Structural and production characteristics and nutritive value of two tropical grasses submitted to different levels of nitrogen

The objective of this study was to evaluate increasing levels of nitrogen on and production and nutritive value of Andropogon and Massai grasses under the edaphoclimatic conditions of the Brazil denominated Middle Nort Region. The design used was completely randomized, with sixteen replicatios. The treatments consisted of nitrogen rates: 0, 150, 300, 450 and 600 kg N ha-1 and two forages, which were evaluated independently. The grasses used were Panicum maximum cv. Massai and Andropogon gayanus cv. Kunth. For Andropogon grass, there was an effect of nitrogen fertilization on thecharacteristics: leaf elongation rate, stem elongation rate, leaf senescence rate and leaf blade final length and, in all cases, the behavior was quadratic. Nitrogen also influenced the morphogenic characteristics: leaf appearance rate, leaf elongation rate, leaf senescence rate, phyllochron, leaf blade final length, number of live leaves per tiller and It was not significant for tiller population density of Massai grass, but the behavior varied according to the characteristic, in which leaf appearance rate and number of live leaves per tiller had decreasing linear behavior, while in phyllochron it increased linearly as nitrogen doses increased. Most of the productive characteristics of the grasses had increasing linear behavior (total biomass production of forage, leaf biomass and stem biomass in Andropogon grass, while in Massai grass, only the total forage production and dead material accumulation had an increasing linear behavior with increase of N doses. Quadratic effect only for masai leaf biomass behavior and there was practically no production of stems, affecting the leaf-stem ratio ) while the highest efficiency of nitrogen utilization was achieved with the application of 150 kg N ha-1 for the two grasses. Nitrogen fertilization also favored the nutritive value of the two grasses positively, with linear increase for crude protein and mineral matter contents, only in Massai, and reduction in the contents of neutral detergent fiber, acid detergent fiber and hemicellulose of grasses. Increasing doses of nitrogen fertilization favors the production, structure and chemical composition of the two grasses. However, the highest efficiency of use is obtained with the application of 150 kg N ha-1.