Spotted knapweed and grass response to herbicide treatments

Picloram at 0.28 kg ai ha(-1), clopyralid plus 2,4-D at 0.21 kg ai ha(-1) p lus 1.12 kg ai ha(-1), or dicamba plus 2,4-D at 0.56 kg ai ha(-1) plus 1.12 kg ai ha(-1) were applied to spotted knapweed (Centaurea maculosa Lam.) at the spring-rosette. bolt, bud, flower, or fall-rosette growth stages in 19 91 on 2 sites in Montana. Treatments (3 herbicide treatments, 5 growth stag es) were applied in a randomized-complete-block design and replicated 3 tim es at each site. Effects of herbicides on mature and seedling spotted knapw eed density depended upon spotted knapweed growth stage at the time of appl ication and the number of pears after application, Picloram consistently re duced mature spotted knapweed density to low levels (<5 plants m(-2)), rega rdless of growth stage, and its effect persisted through 1994. Clopyralid p lus 2,4-D applied at the bolt or bud stage reduced spotted knapweed densiti es similar to that of picloram (95%) at the Avon site, while providing abou t 50% reduction in density 3 years after application at Missoula, This trea tment may provide an alternative to picloram in environmentally sensitive a reas. Dicamba plus 2,4-D was most effective when applied during the bud and bolt growth stages, and least effective when applied during the spring- an d fall-rosette stages. In most situations, picloram and clopyralid plus 2,4 -D provided gl eater control of spotted knapweed than dicamba plus 2,4-D. H erbicide treatments increased perennial grass biomass from 173 kg ha(-1) in the nontreated controls to 494, 880, and 1,309 kg ha(-1) for dicamba plus 2,4-D, clopyralid plus 2,4-D and picloram, respectively.