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.
Where and When It Grows
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. This plant gets its name from the milky juice that oozes out quickly when any plant part is broken. The plant starts growing in the early spring.
How It Affects Livestock
An average-sized sheep that eats 30-100 gms 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 2 to 4 days. Although many milkweeds contain resinoids, most of the ones that cause fatal poisonings contain cardenolides (cardiac glycosides). These cardenolides are similar to digoxin causing electrolyte balances in heart muscle resulting in arrhythmias and cardiac failure.
Signs and Lesions of Poisoning
Depression, weakness, and staggered gait
Difficulty in breathing with expiratory grunting sounds
Dilation of pupils
Rapid, weak pulse or other cardiac arrhythmias
Loss of muscular control
Congestion of visceral organs
Renal tubular degradation and necrosis
How to Reduce Losses
Animals usually do not eat milkweed unless good forage is scarce or under conditions where plants freeze, etc. 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 contaminated with milkweed should not be fed to sheep or cattle. Milkweeds can be controlled with 2,4-D plus picloram (0.5 kg ae/Ac) or glyphosate at a spot spray. Follow all precautions for handling herbicides.