Locoweed is a common name for a group of toxic plants that pose a significant threat to grazing livestock, particularly throughout the arid and semi-arid regions of North America. This chronic poisoning, often referred to as “locoism” (Spanish for “crazy”), has been a persistent source of economic loss for ranchers since the 19th century. It primarily affects cattle, horses, and sheep that graze on western rangelands. Managing livestock health requires understanding the plant’s nature, its toxic mechanism, and its effects on animals.
Identification and Geographic Distribution
Locoweed generally refers to species within two specific plant genera of the pea family (Fabaceae): Astragalus (milk vetch) and Oxytropis (crazyweed). While these genera contain numerous species, only a small number produce the dangerous toxin. These plants are typically low-growing perennials, found in tufts or clumps, featuring compound leaves and flowers that resemble sweet peas.
Visually differentiating toxic species from non-toxic ones is challenging due to shared physical traits. Oxytropis species typically have a pointed, beaked lower petal, while Astragalus species have a blunter keel. The primary geographic range is the plains, foothills, and mountains of the Western and Southwestern United States. Because locoweeds are hardy, they are often the first plants to grow in the spring and remain palatable when other forage is dormant, increasing the likelihood of consumption.
The Toxin That Causes “Loco” Syndrome
The primary source of danger in locoweed is an indolizidine alkaloid called Swainsonine. This toxin is not produced by the plant itself but by a symbiotic fungus (endophyte) that lives within the plant’s tissues. The fungus, often identified as Undifilum oxytropis, produces the alkaloid, which permeates all parts of the host plant, including the seeds and leaves. This explains why toxicity levels vary widely, even among plants of the same species, depending on the endophyte’s presence and concentration.
Once ingested, Swainsonine interferes with normal cell function by inhibiting alpha-mannosidase, an enzyme responsible for breaking down complex sugars within the cell’s lysosomes. By blocking this process, the toxin causes partially metabolized sugars to accumulate within the lysosomes, leading to a condition similar to a lysosomal storage disease. The resulting cellular damage, characterized by vacuolation, affects nearly all body systems, including neurons, liver cells, and kidney tissue, forming the basis of the poisoning syndrome.
Observable Effects in Animals
Chronic locoweed consumption progresses slowly over several weeks of continuous grazing. Neurological symptoms are the most recognizable, manifesting as an irregular, staggering gait, head bobbing, and loss of muscular coordination. Affected animals may also exhibit erratic behavior, such as extreme nervousness, depression, or a violent reaction when stressed or handled. In horses, these neurological effects are often more severe and can lead to permanent impairment, making them unsafe to ride or work.
Reproductive and Physical Damage
The toxin causes significant reproductive and physical damage. Pregnant animals often experience reproductive dysfunction, including abortions, stillbirths, and the birth of offspring with congenital defects. Males can be affected, showing reduced libido and interference with sperm and egg production. Physical effects include gradual loss of body condition, emaciation, and, in cattle grazing at higher altitudes, the development of congestive right heart failure.
Strategies for Control and Prevention
Management involves integrated strategies focused on minimizing animal exposure and reducing plant populations. The most effective approach is denying livestock access to areas with high locoweed concentration, particularly during early spring and fall when the plants are green and more palatable than dormant grasses. Maintaining conservative stocking rates prevents animals from being forced to consume the toxic plants when desirable forage becomes scarce.
Herbicides, such as clopyralid, picloram, and metsulfuron, can be used to control existing locoweed populations. Application is most effective when the plants are actively growing or budding. However, the long-term viability of locoweed seeds means that plants can re-establish when environmental conditions favor growth.
For an animal showing signs of poisoning, the only course of action is immediate removal from the infested pasture. While some physical symptoms may resolve after removal, the chronic neurological damage is often irreversible, especially in horses. Treatment is generally supportive rather than curative.