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Herbicidal control of spotted knapweed (Centaurea maculosa Lam.) on rangeland in the western United States has been most effective using residual herbicides, such as picloram (4-amino3,5,6-trichloropicolinic acid). However, when residual herbicides cause concerns in riparian areas and for non-target forbs, management practices that use herbicides with lower soil persistence need to be developed. The objective of this study was to quantify the interaction 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 Nfertilizer rates (0, 50, 100, 150, and 200 kg ha -1 ) were applied to 2 spotted knapweed infested rangeland sites in a factorial combination arranged in a randomized-complete-block design during the summer of 1996 in Montana. Spotted knapweed, other forb, and grass density and biomass were measured at peak standing grass crop in 1997 and analyzed using analysis of variance. 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 ha -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 biomass. At Hyalite Creek, 2,4-D at 0.6 kg ai ha -1 reduced spotted knapweed density by 30%, and rates greater than 0.6 kg ai ha -1 reduced it by 75%. Spotted knapweed biomass was reduced by 75% at all herbicide rates tested. N-fertilizer 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 Nfertilizer affected forbs. At Hyalite Creek, 2,4-D and N-fertilizer interacted 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 spotted knapweed without affecting early season forbs.

Spotted Knapweed, Forb, and Grass Response to 2,4-D and N-Fertilizer