared to be heavier and were gaining weight from the day of conception which may partly explain the tendency to grow (see Fig. 2). This group may also be composed of cows with longer DOPC which conceived when gaining weight. The older H cows continued to drop in weight even after conception. At around day 75 after conception, the late conceivers started to gain some weight indicating again pregnancy and increased BW go together i f preceded by longer DOPC. The early conceiving old H's were not making any signif i -cant increases up to day 125. (None of the regression coefficients were significant for this group, see Table 8). Among the CB's the early conceivers of both ages were heavier than the late conceivers (see Fig. 3 ). Among the early conceivers the older CB started to put on weight after day 50. The late conceivers started to gain in BW after day 100 in pregnancy. In milk weight the Age 1 early conceiving H were the high producers over the period regressed (see Fig. 5). Among the CB's the early con-ceivers of both ages produced more milk after conception than the late conceivers (see Fig. 6) confirming the Age X DOPC interaction detected by the analysis of variance. 65 PREGNANT CB BODY WEIGHT figure 3 1600 co 3> 1400 TJ O n 8 1200 10001 1 2 3 4 Age1 early conceiving Age1 late conceiving Age2 early conceiving Age2 late conceiving 5 0 100 150 2 0 0 Number of days pregnant OPEN COWS MILK WEIGHT figure a b c d Age1 H Age1 CB Age 2 H Age 2 CB 40 50 100 150 200 Number of days in milk PREGNANT H MILK WEIGHT figure 5 80 1 Age1 early conceiving 2 Age1 late conceiving 3 Age2 early conceiving 4 Age2 late conceiving to I 60 E a 40 20 50 100 150 200 Number of days pregnant PREGNANT CB MILK WEIGHT figure 6 80 CO _Q T Age 1 early conceiving 2 Age 1 late conceiving 3 Age 2 early conceiving 4 ' Age 2 late conceiving I 60 a 40 20 50 100 150 200 Number of days pregnant 69 CONCLUSION Within the scope of the data used in this study, the following observations and conclusions were made: 1. In the Pregnant Cow group the number of days open before concep-tion has no bearing on the amount of milk produced during the period following conception. On the other hand, the number of days open at conception was found to be associated with daily body weight of the cows. Those cows which were open longer than 123 days were heavier during preg-nancy than those who conceived earlier than 123 days by 159.21 lbs. This was due to the observed result that pregnant or not a cow begins to gain weight after a certain period of lactation. 2. With advancing stage of lactation the younger Hblstein cows' body weight declined up to day 100 in lactation and after that started rising. The daily milk weight for this group declined steadily starting from recording, 4 days after calving and i t was the lowest in the group. The younger cows dropped in body weight faster and started regaining faster than the older Hblsteins. With increasing number of days in milk over the period studied, the older tolsteins were producing at a higher level. The body weight of both younger and older crossbreds was not affected by the stage of lactation within the period studied. Their milk production over the period studied decreased with advancing stage of lactation, towever, the linear regression coefficient was not significant for number of days in milk. Number of days in milk accounted for 2.9% and 18.4% of the body weight and milk weight variation respectively. 70 3. Pregnancy was found to have a depressing effect on daily milk yield. This was in agreement with the work of Ragsdale et_ al_. (1924 ) and that of Smith and Legates (1962 ) which indicated that milk production declines 16 to 20 weeks following conception. With declining milk weight there was an increase in body weight. The R accounted for by days preg-nant for body weight and milk weight on daily basis for the period studied were 30.8% and 61.4% respectively. 4. In A l l , Open and Pregnant Cow groups the older cows were heavier by 143.22, 133.12 and 125.06 lbs and produced 3.72, 6.98 and 12.14 lbs more milk daily than the younger cows respectively. This result is in agreement with that of Puri and Sharma (1965) who showed that the amount of milk produced in subsequent lactations is higher when compared to f i rst lactation yield. 5. Among All and Open Cow groups the Holsteins weighed more by 223.00 and 215.04 lbs and produced 5.20 and 5.26 lbs more milk respectively when compared with the crossbreds. This result agrees with Hollen et a l . (1969) who showed that average body weight and milk yield for Holsteins to be greater compared with Holstein sired crossbreds. Among the Pregnant Cows the Holstein were lighter by 49.39 lbs and produced 7.74 lbs less milk than the crossbreds. This is assumed to be due to the unusually light sample of Holstein cows which happened to be pregnant when this study was made, since the result has no other apparent explanation. 6. The Crossbreds were found to have 4.74 less open day to concep-tion than the Holsteins. This indicates more efficient reproductive per-formance by the crossbreds and is in agreement with Hollen e_t al_. (1969 ) findings and tends to confirm the heterotic effect'iupon reproductive 71 performance. 7. The high positive (3.53 ± 1.44) subclass mean for the old cross-breds seems to indicate that the management was delaying their rebreeding after calving purposely. This seems to be in line with the management's plan to remove all the crossbreds from the herd. 2 8. From the R values obtained, days in milk and days pregnant seem to have the major effects upon body weight and milk yield. The other 2 factors studied and given above, even though all have smaller R , are statistically valid and biologically important. 72 LITERATURE CITED 1. Amir, S. and Kali, J . (1974). The relation between weight loss after calving and conception in the dairy cow. Dairy Sci. Handbook 7: 1-12. 2. Balch, C.C. and Line, C. (1957). Weight change in grazing cows. J . Dairy Res. 24: 11-19. 3. Bath, D.L.,- Ronning, M., Lofgreen, G.P. and Meyer, J.H. (1966). Influence of variation in ruminal contents upon estimates of body weight change of dairy cattle. J . Dairy Sci. 49: 830-834. 4. Bereskin, B. and Touchberry, R.W. (1966). Some relationships of body weight and age with f i rst lactation. J . Dairy Sci. 49: 869-873. 5. Brinks, J .S . , Clark, R.T., Kieffer, N.M. and Ouesenberry, J.R. (1962). Mature weight in Hereford range-cows-heritability, repeatability and relationship to calf performance. J . Anim. Sci. 21: 501-504. 6. Brody, S. (1945). 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Title | The effect of age, breed, days open, stage of lactation and pregnancy upon daily body weight and milk weight in lactation dairy cattle |
Creator |
Yegezu, Zegeye |
Publisher | University of British Columbia |
Date Issued | 1977 |
Description | The effect of age, breed, days open at conception, stage of lactation and pregnancy upon daily body weight and milk weight in lactating dairy cattle were studied. The cattle used in the study were those of the University of British Columbia Research Farm at Oyster River, B.C. 76 Holstein and 60 Holstein X Ayrshire crossbred milking cows at different stages of lactation and both either open or pregnant, were used in this study. The study was carried out during the months of May and June of 1975. The procedure involved weighing the animals daily after the afternoon milking and recording the daily milk yield along with the weight of each cow. The cows were classified into three groups, namely All, Open and Pregnant, the first group being a combination of the last two. For All and Open cows, the effect of age, breed, age by breed and number of days in milk on body weight and milk weight were investigated. For the Pregnant cows, the effect of age, breed, number of days open at conception, age by breed, age by number of days open at conception, breed by number of days open at conception and number of days pregnant on body weight and milk weight were investigated. Using the number of days open at conception as a dependent variable, the effect of age, breed and age by breed were investigated. The results showed that among All, Open and Pregnant cow groups, the older cows were significantly (P <0.05) heavier and were producing significantly (P <0.05) more milk than the younger cows. Age accounted for 0.9%, 1.5% and 0.8% of the body weight variation and for 0.1%, 0.5% and 1.3% of the milk weight variation in All, Open and Pregnant cow groups respectively. Among the All, Open and Pregnant groups, the older cows weighed more by 143.22, 133.12 and 125.07 lbs and produced 3.72, 6.98 and 12.14 lbs more milk per day respectively than the younger cows. Among All, Open and Pregnant groups, a significant breed effect (P <0.05) on body weight and milk weight was observed. In the three groups, breed accounted for 2.3%, 3.8% and 0.1% of the body weight variation and for 0.1%, 0.3% and 0.6% of the milk weight variation respectively. Among All and the Open cow groups the Hoi steins were heavier by 223.00 and 214.04 lbs and were respectively producing 5.20 and 5.26 lbs more milk daily than the crossbreds. Among the Pregnant cow group the Hoi steins were lighter by 49.33 lbs and produced 7.74 lbs less milk daily than the crossbreds. The number of days open at conception was a significant source of variation (P <0.05) affecting body weight but not milk weight in the Pregnant cow group. It accounted for 1.3% of the body weight variation. Those cows who stayed open longer than 123 days were heavier by 159.21 lbs than those who became pregnant earlier or on day 123 after calving. Age by breed for the All cow group was significant (P <0.05), accounting for 0.1% of the body weight variation. This interaction was not significant for body weight of the other two groups and for milk weight of all the three groups. Age by number of days open at conception was significant (P <0.05) for the Pregnant cow group milk weight and accounted for 1.2% of the variation. This interaction was not significant for body weight. Breed by number of days open at conception was not significant for the Pregnant cow group body weight and milk weight. Number of days in milk for All and Open cows had a significant (P <0.05) effect on both body weight and milk weight. It accounted for 10.2% and 2.9% of the variation in body weight and 35.6% and 18.4% of the variation in milk weight in All and Open cow groups respectively. In the Pregnant cow group, the number of days pregnant resulted in significant (P <0.05) effect on both body weight and milk weight. It accounted for 30.8% and 61.4% of the body weight and milk weight variation respectively. When number of days open at conception was used as a dependent variable, age, breed, and age by breed showed a significant (P <0.05) effect in the pregnant cow group. Age, breed, and age by breed accounted for 0.3%, 0.4% and 0.9% of the variation respectively. |
Subject |
Milk yield -- Case studies |
Genre |
Thesis/Dissertation |
Type |
Text |
Language | eng |
Date Available | 2010-02-22 |
Provider | Vancouver : University of British Columbia Library |
Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
DOI | 10.14288/1.0094217 |
URI | http://hdl.handle.net/2429/20725 |
Degree |
Master of Science - MSc |
Program |
Animal Science |
Affiliation |
Land and Food Systems, Faculty of |
Degree Grantor | University of British Columbia |
Campus |
UBCV |
Scholarly Level | Graduate |
AggregatedSourceRepository | DSpace |
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