Response of Delphinium occidentale and Associated Vegetation to Aminocyclopyrachlor

Duncecap larkspur (Delphinium occidentale [Wats.] Wats.) is an important perennial weed on high-elevation rangelands because of significant cattle losses due to toxic alkaloids in the plant. Aminocyclopyrachlor was evaluated at six rates between 17.5 and 315 g ai · ha− 1 for larkspur control alone, and in combination with chlorsulfuron or metsulfuron, at a high-elevation location in Wyoming. Aminocyclopyrachlor-containing treatments were compared with 1120 g ai · ha− 1 picloram and 63 g ai · ha− 1 metsulfuron-methyl. Herbicides were applied to two sites in a randomized complete block design with four replicates each on June 18, 2010 at the first site and June 28, 2011 at the second site. Larkspur mortality, plant species richness, vegetation cover, and grass biomass data were collected 1 yr after treatment. Cover data were used to calculate vegetation diversity and to assess changes in species composition associated with herbicide application. A four-parameter log-logistic model was used to evaluate larkspur mortality, species richness, and vegetation cover in response to aminocyclopyrachlor rate. Ninety-percent larkspur reduction was obtained with aminocyclopyrachlor applied alone at rates of 168 − 303 g ha− 1, depending on site. Mixture of aminocyclopyrachlor plus chlorsulfuron at a 2.5:1 ratio required 102 − 127 g ha− 1 of aminocyclopyrachlor to reduce larkspur 90%. Aminocyclopyrachlor plus metsulfuron was the most effective herbicide combination for larkspur control of those we evaluated, requiring 47 + 15 g ha− 1, respectively, to reduce larkspur 90%. Species richness and diversity were reduced by herbicide rates required to effectively control larkspur. Graminoid biomass was not significantly impacted by herbicide or rate. Aminocyclopyrachlor may be a useful tool for duncecap larkspur control. Addition of chlorsulfuron or metsulfuron to aminocyclopyrachlor increased larkspur control but had a greater impact on associated nontarget vegetation.