Centaurea stobe or C. maculosa
Spotted knapweed is a biennial or short-lived perennial plant that grows from 1 to 4 feet tall. It reproduces by seed and has a thick taproot. Seedlings develop the first year into rosettes of narrow, deeply lobed leaves that are up to 6 inches long. The upper leaf surface is rough. Flowering plants produce one to many stems with numerous branches. Stem leaves are smaller and linear, arranged alternately along the stem. A single flowerhead is produced at the end of each branch. Bracts at the base of the flowerhead are black-tipped which gives them a spotted appearance when viewed from a distance. The flowers are pink to light purple in color and mature into brown seeds tipped with a plume of soft tawny bristles.
Type and Class of Livestock: Sheep and goats.
Grazing Objective: Graze to prevent seed production and reduce biomass.
Growth Stage for Treatment: Graze spotted knapweed heavily during the rosette or bolting stage. Livestock prefer young, smaller plants, but will usually readily consume it at all growth stages. Two grazing periods per year, once during rosette to bolting stage and again in the bud stage, provide the best control. Stem reductions, smaller plants, and lower seed production can occur after three to six consecutive years of grazing.
Potential Effectiveness: Sheep and goats readily graze spotted knapweed, considered to be moderately good forage for livestock. Sheep tend to strip leaves and avoid the fibrous stems of mature plants. Grazing can reduce plant vigor, density, size, flower stems, and seed production. It may be necessary to manage grazing based on degree of utilization of desirable species. Palatability may be reduced as the plant ages because of reduced forage value and the presence of a bitter-tasting compound called cnicin. Sheep digestive systems may suffer if diets are composed of more than 70% spotted knapweed. Grazing is most effective when combined with herbicide treatments.
Kennett, G.A., J.R. Lacey, C.A. Butt, K.M. Olsen-Rutz, and M.R. Haferkamp. 1992. Effects of defoliation, shading and competition on spotted knapweed and bluebunch wheatgrass. Journal of Range Management 45:363-369.
Lacey, J.R., K.M. Olsen-Rutz, M.R. Haferkamp and G.A. Kennett. 1994. Effects of defoliation and competition on total non-structural carbohydrates of spotted knapweed. Journal of Range Management 47:481-484.
Launchbaugh, K. and J. Hendrickson. 2001. Prescription grazing for Centaurea control on rangelands. In: L. Smith [ED.] The First International Knapweed Symposium of the Twenty-First Century. Coeur d’ Alene, ID. p. 27-32. Available at: http://www.sidney.ars.usda.gov/knapweed/images/proceed.pdf. Accessed 03 September 2006.
Maxwell J.F., R. Drinkwater, D. Clark, and J.W. Hall. 1992. Effects of grazing, spraying, and seeding on Knapweed in British Columbia. Journal of Range Management 45:180-182.
Olson, B.E., and J.R. Lacey. 1994. Sheep: a method for controlling rangeland weeds. Sheep Research Journal (Special Issue):105-112.
Olsen, B.E., and R.T. Wallander. 1997. Biomass and carbohydrates of spotted knapweed and Idaho fescue after repeated grazing. Journal of Range Management. 50:409-412.
Olsen, B.E., and R.T. Wallander. 2001. Sheep grazing spotted knapweed and Idaho fescue. Journal of Range Management 54:25-30.
Olsen, B.E., R.T. Wallander, and J.R. Lacey. 1997. Effects of sheep grazing on a spotted knapweed-infested Idaho fescue community. Journal of Range Management 50:386-390.
Sheley, R.L., J.S. Jacobs, and J.M. Martin. 2004. Integrating 2,4-D and sheep grazing to rehabilitate spotted knapweed infestations. Journal of Range Management 57:371-375.