Like other locoweeds, woolly locoweed (Astragalus mollissimus, Fabaceae) gets its common name from a peculiar effect it has on grazing animals. This plant produces a toxin known as locoine, an alkaloid that causes sheep, cattle, and other herbivores to “go loco” and exhibit erratic behavior if they consume enough of it.
This southwestern perennial also takes up large amounts of selenium from the soil. Although trace amounts of this element are biologically essential, it too is toxic in large doses. Both locoine and selenium can even cause death if a sufficient amount is consumed.
Woolly locoweed isn’t all bad, however. As with other members of the pea family, this plant is important for its ability to fix nitrogen. Symbiotic bacteria in its roots remove elemental nitrogen from the air and convert it to a form that can be used by the plant. When the plant dies, this biologically available nitrogen fertilizes the soil and can then be used by other plants and the animals that feed on them.