Geyer larkspur is a native perennial forb that is toxic to cattle. Herbicide control of Geyer larkspur is variable and depends on the growth stage of the plant when the herbicide is applied. The objectives of this study were to 1) evaluate aminopyralid, aminopyralid + florpyrauxifen-benzyl, aminopyralid + 2,4-D, aminopyralid + metsulfuron-methyl, metsulfuron-methyl, triclopyr, and triclopyr + 2,4-D for efficacy in controlling Geyer larkspur; 2) determine whether plant growth stage (vegetative or flowering) at the time of herbicide application influences herbicide effectiveness; and 3) determine whether herbicide treatment alters the norditerpenoid alkaloid content of Geyer larkspur. Plots were established in eastern Wyoming in 2021 and northern Colorado in 2022. Herbicide application at the different phenological stages did not affect Geyer larkspur density at the Wyoming site (P = 0.1065; data not shown). Geyer larkspur density at the Wyoming site was reduced by all herbicide treatments 1 yr after treatment (YAT) at the vegetative stage and by all herbicides except triclopyr 2 YAT (P = 0.0249). Geyer larkspur density at the flowering stage was reduced by all herbicides except metsulfuron-methyl, triclopyr, and triclopyr + 2,4-D at 1 YAT and by triclopyr and triclopyr + 2,4-D at 2 YAT. In contrast, there were no differences in Geyer larkspur density across treatments at the Colorado site (P = 0.9621). Precipitation was below average several months prior to herbicide application, which may have affected herbicide effectiveness. The metsulfuron-methyl treatment resulted in the highest total alkaloid concentrations of Geyer larkspur at the vegetative stage and the lowest concentrations at the flowering stage at the Wyoming site. Efforts to control Geyer larkspur in semiarid rangelands can be effectively accomplished by applying aminopyralid herbicides at either the vegetative or flowering growth stage provided environmental conditions prior to herbicide application are sufficient for plant growth and uptake of the herbicide.
