Indigenous to the Rocky Mountain Area, it prefers open prairie, and foothills, often in well drained soils of decomposing granite.
Cattle, horses, sheep, elk, deer
Locoweed is poisonous at all times, even when dried. Swainsonine, an indolizide alkaloid found in all parts of the plant, inhibits the enzymes alpha-mannosidase I & II which are essential for normal carbohydrate and glycoprotein metabolism in cells. As a result, these carbohydrates(oligosaccharides)accumulate in the cells of the brain and most other organs with the result that normal cell function is impaired. Depending on the duration of locoweed consumption, the affected cells can be permanently damaged. Swainsonine is secreted in the milk of lactating animals and will therefore affect the young animal suckling its mother.
Other Astragalus and Oxytropis Species Reported to Cause Locoism
Astragalus lentiginosis (Spotted loco)
A. mollissimus (Woolly, purple loco)
A. wootonii (Wooton loco)
A. thurberi (Thurbers loco)
A. nothoxys (Sheep loco)
A. dyphysus (Blue loco, rattlewood)
A. earlei (Earle's loco)
A. argillophilus (Halfmoon loco)
Oxytropis lambertii (Purple Point loco)
O. bessyi (Bessy point vetch)
Current evidence indicates that swainsonine is in fact produced by one or more fungi that grow within the tissues (endophytes) of the locoweed. Locoweed that does not contain the endophyte Undifilium oxytropis(formerly Embelisia)does not produce the toxic alkaloid.
Cook D, Gardner DR, Lee ST, Pfister JA, Stonecipher CA, Welsh SL. A swainsonine survey of North American Astragalus and Oxytropis taxa implicated as locoweeds. Toxicon. 2016 Aug;118:104-11. doi: 10.1016/j.toxicon.2016.04.033. Epub 2016 Apr 13. PMID: 27085305.
Cook D, Gardner DR, Pfister JA, Lee ST, Welch KD, Welsh SL. A Screen for Swainsonine in Select North American Astragalus Species. Chem Biodivers. 2017 Apr;14(4). doi: 10.1002/cbdv.201600364. Epub 2017 Mar 23. PMID: 28155255.
Perennial herbaceous plants with long tap root. Leaves are grouped basally 8-12inches long, odd-pinnate compound and covered with silvery hairs. The flowers are borne on a leafless stalk in a raceme. The flower is white, pea-like, with purple-tipped, pointed keel. The seed pods are erect, stalkless, with a short beak that splits open to release numerous smooth brown seeds. Seeds may remain viable in the soil for 50 years or more. Oxytropis lambertii, purple locoweed, is very similar in its habitat and distribution to white locoweed. It differs from white locoweed in that it has purple flowers, generally fewer and smaller leaves and tends to flower immediately after white locoweed is finishing blooming. In some areas the purple has cross polinated with the white loco forming colonies of plants with varying shades of flower color.
Decreased appetites lead to weight loss. The effects of locoweed on the intestinal tract may lead to malabsorbtion of essential minerals and vitamins. Decreased growth rates resuling in lower weaning weights is a typical finding in calves exposed to locoweed. The decreasd growth rate in locoed calves may also be due to the effect of swainsonine on the thyroid decreasing growth hormone production.
Calves, lambs, and foals may be born with deformed legs. Abortions and fetal death are common. Hydrops may develop in cows consuming locoweed. Some calves may be born weak and die shortly after birth.
There is no effective treatment for locoweed poisoning. Animals should be moved from locoweed pastures. Recovery depends on the duration and severity of the lesions. Locoed horses should be considered permanently affected.
Congestive heart failure (Brisket disease) occurs in cattle consuming locoweed growing at higher altitudes.
Abnormal behavior including sudden changes in temperament, aggressiveness, ataxia, falling over unexpectedly, violent reaction to routine mangement practices such as putting a halter on, and cattle refusing to go through a chute, are typical of locoed animals. Some horses become very depressed and sleepy. Horses often show more severe neurological effects of locoweed poisoning than cattle and sheep. The unpredictable behavior makes the animals dangerous to work around or ride.
Decreased fertility in cattle characterized by decreased conception rates and lower calving percentages is the major problem encountered with locoweed poisoning. Semen fertility is decreased in bulls and rams consuming locoweed. Semen of affected bulls and rams may have increased numbers of sperm with proximal droplets, detached or bent tails, and poor motility. Reduced libido, changes in behavior and weight loss affect breeding soundness. Reproductive efficiency generally returns to normal about 70 days after the animals are prevented from eating locoweed.
Liver function is often affected by the swainsonine. Liver serum enzymes are often elevated reflecting generalized liver damage.
Serum can be submitted for the detection of swainsonine. The serum half-life of swainsonine is less than 24 hours. The serum should therefore be collected from the animal while it is still grazing the locoweed, and not after it has been off of locoweed pasture for several days. A typical serum profile of a locoweed poisoned animal is a detectable level of swainsonine in the blood, decreased alpha-mannosidase activity, and increased aspartate aminotransferase (AST). Thyroid hormone levels are often depressed. Detection of vacubles in the cytoplasm of lymphoytes is suggestive of locoweed poisoning. Vacuoles in in the cells of the brain, liver, lymph nodes, thyroid gland, and uterus at post mortem examination are a characteristic of locoweed poisoning.
Although horses, cattle and sheep are thought to develop an addiction for locoweeds, they in fact become habituated as there is no dependence on the plants as there would be the case in of addiction. Locoweeds are palatable and of similar nutrient value to alfalfa which helps explain why animals eat them even when normal forages are present.
Through social facilitation, animals learn to eat locoweed from each other. Once one animal starts to eat locoweed others follow. Calves learn to eat locoweed from their dams. Removing animals that are locoweed eaters from the herd will reduce the chances of other animals 'learning' to eat the plant through social observation!
1. Ping L, Child D, etal. Culture and Identification of Endophytic Fungi from Oxytropis glabra DC. ACTA Ecologica Sinica. 2009, 20: 53-58
2. Pryor BM, Creamer R, et al. Undifilum, a new genus for endophytic Embellisia oxytropis and parasitic Helminthosporium bornmuelleri on legumes. Botany, 2009, 87:178-194.
3. Ralphs MH, Creamer R, et al. Relationship Between the Endophyte Embellisia spp. and the Toxic Alkaloid Swainsonine in Major Locoweed Species (Astragalus and Oxytropis). J Chem Ecology 2008, 34: 32-38.
White locoweed (Oxytropis sericea)
Purple locoweed (Oxytropis lambertii)
Purple locoweed leaf and flowers
Wooly locoweed (Astragalus mollissimus)
Wooly locoweed leaves and flowers