Locoweed, the common name of a number of different plants that poison sheep, cattle, and horses on the Great Plains. Some, such as the poison larkspur, are poisonous in themselves. Others are poisonous because they concentrate selenium that exists in the soil. Among these are the milk vetch and white locoweed. The poison affects the nervous system, causing an acute or chronic disorder called loco disease.

Loco is the Spanish word for “mad” or “insane.” The symptoms of loco disease include staggering, falling, defective vision, nausea, constipation, and loss of appetite. Affected animals may suddenly jump for no reason, or run into obstacles. The acute form may end fatally in three days. Animals suffering from the chronic form of the disease may waste away for weeks or even for months.

A number of plants of the genera Astragalus and Oxytropis, of the pea family, Leguminosae, are locoweeds. Milk vetch is A. canadensis; white locoweed, O. lamberti.