Movement dynamics and energy expenditure of yearling steers under contrasting grazing management in shortgrass steppe

Authors: Jorns, Tamarah; Derner, Justin; Augustine, David; BRISKE, DAVID - Texas A&M University; Porensky, Lauren; SCASTA, DEREK - University Of Wyoming; BECK, JEFFREY - University Of Wyoming; LAKE, SCOTT - University Of Wyoming

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2022
Publication Date: 10/3/2022
Citation: Jorns, T.R., Derner, J.D., Augustine, D.J., Briske, D., Porensky, L.M., Scasta, D.J., Beck, J.L., Lake, S. 2022. Movement dynamics and energy expenditure of yearling steers under contrasting grazing management in shortgrass steppe. Rangeland Ecology and Management. 85:38-47. https://doi.org/10.1016/j.rama.2022.09.001.
DOI: https://doi.org/10.1016/j.rama.2022.09.001

Interpretive Summary: Moving animals between paddocks (pastures) in a rotational grazing system may be responsible for lower weight gains as these animals have more movement and associated energy expenditures. We evaluated this using yearling steers in a shortgrass steppe rangeland with a season-long grazing (no movement of animals between paddocks) and a 10-pasture rotational grazing system. Pedometers (like a human fit bit) were attached to a back leg of yearling steers to determine number of steps and associated energy expenditures. Number of steps were similar for steers in each grazing system as was the energy expended. Steers in the rotational grazing system took fewer steps in the beginning of the grazing season but more at the end of the grazing season. This pattern suggests that potential weight gain was depressed due to lower selectivity for high quality forage early in the grazing season, and more expended energy later in the season for these animals to find sufficient amount and quality of forage. Our findings suggest that forage quality rather than movement of animals is the primary driver of differences in livestock gains between the two grazing systems.

Technical Abstract: A potential mechanism for lower livestock weight gains with rotational grazing is the additional movement and associated energy expenditures incurred with rotation of animals among paddocks. We evaluated these metrics in 2016 and 2017 using pedometers affixed to free-ranging naïve yearling steers grazing semiarid, shortgrass steppe under contrasting grazing management treatments with the same stocking rate: traditional season-long (mid-May to October) grazing management (TRM) and collaborative adaptive rangeland management (CARM) at a ranch-scale (2,600 ha: 20, 130 ha paddocks for each treatment). Mean daily number of steps by steers in paddocks during the grazing season, excluding those associated with moves between paddocks, were 3.0% lower (2016) and 7.8% greater (2017) for CARM, but energy expenditures did not differ significantly between treatments in either year. Daily step counts decreased in TRM as the grazing season progressed. Step counts decreased from day one to day eight in CARM paddocks following rotation of steers. Steers in the TRM treatment took more steps daily than CARM steers in the first third of the grazing season, but this reversed in the last third of the grazing season. These findings suggest that observed 12-16% reductions in livestock weight gains with CARM were not influenced by differences in total grazing season steps as energy expenditures of steers did not differ. Two additive influences of within-season steer movement dynamics suggest that forage quality was the primary driver for the decrease in weight gains in CARM. First, fewer steps in the early growing season, when forage quality is highest, indicates reduced selectivity for nutrient-rich patches. Second, more steps by yearlings in the late growing season suggests that these heavier animals expending more energy for maintenance were searching to satisfy gut fill as forage quantity and quality on offer per steer was limiting with the 10-fold higher stocking density.