Common Name
Purple locoweed, Lambert's crazy weed
Botanic Name
Oxytropis lambertii Pursh
Plant Family
Decomposing granite soils of foothills and prairies.
Intermountain States, often in the same areas as white locoweed
Animals Affected
Cattle, sheep, horses, elk
Toxic Principle
Locoweed is poisonous at all times, even when dried. Swainsonine, an indolizide alkaloid found in all parts of the plant, inhibits the enzyme alpha-mannosidase which is essential for normal sugar metabolism in cells. These sugars (oligosaccharides) therefore 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. Swainsonine is actually produced by an endophytic fungus (Embelisia spp.) that lives within the plant stems, leaves and flowers. The fungus does not affect the locoweed plant itself, and it is thought to benefit the plant by making it tolerant of drought conditions.
Perennial herbaceous plants with long tap roots. Leaves are grouped basally, 6-8 inches long, odd-pinnate, compound and covered with silvery hairs. The flowers are borne on a leafless stalk in a raceme. The flower is purple, pea-like, with a 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. Purple locoweed tends to flower after white locoweed is finishing blooming. In some areas the purple locowed has cross polinated with the white locoweeds forming colonies of plants with varying shades of flower color.
Decreased appetite leads 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.
Congenital Defects
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 specific treatment. Affected animals will show some improvement once removed from locoweed, but nervous system damage is likely to be permanent. reproductive performance returns to normal once locoweed is not consumed.
Nervous System
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.
Reproductive System
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. Reproductive efficiency generally returns to normal once the animals are prevented from eating locoweed.
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 vacuoles 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.
Special Notes
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.
Purple locoweed -Oxytropis lambertii