Adaptive management of stocking rates to reduce effects of drought on cow‐calf production systems in semi‐arid rangelands

The simple method we developed to stochastically generate monthly precipitation produces a time series of precipitation values that were representative of general historical precipitation patterns and provided realistic levels of uncertainty in simulated forage production to evaluate alternative management strategies. We used simple ecological sustainability simulator (SESS), modified to represent each of five management strategies (sets of decision rules) for adjusting stocking rates, to identify strategies that could reduce effects of drought on cow‐calf production systems in semi‐arid rangelands. We parameterized the model to represent a region of extensive cow‐calf production in the northeastern portion of the Mexican state of Coahuila, and evaluated animal performance (animal body condition, cow mortality, and calf production) and range condition resulting from each strategy under random precipitation conditions typical of the region. To evaluate the validity of the randomly generated precipitation, we conducted the simulations under an historical (1950–1994) precipitation regime. The five management strategies included one with no adjustments to stocking rate (CONTROL, most common current practice), one with stocking rate adjustment rules based on changes in animal body condition (BCS), and three with different stocking rate adjustment rules based on various comparisons of recent‐year precipitation with long‐term mean precipitation during the growing season (March–November) (REPLA, PPT 1Y, PPT 2Y). Each strategy was evaluated at each of three initial base‐level stock cow stocking rates (125, 250 and 500 AUY·5000ha^‐1). Stocking rate adjustments consisted of partial or total de‐stocking, with the re‐establishment, or not, of the initial number of stock cows before the beginning of the subsequent breeding season. Results of 45‐year simulations under both random and historical precipitation suggest CONTROL and PPT 2Y (based on comparison of current year and previous year precipitation with the long‐term mean) strategies, combined with the high base‐level stocking rate, are the worst and best, respectively. Under the historical precipitation regime, in the last period of time simulated (1980–1994) these two strategies resulted, respectively, in poor (0.5) versus good range condition (1.0), animal body condition scores of 2.6 versus 5.2, annual cow mortalities of 76 versus 5%, and calf production rates of 0.9 versus 10.6kgha^‐1 year^‐1 at weaning. The PPT 1Y strategy (based on comparison of current year precipitation with the long‐term mean) produced results fairly similar to PPT 2Y, with BCS and REPLA strategies producing results intermediate between CONTROL and PPT 2Y. Our results suggest it is advantageous to adjust stocking rates based on precipitation during the current growing season since it improves cattle production without damaging range condition. Considering the practical feasibility of the strategies, we recommend the PPT 1Y strategy because it maintains range condition at moderate stocking rates, results in good animal performance and does not require total de‐stocking of the ranch.