Chairperson, Graduate Committee: Bret E. Olson.Cheeseman, Melany2013-06-252013-06-252006https://scholarworks.montana.edu/handle/1/1063Spotted knapweed (Centaurea maculosa Lam.), an invasive forb from Eurasia, infests over 1.7 million hectares in Montana and costs millions of dollars in forage losses each year. Some wildlife and livestock graze this species, however, most large herbivores, including cattle and horses, avoid spotted knapweed and prefer native plants. Some herbivores may avoid or reduce their intake of spotted knapweed because the plant contains cnicin. Cnicin is a bitter-tasting sesquiterpene lactone that may cause negative post-ingestive consequences in the rumen. If an appropriate supplement and/or antitoxicant can be identified that will reduce the negative post-ingestive effects of cnicin to rumen microbes, sheep and cattle may consume more spotted knapweed and grazing may help control this invasive species. I determined the effects of cnicin, rates of an anti-toxicant (polyethylene glycol), and a nutrient/energy supplement, with or without polyethylene glycol, on sheep and cattle rumen microbial activity, efficiency, and mass using a modified in vitro system. Sheep and cattle rumen microbes were negatively affected by cnicin. Polyethylene glycol, at the rates provided, had limited effects on sheep and cattle rumen microbial response to spotted knapweed plant parts. The nutrient/energy supplement, with or without polyethylene glycol, enhanced sheep and cattle rumen microbial efficiency. Because the nutrient/energy supplement may reduce some of the negative effects of cnicin on rumen microbes, sheep and cattle may consume more spotted knapweed in the field when provided with the supplement. I hypothesized that providing a nutrient/energy supplement, with or without polyethylene glycol, would increase sheep and cattle intake and time spent consuming spotted knapweed in a drylot, and in the field in July and August. Animal behaviors were recorded by focal animal sampling. Providing a nutrient/energy supplement, with or without polyethylene glycol, did not enhance sheep and cattle intake or time spent consuming spotted knapweed in a drylot, or in the field in July and August. Therefore, supplementing sheep and cattle with the nutrient/energy supplement, with or without polyethylene glycol, provided in these trials, may not be an appropriate method for managing the spread of spotted knapweed.enSpotted knapweedNoxious weedsGrazingManagementEcologyThesisCopyright 2006 by Melany Cheeseman