Locoweed (also crazyweed and loco) is a common name in North America for any plant that produces swainsonine, a phytotoxin harmful to livestock. Worldwide, swainsonine is produced by a small number of species, most in three genera of the flowering plant family Fabaceae: "Oxytropis" and "Astragalus" in North America, and "Swainsona" in Australia. The term locoweed usually refers only to the North American species of "Oxytropis" and "Astragalus", but this article includes the other species as well. Some references may list "Datura stramonium" as locoweed.

Locoweed is relatively palatable to livestock, and some individual animals will seek it out. Livestock poisoned by chronic ingestion of large amounts of swainsonine develop a medical condition known as locoism (also swainsonine disease, swainsonine toxicosis, locoweed disease, and loco disease; North America) and pea struck (Australia). Locoism is reported most often in cattle, sheep, and horses, but has been reported also in elk and deer. It is the most widespread poisonous plant problem in the western United States. Agricultural Research Service and New Mexico State University scientists have been collaborating since 1990 to help solve the problem that locoweed presents to livestock farmers. The research involved identifying the fungal species that produces the locoweed toxins, pinpointing levels of toxicity in animals once they have ingested locoweed, observing the effects of locoweed toxins on livestock’s reproduction and grazing preferences, etc. Together, the scientists assembled a grazing management scheme to help farmers avoid the poisonous locoweed.

Most of the 2000 species of "Astragalus", including many that are commonly known as locoweeds, do not produce swainsonine. Some species, including a few that produce swainsonine, accumulate selenium. This has led to confusion between swainsonine poisoning and selenium poisoning due to this genus.

Because "O. sericea" is both frequently encountered and relatively palatable to livestock, it is an important cause of economic losses in livestock production. Keeping livestock away from locoweed infested pasture in spring and fall when grass and other forbs are not actively growing is recommended. Another suggested remedy is to provide palatable supplemental nutrients if animals are to be kept in infested pasture. These remedies take into account livestock preference for locoweed during seasons when grass is dry and not very nutritious. Conditioned food aversion has been used experimentally to discourage livestock from eating it. In horses, a small study has shown promising results using lithium chloride as the aversive agent.