Grass defoliation intensity, frequency, and season effects on spotted knapweed invasion
Authors: J. S. Jacobs, and R. L. Sheley
Journal: Journal of Range Management
Summary of Methods: Density, cover and biomass of spotted knapweed was examined on Idaho fescue, bluebunch wheatgrass dominated sites in outside of Bozeman, Montana. This study was designed to examine the effects of grazing on spotted knapweed establishment. Idaho fescue plants within 1 m2 plots were clipped at four levels (0, 30, 60 and 90% defoliation), two seasons (spring and summer) and three frequencies (1, 2 and 3 at 14-day interval) over a period of three years for a total of 24 treatments replicated 4 times at 2 sites classified as Idaho fescue and bluebunch wheatgrass habitat types. Density of spotted knapweed was measured by counting the number of rosettes and flowering stems within a 10 by 50 cm plot. Cover of spotted knapweed and grass cover were estimated within the plot frame using 6 cover classes. Biomass of knapweed and grass was harvested within a 0.5 m2 hoop in late July the year following simulated grazing treatments.
Article Summary / Main Points: Defoliation levels of less than 60% did not increase spotted knapweed invasion. Spotted knapweed cover was highest when surrounding grasses were defoliated at 90%, though utilization levels greater than 60% caused an increased in spotted knapweed cover. Increased frequency of defoliation caused greater levels of spotted knapweed cover and density. Clipping treatment had no effect on the biomass of spotted knapweed at one of the study sites, where spotted knapweed was the dominant species. Season of clipping did result in increased spotted knapweed density and cover at this site. Early-season, infrequent use allows adequate recovery for grass plants and defoliation after plants have matured minimize the impact on native grass communities.
Vegetation Types: Mountain Shrublands
MLRA Ecoregions: 44B Central Rocky Mountain Valleys
Article Review Type: Refereed
Article Type: Experimental Research
Keywords: centaurea maculosa, weed invasion, grazing management, idaho fescue, festuca idahoensis, defoliation frequency
Annotation: This was a clipping study designed to simulate livestock utilization on Idaho fescue plants. Managers of rangelands with the treat of weed invasion need to keep in mind intensity, season and frequency of use when developing a grazing plan. Maintaining a use level of 60% or less does not affect competitive ability of the grass community.