The growth of crops in a mixture is more variable and difficult to predict than that in pure stands. Light partitioning and crop leaf area expansion play prominent roles in explaining this variability. However, in many crops commonly grown in mixtures, including the forage species alfalfa, the sensitivity and relative importance of the physiological responses involved in the light modulation of leaf area expansion are still to be established. This study was designed to assess the relative sensitivity of primary shoot development, branching and individual leaf expansion in alfalfa in response to light availability. Two experiments were carried out. The first studied isolated plants to assess the potential development of different shoot types and growth periods. The second consisted of manipulating the intensity of competition for light using a range of canopies in pure and mixed stands at two densities so as to evaluate the relative effects on shoot development, leaf growth, and plant and shoot demography. Shoot development in the absence of light competition was deterministic (constant phyllochrons of 32·5 °Cd and 48·2 °Cd for primary axes and branches, branching probability of 1, constant delay of 1·75 phyllochron before axillary bud burst) and identical irrespective of shoot type and growth/regrowth periods. During light competition experiments, changes in plant development explained most of the plant leaf area variations, with average leaf size contributing to a lesser extent. Branch development and the number of shoots per plant were the leaf area components most affected by light availability. Primary axis development and plant demography were only affected in situations of severe light competition. Plant leaf area components differed with regard to their sensitivity to light competition. The potential shoot development model presented in this study could serve as a framework to integrate light responses in alfalfa crop models.

The canopy height (CH) at 95% light interception (LI) is a valuable defoliation frequency strategy used to handle variability in herbage accumulation throughout the year, mainly in C4 grasses. Such a strategy has been adopted as an open pasture management index, but defoliation frequency and intensity remain unsolved issues for shade-grown forages. A field experiment was conducted for 2 years to determine the influence of tree canopy (Eucalyptus dunnii) shading and nitrogen availability (0 and 300 kg N ha–1 year–1) on CH at 95% LI of six perennial tropical forage species. The plots were cut at 95% LI, and the height of the residual sward was kept at 50% of the corresponding CH at 95% LI. The shade level ranged from ~40% at the beginning of the experiment to ~60% at the end of summer 2013. Variations in CH at 95% LI occurred because of shading and across seasons. The range of these variations was species-dependent. Overall, species growing under trees showed higher CH, except for Paspalum notatum and Megathyrsus maximus in the first year. There was a significant increase in the length of the sheaths and leaves, as well as a decrease in tiller density and leaf : stem ratio in plants growing under trees. Nitrogen also had an impact on CH; however, its application did not compensate the shade effect on CH. Therefore, our results suggest that greater CH should be considered in case of defoliated, shade-grown plants and that such strategy might change throughout seasons.

Os carboidratos constituem entre 70 e 80 % da dieta dos ruminantes e são fundamentais para o atendimento de suas exigências em energia, síntese de proteína microbiana, componentes do leite e manutenção da saúde animal. As gramíneas forrageiras tropicais apresentam, normalmente, baixos teores de proteína bruta, os quais estão associados a elevados teores de fibra, que pode representar cerca de 60 % da matéria seca consumida. A fibra representa a fração dos carboidratos de digestão lenta ou indigestível do alimento que ocupa espaço no trato gastrintestinal e, em função de sua concentração e digestibilidade, impõe limitações ao consumo voluntário de matéria seca, minerais e energia. A suplementação energética ou protéica pode suprir as necessidades em energia e proteína dos microrganismos e do ruminante, proporcionando condições favoráveis ao ambiente ruminal, maximizando o crescimento microbiano e a utilização dos compostos fibrosos da forragem. A sincronização entre as fontes de carboidratos e as de nitrogênio (N) maximiza a eficiência microbiana e a diminuição da perda de N em forma de amônia e da energia dos carboidratos, promovendo melhoria na digestão da MS e, especialmente, da fração fibrosa da forragem. Neste artigo são analisados os efeitos da suplementação energética e/ou protéica sobre a degradação e o consumo da fibra de gramíneas forrageiras tropicais.