RESPONSE OF SPOTTED KNAPWEED AND GRASS TO PICLORAM AND FERTILIZER COMBINATIONS

Spotted knapweed (Centaurea maculosa Lam.) has reduced forage producti on, increased soil erosion, and lowered biodiversity on millions of he ctares of rangeland throughout the western United States. Objectives o f this study were to quantify the interaction between picloram (4-amin o-3,5,6-trichloropicolinic acid) and fertilizer on spotted knapweed de nsity and grass yield. Four picloram rates (0.0, 0.14, 0.28, and 0.42 kg ha(-1)) and 4 fertilizer rates (N+P: 0.0+0.0, 105+12.2, 21.1+26.4, and 31.7+39.6 kg ha(-1)) were applied to 3 spotted knapweed infested r angeland sites in a factorial combination arranged in a randomized-com plete-block design during the spring of 1994. Grass yield and spotted knapweed density were measured at peak standing grass crop in 1994 and 1995. Data were analyzed as a split-plot in time using analysis of va riance, Picloram and fertilizer did not interact to affect either spot ted knapweed density or grass yield, All picloram treatments reduced s potted knapweed density to nearly zero, By 1995, all picloram treatmen ts increased grass yield by an average of 1,500 kg ha(-1), Fertilizati on did not affect spotted knapweed density, but the highest rates incr eased grass yield on those sites with a substantial residual grass und erstory, Combining fertilizer with picloram may enhance grass yield on sites with a residual of highly productive grasses.