PS12-24. How Sward Forage Structure and Nutritive Value Shape the Performance of Nellore Steers in Tropical Pasture-based Beef Cattle Production Systems.

Abstract This study evaluated how sward forage structure and nutrient contents affects average daily gain (ADG, kg0.75 day-1) of Nellore steers during the backgrounding-finishing phase in tropical pasture-based production systems: 1) extensively managed degraded pasture (DP); 2) intensively managed silvopastoral system (LFS); 3) intensively managed rainfed pasture with moderate stocking rate (RMS); 4) intensively managed rainfed pasture with high stocking rate (RHS); and 5) intensively managed and irrigated pasture with high stocking rate, overseeded in the dry season (IHS). Each year, 30 Nellore steers (330±25 kg; 18±1 months) were randomly assigned to each system with two replicates over two years (2019–2021). Forage measurements per season included: direct sampling to represent production, and hand-plucking to simulate nutrient intake, considering the animals' grazing behavior. To determine ADG, weightings were performed every 28-day within each season by animals. Data was analyzed using R version 4.5.0 (RCORE TEAM, Boston, Massachusetts, USA), being first organized by animal, and subsequently aggregated by paddock, totaling 80 observations. Linear mixed models were fitted to identify the main structural (e.g., height and herbage mass) and nutritive variables [e.g., crude protein (CP) and fiber fractions] associated with ADG (α = 0.05), considering season as a fixed covariate effect, while year and paddocks nested within year as random effects. The final model demonstrated a high precision between observed and predicted values (R2 = 0.743; P < 0.001), with seasons showing a dominant effect (F = 32.8). IHS maintained similar ADG across seasons, whereas in winter, all other systems sharply declined, showing negative gains in DP and LFS. Among sward attributes, pre-grazing forage mass had the strongest positive association (F = 17.0), with gains increasing up to approximately 9,000 kg DM ha-1 before plateauing. CP content exhibited a similar pattern (F = 38.2), with optimal performance at 9–10%. Pre-grazing canopy height showed a negative quadratic effect (F = 16.9), with maximum ADG at intermediate heights of 25–35 cm and reduced performance at both short and tall sward conditions. Pre-grazing leaf proportion was the only variable with a linear positive relationship (F = 4.8), indicating that swards with greater leaf mass participation consistently supported higher ADG across the full range observed. These results indicate that both sward structure and nutritive value greatly influence ADG of grazing animals, but their magnitude depends on the beef cattle system management under tropical conditions. For image description, please refer to the figure legend and surrounding text.