Evaluating the APEX model for alternative cow-calf grazing management strategies in Central Texas

Abstract CONTEXT Simulation tools are increasingly used to inform grazing management decisions by assessing livestock performance, as well as environmental and economic impacts. Ability to represent the grazing of multiple pastures (i.e., paddocks) that differ in soil, hydrology, vegetation, and management is critical for reliable grazing management decision support. OBJECTIVE The main objectives of this study were to: 1) modify APEX (Agricultural Policy/Environmental eXtender) for sub-daily grazing, cow-calf weight gain, and supplemental hay, and 2) evaluate the APEX modifications in terms of simulating biomass, calf weight gain, economic impacts of alternative cow-calf grazing management strategies. METHODS APEX was modified to enhance its ability to simulate alternative grazing management strategies by including sub-daily grazing among multiple paddocks, supplemental hay estimation, and optional simulation of cow-calf weight gain based either on energy or total digestible nutrients (TDN). Simulation results were evaluated against a 5-year experimental data set from Central Texas comparing multi-paddock rotational grazing and conventional continuous grazing. RESULTS AND CONCLUSION The modified APEX model adequately simulated the responses of vegetation biomass (coefficient of determination, R2 = 0.60–0.70), hay consumption (R2 = 0.94–0.95), calf weaning weight (R2 = 0.52–0.65), costs (R2 = 0.98), and profits (R2 = 0.89) to the two grazing treatments across years. Simulations with the energy-based weight gain algorithm showed more pronounced effects on above-ground biomass, whereas the TDN-based algorithm had a more pronounced weight gain response to forage intake and hay quality. No significant differences (p > 0.05) in biomass and calf weaning weight were observed between treatments across the years for measured data and for energy-based APEX simulation; however, the TDN-based algorithm simulated lower calf weaning weight in multi-paddock rotational grazing than in continuous grazing. Measured and simulated data also showed similar profits between the two grazing treatments, but total cost and gross return per ha were greater for continuous grazing. SIGNIFICANCE The model enhancements for sub-daily grazing, cow-calf weight gain, and supplemental hay improved the potential utility of APEX for assessing environmental and economic impacts of alternative grazing strategies at ranch scale.