RESUMO: O objetivo foi avaliar os efeitos das doses anuais de 100 e 200 kg ha-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 perfilhos marcados. Também foi avaliado o acúmulo de forragem e a densidade populacional de perfilhos (DPP). Foi observada interação significativa (P < 0,05) das doses de N e estações do ano para o período de descanso (PD), duração de vida das folhas (DVF), filocrono, 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-1 de N. Em contra partida, no inverno, os pastos adubados com 100 kg ha-1 de N apresentaram maiores valores para DVF, filocrono e TAlC, e menores para o CFF. Não foi observado efeito isolado de doses de N (P > 0,05) para as taxas de acúmulo de forragem; de aparecimento, alongamento e de senescência de folhas, relação folha:colmo e para a DPP. No entanto, o efeito das estações foi significativo para estas variáveis, com os maiores valores observados durante a primavera e verão. O uso de 200 kg ha-1 de N para pastos de capim-ipyporã promove redução da estacionalidade de produção da forrageira.

ABSTRACT: This study examined the effects 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. Significant interaction effects (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-1 N. Conversely, in winter, the pastures fertilized with 100 kg ha-1 N showed higher LLS, SER and phyllochron values, but lower FLL values. There was no isolated effect of N rates (P > 0.05) on forage accumulation rate; leaf appearance, elongation or senescence rates; leaf:stem ratio; or TD. However, the effect of seasons was significant for these variables, with the highest values occurring during spring and summer. The use of 200 kg ha-1 N in Ipyporã grass pastures reduces the seasonality of forage production.

Resumen Este trabajo tuvo como objetivo evaluar los efectos de tres dosis de nitrógeno (N) en las características morfogénicas y estructurales, la masa (MR) y la distribución de las raíces en el perfil del suelo, y la resistencia del suelo a la penetración de las pasturas de guinea Mombasa manejados con pastoreo rotacional. El diseño experimental utilizó bloques al azar con tres dosis de N (100, 200 y 300 kg ha-1) y tres repeticiones. El criterio para interrumpir el rebrote de los pastos fue la altura de 80 a 90 cm del dosel (90-95 % de intercepción de luz por el dosel). Los animales fueron retirados de los potreros cuando el dosel alcanzó el 50 % de la altura prepastoreo. Se evaluó la masa y acumulación del forraje, las características morfogénicas y estructurales del dosel, la MR y distribución en el perfil del suelo, y la resistencia del suelo a la penetración. En pastos fertilizados con 200 y 300 kg ha-1 de N, se observó la mayor aparición foliar (0.090 hojas macollo-1 día-1 y 0.081 hojas macollo-1 día-1), tasas de alargamiento (2.82 y 2.61 cm macollo-1 d-1) y mayor acumulación diaria de forraje (113.8 y 106.6 kg ha-1d-1). El uso de 300 kg ha-1 de N promovió una mayor resistencia del suelo a la penetración a 10 cm de profundidad en el pospastoreo (3.3 MPa). No se observó ningún efecto de las dosis de N en la MR (P>0.05). Por lo tanto, el control de la altura antes y después del pastoreo de los animales en los potreros ayudó a mantener la estructura del pasto y evitar el proceso de compactación del suelo. De acuerdo con los resultados, se concluye que 200 y 300 kg ha-1 de fertilización con N es una estrategia para intensificar los pastos.

Abstract This work aimed to evaluate the effects of three nitrogen (N) doses on the morphogenic and structural characteristics, root mass (RM) and distribution in the soil profile, and penetration soil resistance of Mombasa guineagrass pastures managed with rotational stocking. The experimental design used randomized blocks with three N doses (100, 200, and 300 kg ha-1) and three replicates. The criterion for interrupting the pastures’ regrowth was the height of 80 to 90 cm of the canopy (90-95 % of light interception by the canopy). The animals were removed from the paddocks when the canopy reached 50 % of the pre-grazing height. Forage mass and accumulation, canopy morphogenic and structural characteristics, RM and distribution in the soil profile, and soil resistance to penetration were evaluated. In pastures fertilized with 200 and 300 kg ha-1 of N, the highest leaf appearance (0.090 and 0.081 leaves-1 tiller-1 d-1), elongation rates (2.82 and 2.61 cm tiller-1 d-1) and highest daily forage accumulation (113.8 and 106.6 kg ha-1d-1) were observed. Using 300 kg ha-1 of N promoted greater soil resistance to penetration at 10 cm of depth in the post-grazing (3.3 MPa). No effect of N doses was observed for RM (P>0.05). The pre- and post-grazing height control of animals in the paddocks therefore helped to maintain the pasture structure and avoid the soil compaction process. According to the results, it is concluded that 200 and 300 kg ha-1 of N fertilization is a strategy for intensifying pastures.