Grass defoliation intensity, frequency, and season effects on spotted knapweed invasion

Preventing the invasion of uninfested rangeland is central to managing spot ted knapweed (Centaurea maculosa Lam.). Intensity, frequency, and season of grass defoliation determine the ability of grasses to tolerate grazing and resist weed invasion. We hypothesized that as grass defoliation intensity increases, spotted knapweed cover, density, and biomass would increase, tha t increasing defoliation frequency would increase the intensity effect, and that spring defoliation would cause a greater increase in spotted knapweed than summer defoliation. We hand clipped grasses in 1 m(2) plots at 2 spot ted knapweed infested Idaho fescue (Festuca idahoensis Elmer) sites in west ern Montana. Clipping treatments were 4 intensities (0, 30, 60, and 90% of the foliage), 3 frequencies (1, 2, and 3 at 14-day intervals), and 2 season s (spring beginning in mid-May and summer beginning in mid-July), factorial ly arranged in a randomized-complete-block design,vith 4 replications for a total of 24 treatments per replication. Treatments were applied in 1995 an d 1996. By 1997 grass cover and density were reduced by defoliation intensi ty of 90%. Defoliation frequency greater than once caused a reduction in gr ass cover and density at the 60% intensity. Spring defoliation caused a gre ater reduction in grass cover and density than summer defoliation. Grass bi omass was reduced by the 30% defoliation treatment. Grass defoliation inten sity greater than 60% caused an increase in spotted knapweed cover and dens ity. Defoliation more than once increased spotted knapweed cover. Spring de foliations increased spotted knapweed cover compared to summer defoliations . Spotted knapweed biomass was not affected by defoliation treatments. Our study suggests that an annual single grass defoliation of 60% or less, rega rdless of the season, will not increase spotted knapweed invasion on rangel and.