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Downy brome is a problematic invasive annual grass throughout western rangeland and has been increasing its abundance, spread, and impacts across Montana during the past several years. In an effort to develop effective management recommendations for control of downy brome on Montana rangeland, we compiled data from 24 trials across the state that investigated efficacy of imazapic (Plateau®, BASF Corporation, Research Triangle Park, NC) applied at various rates and timings and with methylated seed oil (MSO) or a nonionic surfactant (NIS). We ran a mixed-model ANOVA to test for main effects and interactions across application rate (70, 105, 141, 176, and 211 g ai ha−1), application timing (preemergent [PRE], early postemergent [EPOST, one- to two-leaf growth stage], and postemergent [POST, three- to four-leaf growth stage]), and adjuvant (MSO, NIS). Application timing and rate interacted to affect downy brome control (P = 0.0033). PRE imazapic application resulted in the lowest downy brome control (5 to 19%), followed by POST application (25 to 77%) and EPOST application (70 to 95%). Downy brome control remained fairly consistent across rates within application timing. Adjuvant (MSO or NIS) did not affect downy brome control (P = 0.2789). Our data indicate that POST application at 105 to 141 g ai ha−1 provides the most-consistent, short-term control of downy brome. Furthermore, applying imazapic to downy brome seedlings shortly after emergence (one- to two-leaf growth stage) provided better control than applying it to older downy brome seedlings (three- to four-leaf growth stage).

No abstract

Picloram at 0.28 kg ai ha-1 , clopyralid plus 2,4-D at 0.21 kg ai ha-1 plus 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 fallrosette growth stages in 1991 on 2 sites in Montana. Treatments (3 herbicide treatments, 5 growth stages) were applied in a randomized-complete-block design and replicated 3 times at each site. Effects of herbicides on mature and seedling spotted knapweed density depended upon spotted knapweed growth stage at the time of application and the number of years after application. Picloram consistently reduced mature spotted knapweed density to low levels (<5 plants m-2), regardless of growth stage, and its effect persisted through 1994. Clopyralid plus 2,4-D applied at the bolt or bud stage reduced spotted knapweed densities similar to that of picloram (95%) at the Avon site, while providing about 50% reduction in density 3 years after application at Missoula. This treatment may provide an alternative to picloram in environmentally sensitive areas. 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-and fall-rosette stages. In most situations, picloram and clopyralid plus 2,4-D provided greater control of spotted knapweed than dicamba plus 2,4-D. Herbicide 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.