The use of nitrogen fertilization in tropical grasslands is a strategy that may reduce the pressure of livestock production on biome areas with humid forests. The objective of this study was to assess the use of different doses of nitrogen in Guinea grass (Megathyrsus maximus (Jacq.) con. Mombasa) cultivated in the humid tropics. Doses of 0, 10, 20, 30, 40, and 50 kg of N ha−1 application−1 were applied during two consecutive years in experimental plots. The experimental area is located in the northeast of Pará, Brazil, with a predominantly tropical climate according to the Köppen classification. The morphogenic, production, and qualitative characteristics of the forage were evaluated. The morphogenic variables, rate of leaf appearance and rate of leaf elongation, in addition to daily accumulation of forage and crude protein content, increased with increasing doses of nitrogen. The morphological structure of the pasture was not modified. The number of cuts increased, whereas the number of days of recovery decreased, in the rainy season with increasing doses of nitrogen. The use of nitrogen fertilization does not alter the structure of the pasture; however, it improves the morphogenic, production, and chemical characteristics of Guinea grass.

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ResumoObjetivou-se com este trabalho avaliar o efeito de diferentes frequências de desfolhação em períodos fixos sob as características estruturais e produtivas do dossel de capim-Tanzânia (Panicum maximum Jacq) em condições climáticas Am no ano de 2014. O delineamento experimental foi em blocos casualizados com seis tratamentos correspondentes a seis frequências de desfolhação (14; 21; 28; 35; 42 e 49 dias) e cinco repetições e os dados foram submetidos a análise de regressão. A interceptação luminosa (IL) aumentou (P<0,05) de maneira assintótica com o aumento dos períodos de descanso, no qual a máxima interceptação de luz foi de 98% e ocorreu aos 42 dias de rebrotação. O índice de área foliar (IAF) e o acúmulo de matéria seca, aumentaram (P<0,05) linearmente variando de 3,6 a 7,1 e de 880,3 a 4051,7 kg.ha -1 de matéria seca de forragem, nas frequências de 14 a 49 dias, respectivamente. A altura variou de 44 a 119 cm nas frequências de 14 a 49 dias, respectivamente. A frequência de desfolhação que representa uma IL de 95% está relacionada ao período de descanso de 29 dias, em que o dossel de capim-Tanzânia apresentou altura em torno de 63 cm, IAF crítico de 5,1 e acúmulo de matéria seca de forragem de 2.160 kg.ha -1 . Palavras-chave: Área foliar; dossel; interceptação luminosa; massa seca; Panicum maximum. AbstractThe objective of the present study was to evaluate the effect of different frequencies of defoliation at fixed periods on the structural and productive characteristics of Tanzania grass (Panicum maximum) under Am climate conditions in 2014. The experimental design was in complete randomized blocks with six treatments corresponding to six frequencies of defoliation (14, 21, 28, 35, 42, and 49 days) and five repetitions. The data was submitted to regression analysis. The light interception (LI) increased (P <0.05) in an asymptotic way with increasing rest periods. The

Variability in climatic conditions of low-latitude tropical grass cultivation can affect forage production dynamics. Pasture ecosystems are complex and preferably studied from a multifactorial point of view through multivariate approaches. Therefore, in this study, we characterized different growing conditions for grasses of the Megathyrsus genus through studies conducted in tropical humid and semi-arid conditions. We applied principal component, canonical correlation, and discriminant function analyses to the measurements of agronomic and agrometeorological variables in six studies with Guinea and Massai grasses. The principal component analysis, through the climatic characterization by the first principal component, reflects the contrast between water availability and nitrogen variables and energy supply. Agronomic characterization occurred through the distinction between the density of tillers, forage accumulation, and increase in height, versus the accumulation of stems and dead material. The canonical correlation analysis generated a correlation coefficient of 0.84 between the agronomic and agrometeorological variables. There was a contrast between the dead material accumulation and the other agronomic variables, while the agrometeorological variables showed characteristics similar to the first principal component. Discriminant function 1, with 70.36% separation power, distinguished the cultivation conditions based on the study locations. Grass cultivars were differentiated by discriminant function 2, with a 19.20% separation power. From a multivariate variability analysis, despite the similarities of radiation and temperature in the regions studied, the availability of water and nutrients and measurements of agronomic variables can aid in future modeling studies on forage production.

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