Scientific Name: Asclepias spp.
Distribution: Throughout the United States, Canada, and Mexico
Type of Poison: Glucosidic substances
Signs of Poisoning:
Growth Characteristics: The milkweed family has a great degree of diversity in appearance. All are erect, unbranched forbs, usually reaching at least 2 feet in height. The plants get their name from the milky juice that oozes out quickly when any plant part is broken. Milkweeds start growing early in the spring.
Flowers/Inflorescence: Flowers are small and are arranged in umbrella-like clusters. They have five petals which are united at the base. Some, such as those of showy milkweed (Asclepias speciosa), are showy and beautiful, while others, like those of whorled milkweed (Asclepias subverticillata), are smaller and more inconspicuous.
Fruits/Seeds: Fruits consist of two pods. These pods can be round and up to 5 inches long, such as in showy milkweed (Asclepias speciosa), but can also be slender and about 2 inches long, as in whorled milkweed (Asclepias subverticillata). When mature, the pods split down one side, allowing the seeds to escape. The flat seeds bear a tuft of silky hair at the top which enables them to float in the wind.
Leaves: Leaves can be alternate, opposite, or whorled, depending on the species. The margins are entire.
Milkweed is often found in sandy soils of plains and foothills. It grows on ranges and abandoned farms, along roadsides, in pastures, in ditches, and in waste places.
Several species of milkweed are poisonous to range animals. Labriform milkweed (Asclepias labriformus) is the most toxic. Other species — in order of toxicity — include western whorled milkweed (Asclepias subverticillata), woolypod milkweed (Asclepias eriocarpa), and Mexican whorled milkweed (Asclepias fascicularis). Leaves or other aboveground parts of the plant are poisonous. Milkweed may cause losses at any time, but it is most dangerous during the active growing season.
Milkweed poisoning occurs frequently in sheep and cattle and occasionally in horses. Most livestock losses are a result of hungry animals being concentrated around milkweed-infested corrals, bed grounds, and driveways. Poisoning may also occur if animals are fed hay containing large amounts of milkweed.
An average-sized sheep that eats 1 to 3 ounces of green leaves of one of the more toxic species is likely to die of poisoning. It may die within a few hours or live two to four days.
Animals usually do not eat milkweed unless good forage is scarce. Livestock owners can reduce losses by keeping sheep out of milkweed along stock driveways when bands are trailed from one range to another. Supplemental feeding usually is beneficial during trailing. Hay that is contaminated with milkweed should not be fed to sheep or cattle.
A.H. Holmgren, B.A. Andersen. Weeds of Utah. Utah Agricultural Experiment Station, Special Report 21, 115 pp. April 1971.
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.