The legumirous locoweeds or milk vetches (genera Astragalus and Oxytropis) have been a severe problem for ranchers in the western half of the United Sates and are considered to be some of the most toxic plants for horses, sheep, goats, and cattle. Loco is the Spanish word for “crazy” and refers to the staggering and trembling behavior of poisoned animals who walk into things and react to unseen objects. The poisonous compounds are an unusual group of alkaloids that affect certain cells of the central nervous system, explaining the behavioral changes observed.