locoweed or crazyweed [Span. loco = crazy], any of several American species of the genera Astragalus and Oxytropus, north-temperate leguminous plants of the family Leguminosae (pulse family), that, when eaten by horses, cattle, or sheep, cause a nervous disorder called loco disease. The locoweeds, perennials native to the West and Southwest, have pealike flowers and pinnately compound leaves. Not all species of these genera have been found poisonous. An Old World plant related to the Astragalus locoweeds is the source of gum tragacanth. Locoweed is classified in the division Magnoliophyta, class Magnoliopsida, order Rosales, family Leguminosae.