Woolly locoweed, Woolly loco

Astragalus mollissimus Torr.

Fabaceae (Legume family)


Woolly locoweed is a stout, many-branched perennial of the Legume family. Its thick, woody root gives rise to stems lying close to the ground.

The leaves of woolly locoweed have 19 to 29 oval to oblong leaflets that are covered with fine, soft, short hairs. The flowers are purple, lavender or yellow, emerging from April through June. They mature into thick, inflated, moon-shaped seedpods.

This plant is poisonous to all species of livestock and wildlife.


Woolly locoweed grows from southwestern South Dakota to Texas and New Mexico. It is common in upland, mesa and mountain areas of the Trans-Pecos and Panhandle regions of Texas.

Toxic Agent

Woolly loco is toxic to cattle, sheep, goats and particularly horses. Its toxin is swainsonine, which causes damage to the brain, liver, digestive organs, placenta and testes. The damage is reversible except in the brain. Locoism occurs in cattle and sheep after they have eaten about 90 percent of their body weight of the plant in a 2- month period. Ruminants usually must eat from 200 to 350 percent of their body weight over several months before death occurs.

In horses, about 30 percent of body weight consumption produces signs.

Signs of Livestock Ingestion

Poisoning signs develop from the involvement of sensory and motor functions. In cattle, general signs include: carrying the head a little lower than normal, a vacant stare, trembling of the head, difficulty eating and drinking, and abortion.

Swainsonine is passed in the milk, possibly leading to unthriftiness in some suckling calves.

In horses, the time between the first clinical signs and death is much shorter than in cattle. The horse is listless, but on being stimulated, becomes excessively excited. Horses with chronic locoism rarely recover and are permanently dangerous to ride.

Management Strategies

Quickly move affected animals to locoweed-free pastures and place them on good feed. Management to reduce locoweed poisoning is most critical in early spring.

Livestock imported from areas where loco does not grow are the most susceptible to poisoning. Native animals generally avoid locoweed when good quality forage is available.

Maintaining good range condition and sound supplemental protein and mineral feeding programs are the best prevention against losses.

Locoweed dominating a particular area can be controlled chemically. Individual plant treatments or broadcast treatments with Grazon P+D® are effective. Locoweed becomes much more toxic and palatable after chemical application until it is completely dried up.