Spotted knapweed, forb, and grass response to 2,4-D and N-fertilizer

Herbicidal control of spotted knapweed (Centaurea maculosa Lam.) on rangela nd in the western United States has been most effective using residual herb icides, such as picloram (4-amino-3,5,6-trichloropicolinic acid). However, when residual herbicides cause concerns in riparian areas and for non-targe t forbs, management practices that use herbicides with lower soil persisten ce need to be developed. The objective of this study was to quantify the in teraction between 2,4-D (2,4-Dichlorophenoxyacetic acid, dimethylamine salt ) and N-fertilizer on spotted knapweed, other forbs, and grass density and biomass. Five 2,4-D rates (0.0, 0.6, 1.1, 1.6, and 2.2 kg ai ha(-1)) and 5 N-fertilizer rates (0, 50, 100, 150, and 200 kg ha(-1)) were applied to 2 s potted knapweed infested rangeland sites in a factorial combination arrange d in a randomized-complete-block design during the summer of 1996 in Montan a. Spotted knapweed, other forb, and grass density and biomass were measure d at peak standing grass crop in 1997 and analyzed using analysis of varian ce. Spotted knapweed density and biomass at Rock Creek were reduced 50% and 65%, respectively, by 2,4-D of treatments of 1.1 kg ai ha(-1) or greater. Spotted knapweed biomass was slightly increased by N-fertilizer at 200 kg h a(-1). Grass density increased by about 50% when treated with 2,4-D of 1.1 kg ai ha(-1) or greater N-fertilizer did not affect grass density or biomas s. At Hyalite Creek, 2,4-D at 0.6 kg ai ha(-1) reduced spotted knapweed den sity by 30%, and rates greater than 0.6 kg ai ha(-1) reduced it by 75%. Spo tted knapweed biomass was reduced by 75% at all herbicide rates tested. N-f ertilizer and 2,4-D interacted to increase grass density at Hyalite Creek; however, grass biomass was not affected. At Rock Creek, neither 2,4-D nor N -fertilizer affected forbs. At Hyalite Creek, 2,4-D and N-fertilizer intera cted to increase aster (Aster eatonii [Gray] Howell) biomass. Death camas ( Zigadenus venenosus Wats.) biomass was increased by N-fertilizer addition. Combining N-fertilizer with 2,4-D may increase long-term control of spotted knapweed when residual herbicides cannot be used. Application of 2,4-D at the bud stage of spotted knapweed growth will provide some control of spott ed knapweed without affecting early season forbs.