Plants Toxic to Horses

Horses can and will live with poisonous plants in their pasture without harm, preferring instead to graze on the more palatable green grass. But when their normal grazing routine is interrupted during travel, they'll tend to snatch at anything green.

How often have you heard of someone’s horse developing colic, getting sick, or even dying while camping? While the origin of toxic syndrome is hard to determine in horses, always consider plant poisoning in cases of sudden illness, especially if your horse’s digestive tract is affected. Horses can and will live with poisonous plants in their pasture without harm, preferring instead to graze on the more palatable green grass. But when their normal grazing routine is interrupted during travel, they’ll tend to snatch at anything green.

Garth and I like to let our horses graze when we travel. We believe fresh green grass is healthy for both their mental and physical well-being. But if you do the same, you need to know what’s safe. Toxic poisoning can be tricky. Illness can strike minutes, hours, days, or even weeks after exposure. Symptoms may vary depending on the time of year, condition of the plant, and the amount a horse ingests. In some plants, only the immature form, or the berry or root will contain toxin. Some are deactivated by drying, while others remain very potent when dry.

Following are a few of the toxic plants I saw while on a ride in Pennsylvania:

Members of the rhododendron plant/mountain laurel. The leaves of these plants are toxic, affecting heart, nerves, and muscles; these plants can also cause colic, diarrhea, and excess salivation.

Black walnut. Just standing in black-walnut shavings can cause problems, particularly acute lameness, limb swelling, and laminitis. Usually, this occurs when shavings consist of more than 20 percent black walnut.

Red maple. Wilted and dry leaves are toxic. Maple poisoning causes progressive hemolytic anemia (a blood disorder), respiratory distress, depression, dark-red to brown urine, and death.

Milkweed. All parts are toxic, and can cause colic, muscle tremors, respiratory difficulty, irregular heartbeat, central nervous system irritation, and uncoordination. (Note: Horses don’t tend to eat this plant unless it is accidentally baled into hay.)

Cherry, peach, and plum trees. Plants in this family that are stressed by drought, frost, or wilting contain cyanide. The danger is greatest three to four days after branches have been cut or blown down. Symptoms include anxiety, breathing problems, staggering, convulsions, collapse, and sudden death.

Buttercups. The juice of this plant can cause mouth, skin, and intestinal ulcers, excessive salivation, colic, and diarrhea. Buttercups don’t tend to be palatable to horses, and the plant in its dry form isn’t toxic.

Moldy clovers. Red clover and some legumes may be infected by a parasitic fungus known as Rhizoctonia. Molds grow best in wet weather and high humidity, and appear as dark spots on affected plants. The mold itself isn’t necessarily toxic, but rather the mold is associated with growth of mycotoxins.

For red clover, the principle toxin is slaframine. (Hay made with red clover can maintain toxicity for several years, as the toxin is passed in the seeds.) After exposure, horses may refuse to eat because of tongue and lip swelling, creating excess salivation. Other plants infected with mold-related poisoning, such as seen with fescue toxicity, may cause abortion of pregnancy. Mycotoxins may also cause liver damage and photosensitization.

To learn more about toxic plants, contact your local County Extension office, check with your veterinarian, and visit agriculture colleges, libraries, and horticultural societies.

You can also visit these websites: Colorado State University-Fort Collins, (go to the Teaching link, and click on Guide to Poisonous Plants); Cornell University,; the National Animal Poison Control Center,; and the American Society for the Prevention of Cruelty to Animals, (If you suspect your horse has been exposed to a toxic plant, you can also call this organization at 888/426-4435; it’s open 24/7; there’s a $50 consultation fee.)

On a happier note, I read a very good book this summer, The Horses of Proud Spirit, by Melanie Sue Bowles. Bowles shares heart-touching stories about horses lucky enough to cross paths with a couple in Florida who love and care for them.

The book would be sure to please any horse lover on your Christmas list. You can order it from your local bookstore, or visit TTR

Kathy Rumsmoke, 67 Stewart Rd., Horseheads, NY 14845; (607)594-2191;;

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