Technical Abstract: Current management recommendations for grazing cattle on larkspur-infested ranges are based primarily on the concentration of MSAL-type alkaloids. Delphinium barbeyi is one of the more problematic species of tall larkspur plants due to its high concentration of MLA. However, the most abundant norditerpenoid alkaloids in most D. barbeyi populations are the less toxic MDL-type alkaloids, either deltaline or 14-OAD. Previous research using a mouse model, demonstrated that the MDL-type alkaloids have an additive affect on the toxicity of MLA. In this study, experiments were performed to determine if MDL-type alkaloids affect the overall toxicity of tall larkspur plants in cattle. The research from this study demonstrates that MSAL-type alkaloids such as MLA cause greater toxicity than MDL-type alkaloids and are the primary factors responsible for the toxicity of larkspur plants. Consequently, for a larkspur plant to be toxic to livestock, a sufficient quantity of MSAL-type alkaloids is required. However, MDL-type alkaloids appear to potentiate the overall toxicity of the MSAL-type alkaloids and should be considered when predicting potential toxicity of larkspur populations. Consequently when chemical analyses are performed on larkspur plants to assess their toxic potential, the concentration of both the MSAL-type and total alkaloids should be determined, with more weight given to the MSAL-type alkaloids.