RESUMO
Beef cow-calf farming systems are assumed to be resilient to biological disturbances that induce variations in herd demography; however, this hypothesis has not been fully investigated to date. Modeling is an interesting approach to study farming system resilience and to evaluate the impact of biological disturbances, taking into account interactions between system components, including biological variability and management practices. Our objective was to evaluate the resilience of beef cow-calf farming systems to variations in fertility and mortality using a modeling approach. We studied the direct effect of variations in demographic parameters on production objectives without explicitly representing the causes of the variations. We developed a stochastic model to represent the population dynamics of a beef cow-calf herd with breeding by natural service and biological processes occurring at the animal level. The model was validated by comparing observed and simulated distributions of the calving-to-calving interval, which were found to be consistent. Resistance was evaluated by the proportion of simulations where the objective in terms of number of weaned calves is reached even when there is a disturbance that persists for 10 yr. Reversibility was evaluated by the time needed to return to the predisturbance production level. Beef cow-calf farming systems did not appear to be resistant to variations in mortality and infertility rates except when increases in the infertility rates were low (0.02 for cows and 0.03 for heifers). Critical situations, consequently, may emerge with regard to farm production if management practices are not adapted. Reversibility was observed for disturbances that persist for up to 5 yr. However, the system needed 2 to 3 yr to recover its predisturbance production level and up to 4 yr after an increase in cow infertility of 0.12.