Sign Up for Newsletters

Milkweed Toxicity in HorsesBy Kentucky Equine Research Staff · August 8, 2017

While monarch butterflies require milkweed to complete their life cycle, horses are best served by ignoring the ubiquitous plant. Several species of milkweed, a well-known perennial plant, cause poisoning in horses and other livestock, usually when more palatable plants are not available.

Milkweed plants prefer sandy soils and are often found along roadways and waterways. They can be opportunistic, thriving in overgrazed areas.

Plants have erect stems, and most species exude a milky sap from leaves and stems when broken or fractured. Plants produce pods, which contain many seeds, each with silky white filaments attached. Both narrow-leafed (1-1.5 inches) and broad-leafed (2.5-5 inches) species have been identified, and the narrow-leafed varieties appear to be most toxic.

The principle toxin in milkweed is galitoxin, and it is found in all vegetative parts of the plant, including the leaf, stem, and root. Additionally, milkweed contains cardiac glycosides, which also contributes to toxicosis.

Ingestion of as little as 0.1% of body weight—or 1 lb (0.45 kg) for a 1,000-lb (450-kg) horse—of narrow-leafed milkweed may cause toxicity and death. Clinical signs of toxicity include profuse slobbering, incoordination, colic, irregular heartbeat, and violent seizures.

Because of widespread distribution, milkweed is likely to find its way into paddocks and pastures intended for horses. Like most toxic plants, the best defense against milkweed propagation is a weed-management strategy that includes frequent mowing and application of safe, effective herbicides. Proliferation of milkweed in hayfields is also problematic, as the toxins in milkweed are not lost once plants dry. Hence, contaminated hay is potentially dangerous to horses.

Have you found milkweed in your turnout area? Can you not rid the area of the weed? Is there little palatable forage in the turnout space?

Horses can be fenced off from the area and fed hay, hay cubes, or hay pellets to satisfy forage requirements. Coupled with an appropriate feed or balancer pellet, horses can thrive on this diet. For a free nutrition consultation for your horse, contact a nutrition advisor today!