Evaluation of herbicides for Geyer larkspur (Delphinium geyeri) control

Abstract

Geyer larkspur is a native perennial forb toxic to cattle. Herbicide control of Geyer larkspur is variable and most likely attributable to the growth stage of the plant when 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 if plant growth stage (vegetative or flowering) at the time of herbicide application influences herbicide effectiveness; and (3) determine whether herbicide treatment alters norditerpenoid alkaloid content of Geyer larkspur. Plots were established in eastern Wyoming (2021) and northern Colorado (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 year after treatment (YAT) at the vegetative stage and by all herbicides except for triclopyr 2 YAT (P = 0.0249). At the flowering stage, Geyer larkspur density was reduced by all herbicides except metsulfuron-methyl, triclopyr, and triclopyr + 2,4-D at 1 YAT and 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. Control efforts of Geyer larkspur in semiarid rangelands can be effectively accomplished with aminopyralid herbicides across vegetative and flowering growth stages provided environmental conditions prior to herbicide application are sufficient for plant growth and uptake of the herbicide.