PS12-23. Seasonal Dynamics of Nellore Steers in Tropical Pasture-based Beef Cattle Production Systems.

Abstract Seasonal fluctuations in forage production are a major constraint to beef cattle productivity in tropical grazing systems. This study aimed to measure the effect of seasonality on the productivity (live weight gain per hectare, LWGha, kg ha-1 day-1) of Nellore steers during the backgrounding-finishing phase across five tropical pasture-based production systems: 1) extensively managed degraded pasture (DP); 2) intensively managed silvopastoral system (LFS); 3) intensively managed rainfed pasture with a moderate stocking rate (RMS); 4) intensively managed rainfed pasture with a high stocking rate (RHS); and 5) intensively managed and irrigated pasture with a high stocking rate, overseeded in the dry season (IHS). All systems were located in Southeast Brazil and included 30 Nellore steers (330±25 kg; 18±1 months), randomly assigned to treatments with two replicates over two years (2019–2021). Stocking rates were adjusted using the put-and-take technique. Grazing was continuous in DP and rotational (36-day cycles) in the remaining systems. Except for DP, all pastures received liming and corrective fertilization (P, K, S, and micronutrients). Nitrogen fertilization rates were 200 kg N ha-1 yr-1 in LFS and RMS, 400 kg N ha-1 yr-1 in RHS, and 600 kg N ha-1 yr-1 in IHS. Animal performance was evaluates through weighings at the beginning and every 28 days thereafter to determine average daily weight gain (ADG) (kg d-1), stocking rate [number of animal units (AU = 450 kg live body weight) per area (AU ha-1)], and LWGha. The variance of the data was analyzed considering treatments and seasons as fixed effects, and years as random effects. Seasonal trends were analyzed using linear, quadratic, and cubic polynomial contrasts within a random-coefficients model, with the highest polynomial order determined by the significance of the contrasts. Animal productivity varied quadratically across seasons within systems (P < 0.05), with IHS, RHS, and RMS showing higher annual productivity (4.9, 3.2 and 2.2 kg ha-1 day-1, respectively) and lowest variability than LFS and DP average (∼ 0.6 kg ha-1 day-1). During autumn and winter, the reduced forage availability sharply decreased LWGha across all systems (∼ 4.3, 1.6, 0.9 and 0.4 kg ha-1 day-1 for RHS, RMS, LFS and DP, respectively) whereas IHS maintained a stable productivity (2.0 kg ha-1 day-1), indicating that grazing intensification, through improved fertilization, stocking rates, and irrigation substantially increases beef cattle productivity while reducing seasonal fluctuations in animal performance. The competition for natural resources (e.g., light and water) between components (pasture and trees), and low temperatures and soil moisture during the dry season in LFS explains his low performance. For image description, please refer to the figure legend and surrounding text.