Interpretive Summary: Sex-dependent differences for larkspur (Delphinium barbeyi) toxicosis in yearling Angus cattle
By: Jackie Walling
An article published in the March 2019 Issue of the Journal of Animal Science investigates a sex dependent response to toxicity of delphinium barbeyi (larkspur) in Angus cattle. Identifying a genetic basis for larkspur tolerance led to the discovery of sex-dependent responses. Yearling Angus heifers, steers, and bulls were evaluated to determine responses to a standardized 8mg/kg body weight dose of N-(methylsuccinimido) anthranoyllycoctonine type alkaloid.
This study consisted of two experiments over multiple years. Experiment 1 consisted of all animals tested before 2018 (Previous study data used). The focus of this study was Experiment 2 and consisted of 15 bulls and 15 heifers (all born Spring 2017) maintained under the same management conditions. Plant material was delphinium barbeyi collected from Manit, UT in late July 2003. Cattle were dosed and then evaluated 24-hours later by a walk test to determine susceptibility. Blood was drawn before and after dosing for serum alkaloid analyses.
Cattle were hooked to a tractor and walked 5-6km/hr until signs of muscle weakness showed (max 40min). A 0 time was given for cattle that showed signs before the walk test (susceptible) and those that walked 40+ minutes were given a 40-minute walk time (tolerant). Both values were censored in some of the final data analyses.
In Experiment 2, heifers had shorter walk times (more susceptible) with 70% having a zero-walk time compared to bulls. Unadjusted walk times were 4.1 (heifers) and 11.7 minutes (bulls). Serum alkaloid concentrations did not significantly differ between the two. In both sexes, deltaline concentration was less than methyllycaconitine likely due to deltaline’s shorter half-life.
Results from Experiments 1 and 2 were combined to increase sample size. Data was censored here which increased variability and caused negative estimates. Walk times for heifers, bulls, and steers were -8.89, 13.2, and 15.9 minutes. Bulls and steers did not significantly differ while heifers walked about 23 minutes less than both. Methyllycaconitine was similar in heifers and bulls, but was higher than steers.
Because serum alkaloid concentrations for heifers and bulls were similar, it appears sex does not affect drug metabolization and is thus not the cause for susceptibility. Speculation based on a mice study suggests a lower expression of nicotinic acetylcholine receptors (receptors of larkspur) in heifers may be a cause. This lower expression may be dependent on heifer’s estrogen and is more susceptible to methyllycaconitine.
Overall, yearling Angus heifers have a 3.3-fold increase in poisoning risk compared to bulls and grazing management needs to be carefully addressed. Ongoing research is working to increase specificity of larkspur management recommendations by the use of genetic markers.
For the full article, visit the Journal of Animal Science.