Loco weed is a common term describing several poisonous plant species, primarily belonging to the genera Astragalus and Oxytropis within the pea family (Fabaceae). These herbaceous perennials pose a serious threat to grazing livestock, including cattle, horses, and sheep, across Western North America. Accurate identification is important for ranchers and land managers to prevent “locoism,” a condition resulting from the consumption of their toxins.
Distinct Physical Traits
Loco weeds are typically low-growing plants that form tufts or bushy clumps, often reaching 8 to 30 centimeters in height. The leaves are a key identification feature, as they are pinnately compound, meaning leaflets are arranged on either side of a common stalk. These leaflets often have a grayish-green hue and a fuzzy or hairy texture, contributing to a woolly appearance in species like woolly locoweed (Astragalus mollissimus).
The flowers of loco weed species are pea-like in structure, resembling sweet peas, and grow in spikes or clusters. Flower colors vary significantly depending on the species, ranging from white or cream to purple, blue, or yellow. A differentiating trait between the two main genera is the keel petal. Astragalus species have a blunt keel, while Oxytropis species, often called “crazyweeds,” possess a lower keel petal that is pointed or short-beaked.
Following flowering, the plant develops a fruit that is a pea-like legume, or seed pod. These pods can be conspicuously enlarged and often contain kidney-shaped seeds, which sometimes rattle inside the dried pod when shaken. The seeds produced can remain viable in the soil for many years, allowing the species to persist.
North American Distribution and Environment
Loco weeds are indigenous to the prairies and rangelands of North America, concentrating in the western and north-central regions. Their distribution spans arid and semi-arid regions, including the Great Plains, the Rocky Mountain states, and the Southwestern deserts. These plants commonly grow on mountains, foothills, and plains.
These species are well-adapted to harsh conditions and often thrive in dry grasslands, pastures, and disturbed soil environments. Their drought resistance allows them to remain green and palatable when other forage is dormant or dry, making them an appealing food source for livestock. Overgrazing can inadvertently increase the prevalence of loco weed by eliminating competing, more desirable plant species.
The Toxic Compounds
The danger associated with most loco weeds stems from the production of the indolizidine alkaloid known as swainsonine. This compound is not produced by the plant itself, but by a symbiotic fungus, Undifilum oxytropis, which grows inside the plant’s tissues. When ingested, swainsonine acts as a potent inhibitor of several lysosomal enzymes, particularly alpha-mannosidase. The inhibition of this enzyme prevents the normal breakdown of complex sugars, leading to the abnormal accumulation of partially metabolized molecules within cells. This accumulation causes cellular vacuolation, especially in nerve cells, resulting in the neurological damage and staggering gait characteristic of “locoism.”
The effects of swainsonine poisoning can be chronic, affecting nearly all body systems, including reproductive function. A separate form of toxicity occurs in certain Astragalus species, known as “selenium accumulators.” These plants absorb high concentrations of selenium from soils naturally rich in the element, such as those found in parts of the Dakotas, Montana, and Wyoming.
Selenium poisoning, or selenosis, can manifest in two main syndromes: the acute form, “blind staggers,” or the chronic form, “alkali disease.” Symptoms of alkali disease include loss of hair, deformation of hooves, and general emaciation. This presentation differs from the neurological effects caused by swainsonine poisoning.