Loco weed refers to a group of plants known for their toxicity to grazing animals, particularly livestock. The term “loco” comes from the Spanish word for “crazy,” reflecting the erratic behavior observed in poisoned animals. These plants pose a significant threat in certain regions, causing substantial economic losses in the livestock industry.
Identifying Loco Weed and Its Environment
Loco weed plants are perennial herbs, often growing in tufts or clumps. They typically range from 8 to 30 centimeters (approximately 3 to 12 inches) in height. Their leaves are pinnately compound, meaning they have leaflets arranged along a central stem, and are often covered with silvery hairs.
The flowers of loco weed plants resemble sweet peas and can exhibit various colors, including white, purple, yellow, or blue. These flowers are usually borne on leafless stalks in a raceme. Seed pods are erect, stalkless, and contain numerous smooth brown seeds that can remain viable in the soil for 50 years or more.
The primary genera associated with toxic loco weed in North America are Astragalus and Oxytropis, both belonging to the pea family (Fabaceae). These plants are indigenous to the Rocky Mountain Area and are commonly found across the Western and Southwestern United States, including prairies, foothills, and semiarid desert regions. They often prefer well-drained soils, such as those found in decomposing granite. Locoweeds tend to green up earlier than other range plants in the spring, making them appealing to grazing livestock when other forage sources are scarce.
How Loco Weed Causes Harm
The primary toxic compound in loco weed responsible for its harmful effects is swainsonine, an indolizidine alkaloid. This compound is not produced by the plant itself but by an endophyte, a fungus (Undifilum oxytropis) that lives within the plant’s tissues.
Swainsonine works by inhibiting specific enzymes within animal cells. These enzymes are essential for the normal metabolism of carbohydrates and glycoproteins within cells. When swainsonine inhibits these enzymes, partially metabolized sugars accumulate within cellular organelles called lysosomes. This accumulation leads to the formation of vacuoles, or fluid-filled cavities, inside the cells, which disrupt normal cellular function. The interference with glycoprotein synthesis and processing also affects protein function, including hormones, enzymes, and receptor binding, impacting nearly all body systems.
Effects on Animals
Loco weed poisoning, also known as locoism, affects a variety of grazing animals, including cattle, horses, sheep, goats, elk, and deer. Clinical signs typically appear after several weeks of continuous consumption, as the toxicity is chronic rather than acute. Animals may initially appear lethargic and depressed, becoming reluctant to move. As the condition progresses, more pronounced neurological symptoms emerge, such as tremors, an irregular or stiff gait, lack of coordination, and apparent blindness.
Behavioral changes are also common, ranging from increased nervousness and withdrawal from other animals to aggression and violent reactions to routine handling. Horses, in particular, often display more severe neurological effects and unpredictable behavior, posing a safety risk to humans. Animals may consume loco weed even when other forage is available, as the plant is palatable and can create a psychological “need” for swainsonine. Beyond neurological impacts, locoism can cause weight loss and reproductive issues such as reduced fertility, abortions, and birth defects. Calves, lambs, and foals may be born weak or with deformed legs.
Prevention and Care
Preventing loco weed poisoning primarily involves careful pasture management. Since loco weed often greens up earlier than other forage, restricting animal access to infested areas during this period can reduce consumption. Maintaining conservative stocking rates helps ensure animals have enough desirable forage, minimizing the likelihood they will graze on loco weed due to scarcity. Identifying and removing individual loco weed plants or patches, especially before they seed, can also be effective in reducing their spread.
Once animals begin consuming loco weed, it is important to remove them from infested pastures to prevent further intoxication. While some reversible signs of poisoning may resolve within a few weeks after removal, damage to brain and neural tissues can be permanent. There is no specific cure for loco weed poisoning, and affected animals, especially horses with neurological signs, may have a poor prognosis for full recovery. Veterinary consultation is essential for managing affected animals, focusing on supportive care and addressing specific symptoms.