Long term change in vegetation following herbicide control of larkspur

Abstract

Larkspur (Delphinium spp.) control can reduce cattle deaths on mountain rangelands, but vegetation cover must be maintained to protect watersheds from erosion. The objective of this study was to evaluate the long term impact of herbicides on larkspur control and cover of associated species. Duncecap larkspur (Delphinium occidentale S. Watts) near Oakley Ida., and tall larkspur (D. barbeyi Huth) near Manti Utah, were the target species. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) was applied at 1.1, 2.2, and 4.5 kg ae/ha; glyphosate (N-(phosphonomethyl)glycine) was applied at 0.06, 1.1, and 2.2 kg ai/ha; and metsulfuron (1-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amine]carbonyl ]amino]sulfonyl]benzoic acid) was applied at 0.035, 0.07, and 0.14 kg ai/ha. Picloram at 2.2 kg/ha maintained long-term control of both larkspur species (> 80%) when applied in the vegetative, bud, and flower growth stages. Total grass cover was higher on picloram plots than other treatments. Forb cover declined and bare ground was greater in picloram plots than other treatments at Manti. Metsulfuron controlled duncecap larkspur when applied in the vegetative stage. However, long-term control of tall larkspur at Manti declined as new tall larkspur seedlings established. Glyphosate controlled both larkspurs when applied in the vegetative and bud stages, but it allowed undesirable annual and rhizomatous forbs and shrubs to increase by the end of the study. Grass cover was lower on glyphosate plots than on other treatments. Bare ground was higher on glyphosate plots than other treatments at Oakley, but was intermediate at Manti.