Multidecadal directional shift in shortgrass stocking rates

Abstract

Bement (1969) developed a stocking rate (SR) guide for yearling cattle grazing shortgrass steppe based on relationships among average daily weight gain (ADG, kg · d−1), beef production per hectare (BP, kg · ha−1), and stocking rate (animal unit days, AUD · ha−1) measured in long-term grazing experiments conducted from 1940 to 1963. These analyses identified an optimal biophysical SR of 13.5 AUD · ha−1. Here, we 1) examine modern era (2000−2018) SR results from these same long-term grazing experiments to determine if there has been a shift in the optimal biophysical SR and 2) assess the influence of drought (< 75% of normal precipitation) on the optimal biophysical SR. For all years in the modern era, the optimal SR occurred at 23.2 AUD · ha−1, 72% higher than the value reported by Bement (1969). For the 3 drought yr, the optimum SR was 14.2 AUD · ha−1, which still exceeded the optimal SR by Bement (1969). Our results show the capacity of this shortgrass steppe rangeland to produce livestock weight gains has increased substantially between the Bement and modern eras. This multidecadal directional shift to a higher optimum biophysical SR is likely driven by two nonmutually exclusive factors. First, the plant community changed from dominance by a C4 shortgrass (Bouteloua gracilis) in the Bement era to codominance with a more productive C3 midgrass (Pascopyrum smithii) in the modern era. This change has resulted in pasture-level forage production increasing notably between the two eras. Second, the entry weights and genetic growth potential of yearling steers increased over the 8 decades and may have influenced the efficiency of weight gain for a given amount of forage consumed. Our findings provide guidance for incorporating flexible optimum SR in nondrought and drought years for adaptive grazing management strategies.