Common Name
Woolly Locoweed
Botanic Name
Astragalus mollissimus
Plant Family
Open prairies and foothills
Animals Affected
Cattle, sheep, horses
woolly-locoweed (Astragalus mollissimus)
Toxic Principle
Locoweed is poisonous at all times, even when dried. Swainsonine, an indolizidine 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. Other Astragalus and Oxytropis Species Reported to Cause Locoism Astragalus lentiginosis (Spotted 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. sericea (White loco) O. bessyi (Bessy point vetch) O. condensata Curent research has shown that swainsonine is produced by an endophytic fungus (Embelisia spp.)that grows within the plant tissues without harming the plant.
Leafy perennial with short stems and pinnately compund, mostly basal leaves with 21-31 leaflets. The leaves appear silvery-blue due to the heavy covering of white hairs. The flowers are rose colored and produced in dense racermes with a dense covering of hairs. The pods are 2-3cm in length, 2 celled, sharp pointed tip with a groove on upper than lower surface.
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.
Congenital Defects
Calves, lambs, and foals may be born with deformed crooked 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. Decreased fertily occurs when locoweed is eaten.
There is no effective treatment for locoweed poisoning. Animals should be moved from locoweed pastures. Recovery depends on the severity of lesions. Locoed horses should be considered permanently affected.
Nervous System
Abnormal behavior including sudden changes in temperament, aggressiveness, atxia, 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 sleep alot. Horses often show more sever 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 charcterized 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.
Hepatic System
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.
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.
Woolly locoweed (Astragalus mollissimus)
Woolly locoweed (Astragalus mollissimus)