Integrating 2,4-D and sheep grazing to rehabilitate spotted knapweed infestations

Abstract

Effective integrated weed management requires understanding the impacts of management strategies applied alone and in combination. The objectives of this study were to determine the effects of 2,4-D and repeated sheep (Ovis aries) grazing applied alone and in combination on spotted knapweed (Centaurea maculosa Lam.) and perennial grasses. We hypothesized that integrating a single spring 2,4-D application would remove the adult plants, repeated sheep grazing would control rosettes, and spotted knapweed density, cover and biomass would decrease, allowing residual grasses to reoccupy the sites. A single spring 2,4-D application at 2.1 kg a.i. ha⁻¹, repeated annual sheep grazing of 95% spotted knapweed or 60% grass utilization, and repeated sheep grazing and 2,4-D combined were applied to small pastures at 2 sites in western Montana beginning in 1997 and continued through 2001. Spotted knapweed rosette and flowering plant density, and spotted knapweed and perennial grass cover and biomass were sampled from 1998 through 2001. Spotted knapweed rosette density was 61.7, 34.3, 44.3, and 0.3 m⁻² in the control, sheep grazing, 2,4-D, and combined sheep grazing and 2,4-D treatments, respectively, at 1 site in 2001. Spotted knapweed flowering plant density increased from 3.7 in 1998 to 10.7 m⁻² in 2002 in the 2,4-D treatment whereas there was no increase in the 2,4-D combined with sheep grazing treatment from 1998 to 2002. Perennial grass biomass was 6.9, 8.4, 25.7, and 19.7 in the control, sheep grazing, 2,4-D, and combined sheep grazing and 2,4-D treatments, respectively, averaged for both sites and 4 years of sampling. Herbicides released perennial grasses from weed competition and changed the weed population from mature, less palatable plants to juvenile plants that were preferred by sheep.

DOI:10.2458/azu_jrm_v57i4_sheley