Adaptive grazing and animal density implications for stocking rate and drought in northern mixed-grass prairie

Abstract

Grazing management is critical for ruminant livestock enterprises affecting both animal performance and land sustainability. Intensification of grazing management is important for increasing food production, however, linear stocking rate models are not ideal in rangeland environments due to complexities such as drought. We conducted a study in a semiarid northern mixed-grass prairie in Wyoming, USA from 2017 to 2021. Grazing treatments were moderate rotational grazing (MRG; 4 heifers), ultra-high density rotational grazing (UHD; 33 cow-calf pairs), and no grazing (NG) organized in a randomized complete block design using 0.4 ha paddocks. Baseline vegetation was measured prior to grazing each year to determine grazing time available using a modified Robel pole coupled with forage biomass clippings. Planned and realized stocking rates were then calculated for all MRG and UHD treatments and potential stocking rates for NG and relativized to effective precipitation (March 1 to timing of sampling in mid-June). For the UHD treatment, cattle time in paddocks ranged from 0.29 to 1 d while for the MRG treatment, cattle time in paddocks ranged from 4.5 d to 12.5 d Effective precipitation ranged from 130 mm in 2020 to 251 mm in 2018 with a five year mean of 205.4 mm. Planned/potential stocking rate ranged from 1.40 to 5.17 Animal Unit Months per hectare (AUMs ha−1) and realized stocking rate ranged from 1.38 to 4.87 AUMs ha−1 across paddocks and years. Vegetation visual obstruction and stocking rate did not differ by treatment any of the years of the study (all p-values > 0.05) with the exception of 2020 when the NG treatment was higher than the MRG and UHD treatments (p < 0.05). Planned and realized stocking rates for both grazing treatments had highly correlated (both r2-values ≥ 0.80) and significant relationships (both p-values < 0.05) with effective precipitation. Five year mean planned or realized stocking rates did not differ by treatment. Accomplishing planned stocking rates proved to be difficult for the UHD grazing scheme that requires rapid moves as indicated by the broader range of ΔAUMs ha−1 (Planned – Realized) in that treatment compared to the MRG treatment. The effect of precipitation on stocking rates, and specifically relative to potential stocking in the NG treatment, was most influential during the significant deficit year but was non-existent during average years or surplus years. Using annual visual obstruction readings (VOR) to adapt stocking rates may be useful for more intense grazing management.