A simulation model was developed to evaluate the productive and economic effect of the variation in feed practices and stocking rate of a cow–calf system in Chile. Winter supplementation at grazing, stocking rate and economic aspects of the system were analysed. The supplementation of straw v. pasture hay at two different stocking rates was evaluated in the temperate pasture zone in La Araucania, Chile. Data were simulated using a decision support tool to help analyse the system. Simulations with the model involved 20 replicates of a factorial combination of two stocking rates (1.0 and 1.4 cows/ha) with differences in the initial weight (‘light’ v. ‘heavy’ with weights of heifers, primiparous and multiparous cows being 340 v. 380, 400 v. 450 and 440 v. 480 kg, respectively), at the same grazing pressure (kg liveweight/ha), two winter supplements (oat straw v. pasture hay) and two levels (6 v. 8 kg straw, and 5.1 v. 6.8 pasture hay). The model was validated with data collected from an experiment conducted with permanent pastures and a beef cattle cow–calf system from 1984 to 1989. The results indicate that there was a significant (P < 0.01) effect of a cow’s weight on the calves at 180 days, and on their reproductive performance, with the heavier cows increasing their calving rate by 20% relative to the lighter group. The stocking rate and the type and amount of supplement, however, did not influence (P > 0.05) the weight of female and male calves at the time of sale. Finally, supplementation with pasture hay, as opposed to oat straw, incurred a larger (P < 0.01) mean cost at an equivalent level of provision of metabolisable energy. Although difficult to analyse under real conditions, the economic and productive benefits of various feed practices and stocking rates were successfully evaluated in the present study using simulation tools.