Prescription grazing by cattle and sheep to manage yellow starthistle on Idaho rangelands. Hovde, Elayne^*,1, Launchbaugh, Karen¹, Wilson, Linda¹, ¹ University of Idaho, Moscow, ID, USA

ABSTRACT- Since the introduction of yellow starthistle (YST: Centaurea solstitialis)to North America in the late 1800's it has become one of the West's most detrimental weeds to rangeland and recreations areas. Currently it has infested millions of acres in California, Washington, Oregon and Idaho. We assessed the effects of prescribed grazing by examining sheep and cattle utilization of YST in 3 phenological stages: rosette, bolt and flowering. Research was conducted with dry ewes and cows in paddocks located on the breaks above the Clearwater River near Lewiston, Idaho. The grazing trial was implemented in a starthistle-infested, previously cultivated, site dominated by exotic and introduced grasses and native perennial forbs. Sheep and cattle grazed at a stocking rate of about 5 AUM/ha during each phenological stage with appropriate ungrazed controls. In the rosette stage, selection by cattle and sheep for YST was similar to other forbs and grasses. Sheep more readily grazed YST than cattle, particularly in the bolting stage. As YST matured, leaves and small branches were selected over the large main stems and thorn armored flowers. In the flowering stage, cattle grazed very few starthistle plants, and preferred areas away from the heavily infested patches. Preliminary analysis indicates significant reduction in YST density that was not associated with any of the grazing treatments. Biological controls were also examined to explain this reduction. YST density, flower production, cover of herbage plants and utilization will be presented.

KEY WORDS: prescribed , centaurea solstitialis, grazing