Evaluation of APEX modifications to simulate forage production for grazing management decision-support in the Western US Great Plains

Authors: CHEN, GONG - Northwest A&f University; Harmel, Daren; Ma, Liwang; Derner, Justin; Augustine, David; BARTLING, PATRICIA - Retired ARS Employee; FANG, Q - Qingdao Agricultural University; WILLIAMS, JIMMY - Texas A&M University; ZILVERBERG, CODY - Producer; BOONE, RANDY - Colorado State University; Hoover, David; YU, Q - Northwest A&f University

Submitted to: Agricultural Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2021
Publication Date: 4/10/2021
Citation: Chen, G., Harmel, R.D., Ma, L., Derner, J.D., Augustine, D.J., Bartling, P., Fang, Q., Williams, J., Zilverberg, C., Boone, R., Hoover, D.L., Yu, Q. 2021. Evaluation of APEX modifications to simulate forage production for grazing management decision-support in the Western US Great Plains. Agricultural Systems. 191. Article 103139. https://doi.org/10.1016/j.agsy.2021.103139.
DOI: https://doi.org/10.1016/j.agsy.2021.103139

Interpretive Summary: Understanding how grazing management decisions influence the productivity and composition of rangeland plant communities is essential for the development of effective strategies to sustainably manage rangelands. Because experimental analysis of grazing management decisions is difficult, expensive, and require long time periods, simulation modelling can more rapidly advance our understanding and stewardship of rangeland ecosystems. Capitalizing on a field study of traditional and collaborative adaptive rangeland management, modifications in the Agricultural Policy/Environmental eXtender (APEX) model were evaluated by comparing simulated forage production to experimental data from 20 pastures (123-137 ha each). APEX modifications included rotational grazing based on a user-defined sequence, within-day movement of cattle, and plant growth for key plant functional groups. These modifications improved the simulation of relative differences in forage production between grazing treatments, across years, and among soil types; however, APEX underestimated forage production occurred in 2015 and 2017 due to overestimating water stress for the warm season perennial grass functional group. Simulation of grazing management scenarios showcased a reduction in forage production as grazing duration in rotational grazing decreased. These modifications were needed to capture the complexity of semiarid environments, enhancing APEX to accurately assess grazing management decisions on forage production in regions such as the Western Great Plains.

Technical Abstract: Understanding how grazing management decisions influence the productivity and composition of rangeland plant communities is essential for the development of effective strategies to sustainably produce multiple ecosystem goods and services on rangelands. Because experimental analysis of grazing management decisions is difficult, expensive, and require long time periods, simulation modelling can more rapidly advance our understanding and stewardship of rangeland ecosystems. Capitalizing on a comparative field study of traditional and collaborative adaptive rangeland management, modifications in the Agricultural Policy/Environmental eXtender (APEX) model were evaluated by comparing simulated forage production to experimental data from 20 pastures (123-137 ha each). APEX modifications included rotational grazing based on a user-defined sequence, within-day movement of cattle, and plant growth for key plant functional groups. These modifications improved the simulation of relative differences in forage production between grazing treatments, across years, and among soil types; however, APEX underestimated forage production occurred in 2015 and 2017 due to overestimating water stress for the warm season perennial grass functional group. Simulation of grazing management scenarios showcased a reduction in forage production as grazing duration in rotational grazing decreased. These modifications were needed to capture the complexity of semiarid environments, enhancing APEX to accurately assess grazing management decisions on forage production in regions such as the Western Great Plains.