Scientific Name: Astragalus spp.
Distribution: Throughout the United States, Canada, and Mexico
Type of Poison: Glucosides of 3-nitropropionic acid
Signs of Acute Poisoning:
Signs of Chronic Poisoning:
Growth Characteristics: An erect to prostrate forb, with stems that are mostly hairy and leafy. Milkvetches emerge from late April to June and reproduce from seed.
Flowers/Inflorescence: Flowers resemble pea flowers but are smaller. Colors vary from white to yellow to blue to purple, and are arranged in a raceme.
Fruits/Seeds: A pea pod (legume) with a papery, leathery, or woody cover.
Leaves: Odd-pinnate leaves (a leaflet at the terminal end of the leaf), usually pubescent.
Milkvetches grow throughout much of the North American continent. Most of the poisonous species grow on the meadows, deserts, and forests in the Rocky Mountain states. Milkvetches emerge from late April to June, depending on elevation and snow melt. Leaves and stems become dry after seed dispersal in July or August. Plants are poisonous from the time they emerge until they dry up or are killed by frost. Poisonous nitro compounds are found in varying quantities in 263 species and varieties of North American Astragalus.
Milkvetches are poisonous plants that affect cattle, sheep, and horses. Cattle of all ages are highly susceptible to poisoning. Even when other forage is available, cattle readily eat milkvetch. Milkvetch poisoning may be mistaken for larkspur poisoning.
The poison in milkvetches acts quickly. Two pounds of green milkvetch may cause acute poisoning or death in a 1,000-pound cow. Some deaths occur within one hour, so fast that cattle show no signs; more often, animals die within three or four hours after eating the plant.
Acute poisoning is characterized by a general muscular weakness. It paralyzes leg muscles so that affected animals fall after the slightest excitement, although they appear bright. The heart beats very rapidly before the animal dies from heart failure.
Chronic intoxication may occur in cattle and sheep from grazing any of the toxic milkvetch species slowly over a period of several days or weeks. This type of intoxication is characterized by respiratory problems and varying degrees of posterior paralysis. This condition occurs in most of the western United States and western Canada.
To reduce losses, prevent animals from grazing these plants for extended periods.
There is no known treatment for milkvetch poisoning.
L.F. James, R.F. Keeler, A.E. Johnson, M.C. Williams, E.H. Cronin, and J.D. Olsen. Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture, Agriculture Information Bulletin 415, 90 pp. 1980.
Pratt, Mindy S. Locoweed. Range Plants of Utah. 2008. Utah State University Extension Service. 14 September 2009. http://extension.usu.edu/rangeplants/htm/locoweed.