Management of three toxic Delphinium species based on alkaloid concentrations

A systematic approach to the taxonomic classification of the tall larkspur complex (Delphinium spp.) has been developed and implemented using molecular genetics, plant morphology, and alkaloid profiles, as shown in other papers in this series. This approach supports the classification of three distinct species (D. glaucum, D. barbeyi and D. occidentale), as the species differ in genetics and toxicity. Toxic alkaloid concentrations over the growing season were integrated with data on diet selection to make management recommendations on a species-specific basis to reduce the risk of poisoning cattle. Alkaloid concentrations in tall larkspurs in excess of 3 mg/g impart moderate or high risk to grazing cattle if sufficient quantities are consumed. D. glaucum is most toxic, with toxic alkaloid concentrations that exceed 3 mg/g throughout the grazing season until late maturity. Cattle should be denied access to dense patches of this species throughout the grazing season until after seed shatter. Concentration of toxic alkaloids in D. barbeyi is highest in vegetative plants, but D. barbeyi is unpalatable to cattle until flowering racemes begin to elongate. We recommend grazing D. barbeyi ranges early in the season when it is not palatable, then removing cattle from early flowering stage through mid-pod stage when cattle are most likely to be poisoned. Cattle can again safely graze D. barbeyi late in the season when the toxic alkaloid concentration typically declines below 3 mg/g. Some populations of D. occidentale and the D. barbeyi×D. occidentale hybrids do not contain toxic alkaloids, and pose little risk of poisoning throughout the year. Toxicity of northern populations of D. occidentale varies from year-to-year for unknown reasons. Cattle losses from D. occidentale are usually less severe than from D. barbeyi, but generally the same recommendations apply as for D. barbeyi. Toxicity sampling is an essential management tool. Tall larkspur populations can be tested for toxicity throughout the growing season. Whenever toxic alkaloid concentrations exceed 3 mg/g in leaves, flowers, or pods, cattle should be removed from the area and not returned until pods begin to shatter and risk of poisoning is lower. In general, tall larkspurs are safe to graze when alkaloid concentrations fall below 3 mg/g because it is difficult for cattle to eat sufficient larkspur to become fatally poisoned.