Grazing strategies, stocking rates, and frequency and intensity of grazing on western wheatgrass and blue grama

Abstract

Stocking rates and grazing strategies may alter botanical composition of rangeland vegetation by altering frequency and intensity of defoliation of individual plant species. We used long-interval time-lapse photography to study frequency and intensity of defoliation of western wheatgrass (Pascopyrum smithii[Rydb.] A. Love) and blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Steud.) tillers under continuous season-long and time-controlled short-duration rotation grazing by steers at 2 stocking rates. Frequency, intensity, and variability of defoliation of both grasses were similar under both grazing systems. Western wheatgrass tillers were grazed more frequently under heavy than under moderate stocking, and in 1990 more herbage was removed the second time a tiller was grazed under heavy stocking. Blue grama tillers were grazed more frequently under heavy than under moderate stocking in both years under rotation grazing, but only in 1990 under continuous grazing; more herbage was removed under heavy stocking the second time a tiller was grazed. Under heavy and moderate stocking, respectively, 19% and 36% of western wheatgrass tillers and 42% and 54% of blue grams tillers were ungrazed throughout the grazing season. Few western wheatgrass tillers were grazed more than twice, and few blue grams tillers were grazed more than once. Stocking rates have much greater potential than grazing systems for altering frequency and intensity of defoliation and subsequent changes in botanical composition of range plant communities. Results of grazing studies support this conclusion.