Effects of Nitrogen Fertilization on Spotted Knapweed and Competing Vegetation in Western Montana

Abstract

The effects of N fertilization on spotted knapweed (Centaurea maculosa Lamarck) and competing vegetation were studied at 2 sites in western Montana during 1981 through 1984. The N was applied 1 time at each site at rates of 56, 112, 224, and 448 kg/ha. Spotted knapweed biomass showed a significant, positive yield response to N at all rates at both sites during the year of application. The only response by spotted knapweed to N in succeeding years was in the second year at Site 1 where a significant response was detected at the 448 kg N/ha rate. Competing vegetation at Site 1 (primarily quackgrass, Agropyron repens (L.) Beauv.) in the first year showed a significant, positive response to N, while competing vegetation at Site 2 (primarily crested wheatgrass, Agropyron cristatum (L.) Gaertn.) did not respond to N in the first year. No response by competing vegetation to N in succeeding years at either site was detected. There was a significant relationship between percent knapweed and N rate at both sites in the year of N application, but not in succeeding years. These results suggest that N fertilization, by itself, as a cultural control approach to knapweed may be impractical, and could contribute toward the increase of knapweed when used in some of the plant communities normally associated with spotted knapweed on rangeland in western Montana.