Grazing of Delphinium occidentale (duncecap larkspur) by susceptible and resistant cattle

Delphinium spp. (larkspurs) often fatally poison grazing cattle. Angus cattle differing in susceptibility to larkspur poisoning were used to assess selection of larkspur (Delphinium occidentale) while grazing. During July 2015, 2016, and 2017, diet selection of 12 cattle (6 susceptible, 6 resistant) was determined for 2 to 3 wk. During 2015 resistant steers ate 6% of their diets as total larkspur compared with 7% (P > 0.34) for susceptible steers. During 2016 there was a treatment × date interaction (P = 0.02) for total larkspur consumption; resistant steers consumed more larkspur on 5 trial days than did susceptible steers. Overall, resistant steers ate 6% of their diets as larkspur compared with 3% for susceptible steers. During 2017 there was a treatment × date interaction (P = 0.03) for total larkspur consumption; resistant heifers consumed more larkspur on 2 trial days than did susceptible heifers. Overall, during 2017 resistant heifers ate 7% of their diets as larkspur compared with 3% for susceptible heifers. The average serum concentration of methyllycaconitine in severely intoxicated, susceptible animals was approximately 700 ng/mL compared with approximately 1,000 ng/mL for severely intoxicated, resistant animals. The only fatalities (2) were in susceptible animals. The diet selection and comparative responses of resistant and susceptible animals on rangeland appear to validate the phenotyping done in the laboratory but must be verified in a controlled setting. Selecting resistant cattle to graze larkspur-infested rangelands may reduce losses; however, further research is required to develop genetic biomarkers to identify such animals.