Author
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Gillen, Robert |
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Sims, Phillip |
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Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/22/2005 Publication Date: 3/1/2006 Citation: Gillen, R.L., Sims, P.L. 2006. Stocking rate and weather impacts on sand sagebrush and grasses: a 20-year record. Rangeland Ecology and Management. 59(2)145-152. Interpretive Summary: Knowledge of the impact of grazing intensity on the abundance of common plant species is fundamental to the sustainable management of rangelands. As grazing intensity increases, the most palatable plants in a pasture often decrease while less useful species increase. Our objective was to determine the impact of grazing intensity on shrub and grass cover in a sand sagebrush grassland of the Southern Plains. Light, moderate, and heavy grazing treatments were applied from 1941 to 1961. The percentage of ground surface covered by grasses and shrub canopy was measured along line transects in 1940, 1942, 1949, 1951, 1955, 1958, and 1961. Canopy cover of sand sagebrush was not changed by grazing intensity. Some grasses increased or decreased in some years but no grass species responded to grazing intensity in a single, consistent direction over the entire length of this 20-year study. Grazing intensity effects were most obvious during years with high precipitation but these effects disappeared during drought. Variability in annual precipitation was the primary factor controlling populations of sand sagebrush and grasses on these rangelands as long as grazing intensity was maintained within the bounds tested in this study. Ranchers should be able to graze at a level that maximizes net profit and still maintain the ecological integrity of these native grasslands. Technical Abstract: Knowledge of the relationships between stocking rate or grazing intensity and abundance of common plant species is fundamental to the sustainable management of rangelands. Our objective was 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, USA. Light, moderate, and heavy grazing treatments, set at 41, 53, and 82 animal-unit-days ha-1 (AUD ha-1), were grazed with straight-bred Hereford steers with an initial weight of 213 ± 11 (s.e.) kg for about 320 d from about 13 November to 29 September of the next year, between 1940 and 1951. From 1952 through 1961 the pastures were grazed year-long by cows and calves at 45, 60, and 87 AUD ha-1. Basal cover of grasses and the canopy cover of shrubs were measured along line transects in 1940, 1942, 1949, 1951, 1955, 1958, and 1961. Canopy cover of sand sagebrush was not affected by stocking rate. Individual grasses 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 20-year study. Stocking rate effects were most obvious under favorable conditions of high precipitation and reduced competition from sand sagebrush but these effects disappeared during drought. Climatic variability and slope gradient exerted the primary controlling influence on sand sagebrush-grasslands in the Southern Great Plains when stocking rates were within the bounds tested in this study. |
