-This study evaluated the biomass flow of massai grass in regrowth subjected to different nitrogen levels (control -no fertilization; 400; 800; and 1200 N kg ha -1 year -1 ) and under rotational stocking with sheep, in a completely randomized design with repeated measures over time. The leaf elongation rate increased with increasing nitrogen levels (N) and the fourth grazing cycle presented a lower value compared with the others. The stem elongation rate responded linearly with increasing levels of N, but it was not influenced by grazing cycles. The senescence rate of leaves remaining before grazing and after grazing were not influenced by the nitrogen fertilization or amended with successive grazing cycles. The phyllochron was influenced only by the nitrogen fertilization, with a decreasing linear response with nitrogen levels. Quadratic response was observed for the average length of leaves with increasing N levels and the grazing cycle 4 presented the lowest value for this variable. For each kilogram N ha -1 year -1 , increases of 0.161 and 0.1604 kg ha -1 day -1 were verified in the herbage growth and accumulation rates, respectively. Nitrogen fertilization favors the biomass flow of massai grass, promoting an expressive increase in the production and accumulation of forage.

ResumoO uso racional do fogo em pastagens naturais pode ser benéfico para os atributos químicos do solo, mineralizando seus nutrientes, tornando-os disponíveis para as plantas forrageiras e favorecendo seu crescimento. Sem os devidos cuidados a queima pode promover uma redução nos teores de matéria orgânica do solo, fato que reduz algumas qualidades físicas e biológicas. O fogo nem sempre promove aumento na produção e qualidade da forragem, sendo a resposta diretamente relacionada com a composição florística da pastagem. Quando há espécies com grande valor forrageiro tolerantes ao fogo, essa prática pode ser benéfica quanto à qualidade da forragem, mas, quando as espécies forem de baixa palatabilidade e baixo valor nutritivo, a qualidade da pastagem pode ser reduzida pelo uso do fogo. O efeito do fogo em COSTA, M.R.G.F. et al. Uso do fogo em pastagens naturais. PUBVET,

The knowledge of gas exchanges in forage plants is essential for a better understanding of the process of forage biomass production in pasture. This study evaluated the gas exchange in massai grass fertilized with increasing levels of nitrogen fertilizer (control - without nitrogen fertilizer; 400; 800 and 1200 kg ha-1 year-1) and under rotational grazing by sheep, in a completely randomized design with repeated measures in time. The rest period was approximately 1.5 new leaves per tiller, as determined in the pre-test at the beginning of the experiment, providing interval of 22; 18; 16 and 13 days for the levels 0.0 - control; 400; 800 and 1200 kg ha-1 year-1 nitrogen, respectively. The animals used to lower the sward height to the recommended residual height were sheep (½ Morada Nova x ½ undefined breed), placed in paddocks of 42.3 m2. As the animals grazed, the height of the sward was monitored with a ruler until the canopy reached the recommended residual height of approximately 15 cm, corresponding to the residual LAI of exit of the animals from the paddock at approximately 1.5, as determined in a pre-test to set up the experiment. The variables stomatal conductance, leaf photosynthesis rate, leaf carbon dioxide concentration, photosynthesis/transpiration ratio, chlorophyll relative index and nitrogen sufficiency index revealed a positive linear response to nitrogen fertilization. Nitrogen fertilization level at 1200 kg ha-1 year-1 caused an increment of 92.3% in leaf photosynthesis rate in relation to the control. The leaf temperature and photosynthesis/conductance ratio were reduced with increasing nitrogen levels. The leaf transpiration showed a quadratic response with maximum point with increasing nitrogen levels. Nitrogen fertilization favor the gas exchange in massai grass up to the last level tested.

The objective of this study was to assess the agronomic and nutritional responses of the elephant grass cultivar Carajas when subjected to fertilization with protected and conventional urea. A randomized block design with 4×2+1 factorial arrangement was adopted. The treatments consisted of four doses (100, 200, 400, and 800 kg N ha-1), two nitrogen sources (conventional and protected urea), and one additional treatment without fertilization. The experiment was carried out from October 2015 to March 2016, in a semi-arid region with a dystrophic yellow latosol type soil. The application of conventional urea was done with each of the multiple cuts performed per year, whereas the protected urea was applied in a single dose. Total senescence rate (TSR), leaf and stem elongation, height, tiller density, total forage biomass, green leaf and stem biomass, and dry matter all increased with increasing N doses. However, the phyllochron, live material/dead material ratio, and agronomic efficiency all decreased with N increasing dose. Plants fertilized with protected urea presented greater TSR and phyllochron. Mineral matter content was higher in plants fertilized with conventional urea. Neutral detergent fiber content was maximized at a dose of 437.63 kg N ha-1. The protected urea resulted in a higher value of leaf/stem ratio at the 400 kg N ha-1 dose, whereas ether extract and crude protein at the 800 kg N ha-1. With application of conventional urea, the highest crude protein value occurred at a dose of 100 kg N ha-1. The morphogenesis, structure, production, and chemical composition of elephant grass ‘Carajas’ were all positively influenced when nitrogen doses were increased, with the recommended optimal dose for use being 400 kg N ha-1 for both sources. The use of protected urea is recommended because of its slow release and need for only a single application.