Nitrogen cycling in tropical grass-legume pastures managed under canopy light interception

Abstract

In grass-legume pastures, grazing management strategies are an essential factor affecting nitrogen (N) cycling. This study assessed the impact of grazing management on N cycling in rotationally-stocked mixed pastures of ‘Marandu’ palisadegrass (Brachiaria brizantha) and ‘Comum’ calopo (Calopogonium mucunoides). Treatments included three grazing management strategies, defined by interruption of the rest period when the canopy reached 90 (90LI), 95 (95LI) and 100% (100LI) of the interception of photosynthetically active radiation. A 2-yr experimental period was adopted. Plant litter responses, forage intake and livestock excretion were evaluated. No differences between grazing management were obtained for existing (294 g OM m−2) and deposited litter (6.7 g OM m−2 d−1, P > 0.10). Compared to the dry season, the litter decomposition rate increased 24.0%, and the half-life decreased 37.8% in the rainy season (P < 0.10). The N cycling via litter (553 g ha−1 d−1) was similar in all grazing management (P > 0.10). Less frequent defoliation (100LI) resulted in reduced proportion of legume intake (P < 0.10, 94.4 vs. 168.5 g kg−1; an average of 90LI and 95LI, respectively), lower N intake (123.1 vs. 194.1 g animal unit−1 d−1) and a lower input of N from biological fixation (73.2 vs. 97.8 kg ha−1 yr−1). Less frequent defoliation should be avoided because it reduces the N intake and N retained by animals, which caused a reduction in N utilisation efficiency by heifers. Thus, 95% light interception is the maximum limit to interrupt the regrowth in palisadegrass-calopo pastures.