A static and deterministic model, synthesizing published experimental information, was used to simulate the productivity and biological efficiency of beef cattle life-cycle production systems in Rio Grande do Sul, differing in heifers' age at first calving (IP, months) and steer age at slaughter (IA, months): "Traditional System" (ST), IP48, IA54; "Improved System" (SM), IP36, with two slaughter ages, IA36 (SM-I) and IA24 (SM-II); "One Year System" (SU), IP24, with three slaughter ages, IA36 (SU-I), IA24 (SU-II) and IA18 (SU-III). For each system, the variation on calving rate (TN) between 50 and 90% was evaluated. ST was based on native pasture. Other systems adjusted seasonal stocking rate in native pasture and used improved winter pastures with those animal categories pretended to be improved. Live weight production showed reduced increments with increasing TN, reaching maximal values of 73, 97, 129, 116, 149 and 151 kg/ha for systems ST, SM-I, SM-II, SU-I, SU-II and SU-III, respectively. Optimum TN increased with IP. Responses to increasing TN between 50% and optimal value, showed a tendency to increase with younger IA of steers: 23,2% (SU-III), 22,2% (SM-II), 21,4% (SU-II), 20,3% (SM-I), 16,8% (SU-I), and 15,1% (ST). Biological efficiency for SU-III system with TN=80% was of 35,1 Mcal ME/ kg of LW. Improving simultaneously, reproductive and productive coefficients, reduced LW production energetic cost in 42,2% with respect to cost in ST with TN=50%.
beef cattle; biological efficiency; productivity; simulation; systems
