Multi-decadal directional shift in shortgrass stocking rates

Authors: Raynor, Edward; Derner, Justin; BALDWIN, TEVYN - University Of Wyoming; RITTEN, JOHN - University Of Wyoming; Augustine, David

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2020
Publication Date: 12/28/2020
Citation: Raynor, E.J., Derner, J.D., Baldwin, T., Ritten, J., Augustine, D.J. 2020. Multi-decadal directional shift in shortgrass stocking rates. Rangeland Ecology and Management. 74:72-80. https://doi.org/10.1016/j.rama.2020.09.005.
DOI: https://doi.org/10.1016/j.rama.2020.09.005

Interpretive Summary: We evaluated the relationships of average daily gain and beef production per unit area for yearling steers in pastures managed with different stocking rates at the USDA ARS Central Plains Experimental Range. We assessed if differences occurred between optimal stocking rate for beef production during initial 24 years (1940-1963) and the last 19 years (2000-2018) of a long-term grazing intensity experiment in the shortgrass prairie region of Colorado. In addition, we evaluated the role of drought on the optimal stocking rate for beef production during the modern period. Relative to the initial 24 years of this experiment, we found a 72% increase in the stocking rate. The influence of drought (<75% of normal precipitation) on the stocking rate was substantial as it was 39% lower than the optimal value for the modern era. Our results show the capacity of this semiarid, shortgrass rangeland to produce livestock weight gains has increased substantially from the initial to the modern eras of this long-term experiment.

Technical Abstract: Bement (1969) developed a stocking rate (SR) guide for cattle grazing shortgrass steppe based on relationships among average daily weight gain of yearling cattle (ADG, kg · d-1), beef production per hectare (BP, kg · ha-1) and stocking rate (AUD · ha-1) measured in trials conducted from 1940 to 1963. These analyses identified an optimal biophysical SR of 13.5 Animal Unit Days (AUD) · ha-1. Here, we 1) examine modern era (2000-2018) SR experiments to determine if there has been a shift in the optimal biophysical SR from Bement (1969), and 2) assess the influence of drought (<75% of normal precipitation) on the optimal biophysical SR. For all modern years combined, the optimal SR occurred at 23.2 AUD · ha-1, or which is 70% higher than the value reported by Bement (1969). When we excluded the three drought years, the optimum SR for normal-to-wet years increased to 25.6 AUD · ha-1, or 10% higher than the SR value for all modern years combined. For the drought years in the modern era, we calculated an optimum SR of 14.2 AUD · ha-1, or 39% lower than the SR value for all modern era years. Our results show the capacity of this shortgrass steppe rangeland to produce livestock weight gains has increased substantially from the Bement (1940-1963) to the modern era. This directional shift to a higher optimum SR is likely driven by two non-mutually-exclusive factors. First, the plant community changed from dominance by a C4 shortgrass (Bouteoula gracilis, blue grama) in the Bement era to co-dominance with a more productive C3 mid-grass (Pascopyrum smithii), western wheatgrass) in the modern era. Second, the entry weights and genetic growth potential of yearling steers increased over the eight decades. Our findings provide guidance for incorporating flexible optimum SR in non-drought and drought years for adaptive management strategies in this rangeland.