Horsetail Milkweed

Asclepias subverticillata



Horsetail milkweed is an erectstemmed plant growing to 5 feet tall. The narrow leaves are in whorls of three or paired opposite, with margins rolled backward. The greenish-white flowers give rise to pods 1 to 3 inches long from May to September. Seeds have tufts of long silky hairs.


Abundant in western Texas, this plant has also been recorded in the South Texas Plains and Gulf Coast Prairie regions. It is frequent in northern Mexico and ranges into Arizona, Colorado and Utah. It often abounds in open pastures and along arroyos, draws, trails, roadsides and bar ditches.

Toxic Agent

The toxic agent involved is suspected to be the resinoid galitoxin. Horsetail milkweed has poisoned sheep, cattle, horses, chickens and turkeys. To ingest a toxic dose, an animal generally must eat 0.2 percent of its body weight in green plant material. A sheep may be killed by eating 2 to 3 ounces.

Signs of Livestock Ingestion

The signs produced by whorled species of Asclepias are different from those produced by other milkweeds. Effects are on the nervous system rather than the cardiac system. They include: Staggering, incoordination, excitement; Head tremors, muscle tremors, convulsions; Star-gazing posture; Depression, labored breathing, dilated pupils, progressing to death.

Clinical signs appear within a few hours of ingestion of a toxic dose, and death follows from 1 to a few days in most fatal cases.

Management Strategies

Animals dislike the taste of milkweeds and seldom graze them unless they are confined to milkweed-infested areas. Most losses result from overgrazing and drought. The plants are most toxic before maturing; somewhat less so as they dry. Horsetail milkweed retains enough toxicity to be dangerous in hay. Although most animals die after reaching the convulsive stage of milkweed poisoning, some recover. Move them to shade, keep them quiet and give them plenty of food and water. No medicinal treatment is specified, but sedatives, laxatives and intravenous fluids may help.