Stocking Rate and Weather Impacts on Sand Sagebrush and Grasses: A 20-Year Record

Understanding how stocking rate or grazing intensity affects the abundance of common plant species is fundamental to the sustainable management of rangelands. We had the unique opportunity to determine the impact of stocking rate on shrub canopy cover and grass basal cover in a sand sagebrush (Artemisia filifolia Torr.) grassland of the Southern Plains, United States. Treatments were imposed over a 20-year time span that included an entire precipitation cycle from wet to dry and back to wet conditions. From 1941 until 1951, continuous stocking treatments of 41, 53, and 82 animal-unit-days ha-1 (AUD ha-1) were imposed with straight-bred Hereford steers (initial weight of 213 kg +/- 11 SE) for about 320 days from mid-November to late September of the next year. From 1952 through 1961, the experimental pastures were grazed yearlong by cow-calf pairs at 45, 60, and 87 AUD ha-1. Canopy cover of shrubs and basal cover of grasses were measured by the line-intercept method in 1940, 1942, 1949, 1955, 1958, and 1961. Canopy cover of sand sagebrush was not affected by stocking rate. Individual grass species exhibited positive and negative responses to stocking rate in some years, but no grass species responded to stocking rate in a single direction over the entire length of this long-term study. Stocking rate effects were most obvious under favorable conditions of high precipitation, but these effects were absent during drought. Climatic variability and slope gradient exerted the primary controlling influences on sand sagebrush-grasslands in the Southern Great Plains when stocking rates were within the bounds tested in this study.