Seasonal change in nutrient composition of spotted knapweed and preference by sheep

Spotted knapweed (Centaurea stoebe ssp. micranthos) is one of the most troublesome rangeland weeds in western North America. As part of a larger investigation evaluating the use of targeted sheep grazing to control spotted knapweed, we examined sheep preference and forage quality of dried spotted knapweed in different phenological stages. We collected spotted knapweed for preference evaluation trials from a sagebrush steppe community near Dubois, Idaho throughout the 1999 growing season and analyzed the plants for nutritional quality. Preference for spotted knapweed was evaluated in a cafeteria trial, where we offered 12 ewes dried and chopped spotted knapweed in rosette, bolting, and flowering/seedset phenological stages. Nutritional value of spotted knapweed was highest in early phenological stages and declined with phenological development. Sheep readily consumed spotted knapweed in all phenological stages; however, rosette and bolting knapweed were generally selected over the flowering/seedset knapweed. Spotted knapweed consumption was related to crude protein, and neutral detergent fiber content of spotted knapweed, yet consumption was not related to cnicin content, a suspected feeding deterrent. This study suggests knapweed is acceptable forage that has adequate nutritional value during the growing season to sustain wild and domestic ungulates.