A static deterministic model was used to evaluate productivity and efficiency of growing and finishing beef steers systems in Rio Grande do Sul State differing at steers slaughter age (IA): 54, 42, 36, 30, 24 and 18 months (IA54, IA42, IA36, IA30, IA24 e IA18, respectively), or in market live weight for buying and sale (cattle growing systems from 150 to 250 kg -R1, from 250 to 350 kg -R2, or finishing cattle from 350 to 450 kg -R3). Feeding management was described for each system based on published reports according to IA, while for systems R1, R2 and R3 it was considered that native pastures were the exclusive forage resource. There was a quadratic response curve between productivity and IA, with a minimum production (65 kg/ha) for IA54 and the highest for IA18 (343 kg/ha). Energetic cost per kg of produced live weight was linearly increased in 1.19 Mcal ME for each month increased at IA. Reducing IA, increased the proportion of the grazing area that was occupied by improved pastures up to 66%. This allowed higher stocking rates and daily gains, as well. For an equal total live weight gain, reducing steer weight for buying and sale, increased system productivity (R1: 113, R2: 81 and R3: 63 kg/ha), so that productive specialization appears as an alternative for improving those systems which depends exclusively on native pastures.
cattle systems; efficiency; growing; productivity; simulation
