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.
Responses of grasses to N fertilization are affected by cutting intensity although little is known regarding the interactions of these factors in warm‐season grasses. Pre‐cutting canopy height, herbage accumulation and changes in the nutritive value of warm‐season grasses in response to four different management strategies were assessed from October 2011 to September 2014. Treatments included two cutting intensities (70 vs. 50% depletion of canopy height set by 95% light interception), two N fertilization levels (zero vs. 300 kg N ha−1 year−1) and six perennial C4 grass species (Axonopus catharinensis; Cynodon spp. hybrid Tifton 85; Hemarthria altissima cv. Flórida; Megathyrsus maximus cv. Aruana; Paspalum notatum cv. Pensacola; and Urochloa brizantha cv. Marandu) grown in monoculture in a factorial experimental design. Canopy height varied among grass species, cutting intensity and N treatments, mainly among seasons, indicating that more than one management target (i.e. canopy height) existed throughout the plant growth cycle for each species. The largest herbage accumulation occurred in the N fertilization treatments for most species, regardless of cutting intensity. Nitrogen fertilization and 50% depletion of canopy height increased the leaf proportion and decreased the neutral detergent fibre content. Overall, N fertilization had a stronger positive impact than cutting intensity on the acid detergent fibre content, dry‐matter digestibility and crude protein content, but the magnitudes of the responses were species‐specific.
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.
Andropogon lateralis is a tall and highly plastic tussock-forming grass native from southern South America. It is a frequent component of Campos and Subtropical highland grasslands that often becomes dominant under lax grazing regimes. The aim of this work was to analyze the response of species diversity and forage production of a natural grassland dominated by A. lateralis to a wide range of grazing intensity. We hypothesized that species diversity and forage production would both peak at the intermediate canopy heights determined by grazing regimes of moderate intensity. A grazing experiment was conducted in a highland grassland with mesothermal humid climate at 922 masl (Atlantic Forest biome, Santa Catarina state, Brazil) that comprised 87 species from 20 families but had 50% of its standing biomass accounted by A. lateralis. Four pre-/post-grazing canopy heights—12/7, 20/12, 28/17, and 36/22 cm (measured on A. lateralis)—were arranged in a complete randomized block design with four replications, and intermittently stocked with beef heifers from October 2015 to October 2017. Andropogon lateralis cover decreased (from 75 to 50%), and species richness increased (15–25 species m−2) as canopy height decreased. Grazing intensity did not affect annual forage production (4.2 Mg DM ha−1). This natural grassland dominated by A. lateralis had a high capacity to adjust to grazing regimes of contrasting intensity, maintaining forage production stable over a wide range of canopy heights. However, to prevent losses in floristic diversity, such grassland should not be grazed at canopy heights higher than 28 cm.
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