Vegetation response to stocking rate in southern mixed-grass prairie

Abstract

Stocking rate directly influences the frequency and intensity of defoliation of individual plants which, in turn, impacts energy flow and plant succession in grazed ecosystems. The objective of this study was to determine the effect of stocking rate on standing crop dynamics and plant species composition of a southern mixed-grass prairie over a 7-year period (1990 through 1996). Long-term (30-year) mean precipitation has been 766 mm per year. Growing conditions were generally favorable for the study period. Yearling cattle (initial weight 216 kg, SD = 12 kg) grazed at 6 stocking rates, ranging from 23 to 51 AUD ha-1, from 14 April to 24 September (162 days). The currently suggested year-long stocking rate is 25 AUD ha-1. Herbage standing crop was measured in July and September every year while species composition was determined in July in even years. Total and dead standing crop declined as stocking rate increased but live standing crop was not related to stocking rate. Slopes of regression lines relating standing crop and stocking rate were constant over years, indicating no response for plant productivity. The major vegetation components, sideoats grama [Bouteloua curtipendula (Mich.) Torr.], shortgrasses, and forbs were not affected by stocking rate over years. Tallgrasses responded by increasing at the lower stocking rates over the study period. However, these grasses contributed less than 5% of the total standing crop. Red and purple threeawn (Aristida longiseta Steud. and A. purpurea Nutt.) increased at all stocking rates from 1990 to 1996 but the increase was greater at the lower stocking rates. This mixed-grass vegetation showed little response to stocking rate over the 7-year study period. The vegetation may have been in equilibrium with previous heavy stocking rates so that little change would be expected at those rates. Increases in grazing sensitive species at lighter stocking rates may occur over longer time intervals.