The Effect of Grazing and Aminopyralid Herbicide Treatment on Spotted Knapweed Infested Rangeland
Landon J Edwards
Committee: Alisa Hove, Bob Eckstein, Amy Boyd
Spotted knapweed (Centaurea maculosa), an invasive weed, has severely affected the productivity of western rangelands by decreasing forage palatability, yield and diversity. Rangeland managers are struggling to control knapweed in a way that is economically and environmentally sustainable. Effects of various weed eradication methods have been reported, but the effect of combinations of different control methods on weed removal is not well understood. This study presents the results of a two-year analysis of the effects of a single anminopyralid herbicide application following multiyear grazing on a population of the spotted knapweed in Missoula, Montana, USA. The research aimed to provide local land managers with a sustainable, effective weed management plan. We hypothesized that one application of herbicide after multiple years of grazing would reduce the density of spotted knapweed more than grazing or spraying would alone. A secondary aim was to determine if the herbicide affected other perennial forbs or grass species. We tested four different treatment combinations (grazed and sprayed (G+S), non-grazed and non-sprayed (NG+NS), grazed and non-sprayed (G+NS) and non-grazed and sprayed (NG+S)), with the grazed plots having been grazed for seven summers prior to the beginning of the study. Ten paired plots were established with five pairs in grazed areas and five pairs in non-grazed areas. One plot in each pair was sprayed with herbicide in summer 2014. In each plot, we measured the total density of spotted knapweed and the percent cover of perennial grass, annual grass, and perennial forbs. Results showed that grazing significantly affected total, nonflowering and seedling spotted knapweed density and spraying significantly affected flowering spotted knapweed density. The combination of treatments was most effective overall. Grazing and spraying had no effect on native perennial grass, silky lupine or common yarrow. Therefore, these treatments can decrease knapweed density while preserving native vegetation.
Key Words: Grazing; Invasive Species; Herbicide; Sheep; Rangeland; Spotted Knapweed; Land Management