Summer diets of sheep grazing spotted knapweed-infested foothill rangeland in Western Montana

Abstract

Spotted knapweed (Centaurea biebersteinii DC.) is a perennial, invasive forb that infests millions of hectares of private and public rangelands in western North America. Previous research indicates that spotted knapweed is nutritious and readily grazed by domestic sheep (Ovis aries), but no studies have investigated prescription grazing of spotted knapweed within different levels of infestation or on a landscape scale. This twoyear study quantified the diets of a ewe-lamb band (n almost equals 800 ewes, 1120 lambs) that prescriptively grazed spotted knapweed-infested foothill rangeland in western Montana. Sheep grazed within light and moderate infestations of spotted knapweed (13% and 36% of vegetative composition, respectively) until perennial grasses were reduced to a 5 to 8- cm residual stubble height. Diets were estimated in mid-June and mid-July by clipping current year's standing crop immediately before and after grazing. Clipped samples were analyzed for CP, NDF, and ADF to estimate nutritive quality. Relative preference indices were calculated to evaluate diet selection by sheep. Sheep ate more spotted knapweed in moderate versus light infestations (64 vs. 26% of their diets, respectively; P<0.01), and spotted knapweed averaged 45% of sheep diets between June and July (P=0.61). Within light infestations, sheep ate fewer graminoids in June than July (17 vs. 55% of their diet, respectively; P<0.01). Sheep diets in moderate infestations averaged 33% graminoids regardless of month (P=0.18). Sheep did not select for graminoids in light infestations in June, but did select for spotted knapweed leaves in moderate infestations during July. Nutritive quality of sheep diets was similar to sheep grazing uninfested rangeland. Relative utilization of graminoids averaged 15%, except under exceptionally hot and dry weather conditions. Relative utilization of spotted knapweed averaged 45%. Previous research suggests that this level of spotted knapweed utilization may render herbicide application uneconomical. My results indicate that sheep can prescriptively graze moderate spotted knapweed infestations in either June or July, but to limit graminoid consumption, light infestations should be grazed in June vs. July.