Response of spotted knapweed and grass to picloram and fertilizer combinations.

Abstract

Spotted knapweed (Centaurea maculosa Lam.) has reduced forage production, increased soil erosion, and lowered biodiversity on millions of hectares of rangeland throughout the western United States. Objectives of this study were to quantify the interaction between picloram (4-amino-3,5,6-trichloropicolinic acid) and fertilizer on spotted knapweed density 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, 10.5+12.2, 21.1+26.4, and 31.7+39.6 kg ha-1) were applied to 3 spotted knapweed infested rangeland sites in a factorial combination arranged in a randomized-complete-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 variance. Picloram and fertilizer did not interact to affect either spotted knapweed density or grass yield. All picloram treatments reduced spotted knapweed density to nearly zero. By 1995, all picloram treatments increased grass yield by an average of 1,500 kg ha-1. Fertilization did not affect spotted knapweed density, but the highest rates increased grass yield on those sites with a substantial residual grass understory. Combining fertilizer with picloram may enhance grass yield on sites with a residual of highly productive grasses.