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

Abstract

The investigations reported in the thesis were designed to discover the importance of leaf area remaining after defoliation for subsequent forage regrowth. In field trials 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 accompanied by a depression of the clover content in the stand. For more detailed investigations of the light use, leaf area index (LAI) and net photosynthesis relationships, orchardgrass (l2actylis 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 field and subjected to defoliation managements, similar to those in 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. Samples of the forage stands and attached soil were removed to the laboratory for the determination of net photosynthesis at a number of light energy (400-700 nm) levels. The LAI and dry matter of these forage stands and their components were then determined. From these data net photosynthesis at varying LAIs was compared between stands of contrasting managements and botanical composition. The results of these laboratory investigations showed that there were differences in the net photosynthesis-LAI response between the defoliation managements only in the pure orchardgrass stands. With orchardgrass stands defoliated at 1 inch the relationship between net photosynthesis and LAI was linear up to an LAI of 17. However, with orchardgrass stands defoliated at 3 inches the response was curvilinear and the optimum LAI was about 12. In the pure white clover stands there were no clearly defined optimum LAIs or difference in response between the stands with differing managements. The grass-clover forage stands showed an optimum LAI of 10 and the presence of some clover may have nullified an effect from the defoliation managements. The data show that the light interception-LAI theory was applicable to grass stands under lenient defoliation. The lower leaves, left after a defoliation, did not contribute greatly to the stands ability to assimilate carbon dioxide and finally they became "parasitic" resulting in the stand having an optimum LAI of about 12. When the defoliation was heavy there was no evidence of leaves becoming "parasitic". Frequency of defoliation at 1 inch had little influence on yield despite differences in the increase in net photosynthesis for a unit increase in LAI. Carbon dioxide fixation efficiency was calculated from the light energy response data. The efficiencies were similar to those published for similar material and showed furthermore only slight differences in light energy use between the forage stands of pure grass, pure clover and their mixture. The differences between the dry matter productivity of the forage stands caused by the management treatments appeared to be a result of an inability of the clover to produce a sufficiently high LAI to compete under the lenient defoliation.

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