Morphogenetic and structural traits of Ipyporã grass subjected to nitrogen fertilization rates under intermittent grazing/Características morfogênicas e estruturais do capim-ipyporã submetido a doses de adubação nitrogenada sob pastejo intermitente

This study examined the efects of annual nitrogen (N) rates of 100 and 200 kg ha (-1) on the morphogenetic and structural traits of Ipyporã grass (Brachiaria brizantha × Brachiaria ruziziensis) under intermittent grazing during the seasons of the year. The experiment was laid out in a randomized-block design with two treatments and three area replicates. Ipyporã grass pastures were grazed by beef cattle. Morphogenetic and structural traits of the forage canopy were evaluated using the marked-tiller technique. Forage accumulation and tiller density (TD) were also evaluated. Signifcant interaction efects (P < 0.05) between N rates and seasons were observed for the rest period, leaf lifespan (LLS), phyllochron, stem elongation rate (SER) and final leaf length (FLL). During winter, the rest period was shorter (40 vs. 50 days) in the pastures fertilized with 200 kg [ha.sup.-1] N. Conversely, in winter, the pastures fertilized with 100 kg [ha.sup.-1] N showed higher LLS, SER and phyllochron values, but lower FLL values. There was no isolated efect of N rates (P > 0.05) on forage accumulation rate; leaf appearance, elongation or senescence rates; leaf:stem ratio; or TD. However, the efect of seasons was signifcant for these variables, with the highest values occurring during spring and summer. The use of 200 kg [ha.sup.-1] N in Ipyporã grass pastures reduces the seasonality of forage production. Key words: Brachiaria, nitrogen fertilization, tissue fow, tropical pastures. O objetivo foi avaliar os efeitos das doses anuais de 100 e 200 kg [ha.sup.-1] de nitrogênio (N) sobre as características morfogênicas e estruturais do capim-ipyporã, em lotação intermitente durante as estações do ano. O delineamento experimental foi em blocos ao acaso, com dois tratamentos e três repetições de área. Os pastos de capim-ipyporã foram submetidos ao pastejo por bovinos de corte. Foram avaliadas as características morfogênicas e estruturais do dossel forrageiro por meio da técnica de perflhos marcados. Também foi avaliado o acúmulo de forragem e a densidade populacional de perflhos (DPP). Foi observada interação signifcativa (P < 0,05) das doses de N e estações do ano para o período de descanso (PD), duração de vida das folhas (DVF), flocrono, taxa de alongamento de colmos (TAlC) e comprimento final das folhas (CFF). Durante o inverno, o PD foi menor (40 vs 50 dias) nos pastos adubados com 200 kg [ha.sup.-1] de N. Em contra partida, no inverno, os pastos adubados com 100 kg [ha.sup.