Pasture nitrogen input through fertiliser or legume integration: effects on canopy structure, forage nutritive value, animal production and nitrogen cycling

Abstract

Nitrogen (N) application or the integration with legumes into pasture systems coupled with grazing management strategies based on canopy structure may provide conditions for enhanced forage nutritive value, greater forage and animal productivity, and increased rates of N cycling. This study assessed canopy structure, forage nutritive value, animal performance and metabolism, and N cycling responses of three pasture treatments, namely, 1) mixed Marandu palisadegrass {Brachiaria brizantha (Hochst. ex A. Rich.) R.D. Webster [syn. Urochloa brizantha Stapf cv. Marandu]} and forage peanut (Arachis pintoi Krapov. & W.C. Greg cv. BRS Mandobi) pastures without N fertiliser (GRASS+LEGUME); 2) monoculture Marandu palisadegrass pasture with 150 kg N/ha/yr (GRASS+N); and 3) monoculture Marandu palisadegrass without N fertiliser (GRASS). Grazing management was similar across pasture treatments, using continuous stocking and a canopy height target of 20 to 25 cm. Concerning the canopy structure and forage nutritive value, herbage mass was greater in GRASS+N and GRASS+LEGUME pasture than in GRASS in summer and spring seasons (p = 0.014). Grass crude protein (CP) and in vitro digestible dry matter (IVDDM) were greatest in the GRASS+N pasture (p < 0.001, and p < 0.001, respectively). Forage peanut had greater CP and IVDDM, and lower neutral detergent fibre (NDF) than Marandu palisadegrass. Concerning animal performance and metabolism, the average daily gain was greater in the GRASS+N and GRASS+LEGUME pastures than in the GRASS (p = 0.081). GRASS+N pasture had the greatest stocking rate and liveweight gain per area (p < 0.001 and p < 0.001, respectively), followed by GRASS+LEGUME pasture. No differences between treatments were found for the dry matter forage intake (p = 0.729); however, GRASS+N and GRASS+LEGUME pastures had greater crude protein and digestible organic matter intakes than GRASS pasture (p = 0.007 and p = 0.083, respectively). Apparent efficiency of N utilisation and microbial protein/CP intake ratio were greatest in the GRASS+LEGUME pastures (p = 0.009 and p = 0.042, respectively). Concerning N cycling, existing litter and litter deposition rate were greatest in GRASS pasture (p = 0.005 and p = 0.005, respectively). Litter decomposition rate was greater, and half-life time was lower in the GRASS+LEGUME and GRASS+N pastures than in the GRASS pasture (p = 0.079 and p = 0.050, respectively). GRASS+N and GRASS+LEGUME pastures had the greatest deposited litter N (p = 0.004). The greatest faecal N excretion, and urinary N excretion per seasons occurred in GRASS+N (p = 0.002, and p < 0.001, respectively). Nitrogen application or the integration of forage peanut in a grass pasture increased green herbage mass and improved forage nutritive value, increased animal performance, and improved the conservation of soil N reserves.