Evaluation of herbicides for Geyer’s larkspur (Delphinium geyeri)

Authors: Stonecipher, Clinton; Welch, Kevin; Derner, Justin; GARDNER, DALE - Retired ARS Employee; RANSOM, CORY - Utah State University; Cook, Daniel

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Geyer’s larkspur is a native perennial forb toxic to cattle. Herbicide control of Geyer’s larkspur is variable and most likely attributable to growth stage of the plant when herbicide is applied. The objectives of this study were to (1) evaluate the herbicides; Milestone, Duracor, Grazon HL, Chaparral, Escort, Remedy Ultra and Crossbow for efficacy in controlling Geyer’s larkspur; (2) determine if plant growth stage at the time of herbicide application influences herbicide effectiveness; and (3) determine whether herbicide treatment alters alkaloid content of Geyer’s larkspur. Plots were established at sites in eastern Wyoming, in 2021, and northern Colorado, in 2022. Herbicides were applied at vegetative or flowering plant growth stage, along with one nontreated control plot. At the Wyoming site, herbicide application at the vegetative stage provided the greatest control of Geyer’s larkspur. Milestone, Duracor, Grazon HL, and Chaparral all provided good control of Geyer's larkspur at the Wyoming site. There was no difference in herbicide control across treatments at the Colorado site. Precipitation was below average several months prior to herbicide application which may have affected herbicide effectiveness. Escort resulted in the highest total alkaloid concentrations of Geyer’s larkspur at the vegetative stage and lowest concentrations at the flowering stage at the Wyoming site. Control efforts of Geyer’s larkspur in semiarid rangelands can be effectively accomplished with Milestone, Duracor, Grazon HL, and Chaparral across vegetative and flowering growth stages provided environmental conditions prior to herbicide application are sufficient for plant growth and uptake of the herbicide.

Technical Abstract: Geyer larkspur is a native perennial forb toxic to cattle. Herbicide control of Geyer larkspur is variable and most likely attributable to 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 at sites 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 YAT at the vegetative stage and by all herbicides with the exception of 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 to the Wyoming site, 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 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.