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

Authors: FANG, Q - Qingdao Agricultural University; Harmel, Daren; Ma, Liwang; BARTLING, PAT - Retired ARS Employee; Derner, Justin; JEONG, JAEHAK - Texas A&M University; WILLIAMS, JIMMY - Texas A&M University; BOONE, RANDY - Colorado State University

Submitted to: Agricultural Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2021
Publication Date: 10/8/2021
Citation: Fang, Q.X., Harmel, R.D., Ma, L., Bartling, P., Derner, J.D., Jeong, J., Williams, J., Boone, R. 2021. Evaluating the APEX Model for alternative cow-calf grazing management strategies in Central Texas. Agricultural Systems. 195. Article e103287. https://doi.org/10.1016/j.agsy.2021.103287.
DOI: https://doi.org/10.1016/j.agsy.2021.103287

Interpretive Summary: Simulation tools are increasingly used to improve grazing management decisions and policy formulation and to assess livestock and economic impacts. Accounting for spatial variability of soil hydrology, vegetation type, and management practices among grazing units is important for assessing grazing system and developing effective grazing strategies via modelling. The main objectives of this research were to: 1) evaluate APEX modifications for multi-paddock rotational grazing and for supplemental hay feeding, 2) evaluate two new cow-calf weight gain algorithms under different grazing systems, and 3) assess the performance of rotational grazing system in response to the number of paddocks, grazing timing, hay supplement, and forage management by combining experimental and simulated data. The APEX model was modified to simulate multi-paddock rotational grazing and to compare cow-calf weight gain algorithms based on either energy or total digestible nutrients. These modifications were necessary to improve its ability to assess alternative grazing systems. Simulation results were evaluated against a 5-year experimental data set from rotational and continuous grazing systems in Central Texas. The modified APEX model adequately simulated the responses of vegetation production, hay consumption, calf weaning weight, costs, and profits to the two grazing treatments across seasons. The energy-based weight gain algorithm simulated larger effects of grazing on vegetation production, whereas the total digestible nutrients algorithm showed a higher sensitivity of weight gain to forage intake and hay quality due to differences in estimating animal demand, maintenance requirements, and weight gain between the two algorithms. No significant differences in measured vegetation production and calf weaning weight occurred between treatments across the years. Similarly, the energy-based algorithm produced no significant differences in these variables between grazing treatments; however, the total digestible nutrients based algorithm simulated lower calf weaning weight in rotational than in continuous grazing systems. Measured and simulated data also showed similar profits per cow-calf between the two grazing treatments, but higher total cost and gross return per ha for the continuous grazing system. The APEX model enhancements for multiple-paddock rotational grazing and cow-calf weight gain improved its utility to assess alternative grazing systems.

Technical Abstract: Simulation tools are increasingly used to improve grazing management decisions and policy formulation and to assess livestock and economic impacts. Accounting for spatial variability of soil hydrology, vegetation type, and management practices among grazing units is important for assessing grazing system and developing effective grazing strategies via modelling. The main objectives of this research were to: 1) evaluate APEX modifications for multi-paddock rotational grazing and for supplemental hay feeding, 2) evaluate two new cow-calf weight gain algorithms under different grazing systems, and 3) assess the performance of rotational grazing system in response to the number of paddocks, grazing timing, hay supplement, and forage management by combining experimental and simulated data. The APEX model was modified to simulate multi-paddock rotational grazing and to compare cow-calf weight gain algorithms based on either energy or total digestible nutrients. These APEX modifications were necessary to improve its ability to assess alternative grazing systems. Simulation results were evaluated against a 5-year experimental data set from rotational and continuous grazing systems in Central Texas. The modified APEX model adequately simulated the responses of vegetation production (coefficient of determination, R2 = 0.61-0.70), hay consumption (R2 = 0.94-0.95), calf weaning weight (R2 =0.50-0.62), costs (R2 = 0.97-1.00), and profits (R2 = 0.85-0.97) to the two grazing treatments across seasons. The energy-based weight gain algorithm simulated larger effects of grazing on vegetation production, whereas the total digestible nutrients algorithm showed a higher sensitivity of weight gain to forage intake and hay quality due to differences in estimating animal demand, maintenance requirements, and weight gain between the two algorithms. No significant differences (p > 0.05) in measured vegetation production and calf weaning weight occurred between treatments across the years. Similarly, the energy-based algorithm produced no significant differences in these variables between grazing treatments; however, the total digestible nutrients based algorithm simulated lower calf weaning weight in rotational than in continuous grazing systems. Measured and simulated data also showed similar profits per cow-calf between the two grazing treatments, but higher total cost and gross return per ha for the continuous grazing system. The APEX model enhancements for multiple-paddock rotational grazing and cow-calf weight gain improved its utility to assess alternative grazing systems.