USDA ARS

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Milkweed (Asclepias spp.) : USDA ARS

Milkweed is a perennial that often bears blossoms and fruit at the same time. The plant may be 0.5-1.0 meters high. Greenish-white flowers are borne in umbrella-like clusters. Leaves may be narrow or broad. Leaves or other above-ground parts of the plant are poisonous. They contain several glucosidic substances called cardenolides that are toxic. Milkweed may cause losses at any time, but it is most dangerous during the active growing season.

Several species of milkweed are poisonous to range animals. Labriform milkweed (Asclepias labriformis) is the most toxic. Other species in order of toxicity include western whorled milkweed (A.subverticillata), woollypod milkweed (A. eriocarpa), and Mexican whorled milkweed (A. fascicularis).

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 also may occur if animals are fed hay containing large amounts of milkweed.

Milkweed

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Toxic Weed: Milkweed

The primary toxic principle, galitoxin, is of the resinoid class. Galitoxin is found in all vegetative parts of the plant. In addition, a group of toxicants known as cardenolides may be responsible for digitalis-like signs that cause or contribute to death. In general, it appears that the broad-leaved species produce cardiotoxic and GI effects while the narrow-leaved species are more commonly neurotoxic. Dosages of whorled milkweed as low as 0.1 % - 0.5% of the animal's body weight may cause toxicosis and, possibly, death. Cattle, sheep and horses are most susceptible. Toxicity is not lost when the plant is dried. Therefore, contaminated hay is potentially toxic.