The Effect of Rainfall on Columbia Milkvetch Toxicity

Daily precipitation patterns were compared to the variation in miserotoxin concentration of Columbia milkvetch (timber milkvetch) sampled sequentially during the spring and summer of 1973 and 1974. On rough fescue grasslands, the substantial increase in rainfall during the April-to-August period of 1974 not only extended toxicity intervals but also increased miserotoxin levels during the prebud growth stage. A large-scale rain storm induced miserotoxin synthesis during the pod stage. Greater soil moisture-holding capacity at one grassland experimental plot prevented a rapid decline in miserotoxin levels when drought conditions developed. In contrast, the toxicity trends on Douglasfir forest sites did not show a response to variations in precipitation and toxin differences between local sites were not significant. Consequently, a predictability equation was developed for Columbia milkvetch toxicity in Douglasfir forests on Gray Luvisolic soils.