Roots of Hazards

A look at some harmful plants for horses and how to get rid of them

From coast to coast, America’s fruited plains are a beautiful sight to behold. Stable managers have laid claim to acres upon acres of this verdant countryside to raise, train and care for their horses. Certainly a lush pasture, blooming with Mother Nature’s finest displays, is easy on the eyes, but sometimes it can be detrimental to a horse’s stomach—or even result in the animal’s death.

Most plant species, if ingested in large enough quantities, have the potential to cause health problems in horses. Says Dr. John Tejzes, a toxicology resident at the California Animal Health and Food Safety Laboratory in Davis, “You have to consider it from the plant’s perspective. It’s their defense mechanism. Whether they taste bad, have thorns or become toxic if eaten in abundance, most plants have some kind of deterrent to keep the animals from overgrazing their species.”

Yet horse owners should be aware that there are certain plant species commonly found in pastures that cause harmful side effects or even death if horses eat little more than a mouthful. Animal science experts have been able to identify a variety of flowers, grasses, trees, vines and shrubs that are toxic to livestock, and horses in particular. Stable managers can protect their animals from accidental poisoning by taking a good look at what’s growing on their land.

Know Thy Species

There are literally tens of thousands of native and non-native plant species growing in North America. Unless you have an interest in a second career as a botanist, it’s best to narrow your self-education to poisonous plants that commonly grow in your region. According to Dr. Michael Knight, medical director of the Animal Poison Control Center in Urbana, Ill., a good way to do this is to enlist the help of your county extension agent, who can tell you which species cause problems for livestock. They also can offer advice on how to eradicate the plant.

Tejzes emphasizes that, for most poisonous plants, it’s the dose that makes the toxin, and not so much the presence of the plant. So, it’s a judgment call in every case whether or not to uproot the vegetation. However, horse owners still need to be aware of the plants that cause critical health concerns, or even death, in small doses. The following is a rundown of some of the most deadly; by no means is it an exhaustive list. Stable managers are strongly advised to contact a local expert to learn about the dangerous plants in their regions.

The Serious Offenders

Genus Species: Prunus spp. (Spp. means that there are multiple species)

Common Names: wild and cultivated cherry trees, such as choke cherry, cherry laurel, apricot, almond, black cherry, wild red cherry

Geographic Location: These trees are found throughout the United States; however, the choke cherry is primarily in the northern states, from the West to East Coast, and the black cherry is primarily east of the Mississippi River.

The Hazard: They contain cyanide, which is released as a gas when the tree or its branches and leaves are wilting or dying; thusly, fall is a particularly high-risk time of year. The poison is concentrated in the seeds, leaves and bark, but a low degree of poison exists in the fruit as well. Only a few mouthfuls in an adult horse can cause severe and fatal complications within a couple hours of ingestion. When the plant is alive and healthy, the cyanide is not as easily broken down when ingested, but the threat of poisoning still exists.

Symptoms: dizziness, difficulty breathing and convulsions

Prognosis: Death is an immediate threat, yet a horse can survive without suffering permanent damage if medical treatment is received soon after ingestion and the horse is strong enough to “ride out” the symptoms.

Genus Species: Nerium oleander

Common Names: oleander, rose laurel, roseniorbeer, adelfa

Geographic Location: They’re found in warmer areas of the United States, such as the West Coast, the South and the Southeast. It’s a naturalized ornamental shrub, so it’s more likely to be found in or nearby landscaped areas.

The Hazard: The entire plant is extremely toxic, with severe or fatal complications arising from as little as one mouthful. It contains myriad toxins that affect the heart, such as cardiac glycoside.

Symptoms: abnormal heartbeat, sweating, weakness, difficulty breathing, body collapse, diarrhea

Prognosis: Immediate death is likely without urgent attention, yet survival is possible with timely, aggressive medical care. Depending on the amount of poison ingested, a survivor might be left with permanently scarred heart tissue, which would affect the horse’s endurance and stamina.

Genus Species: Taxus spp.

Common Names: Japanese yew, English yew, Florida yew, Pacific yew

Geographic Location: These plants are found everywhere in North America, yet they’re rare in the desert Southwest. The ornamental hedge with clusters of tiny, red berries is typically integrated into landscaped areas, but clippings can be inadvertently rolled into feed bales or thrown over a fence into a pasture.

The Hazard: The seeds, stems and leaves contain a poison called taxine, which blocks electrical conduction within the heart, causing asphyxiation and death.

Symptoms: body collapse, tremors, grunting, difficulty breathing, irregular heartbeat

Prognosis: Immediate death is likely without urgent attention, yet survival is possible with timely, aggressive medical care. Depending on the amount of poison ingested, a survivor might be left with permanently scarred heart tissue, which would affect the horse’s endurance and stamina.

Genus Species: Centauria solstitialis/Centauria repens

Common Names: yellow star thistle, star thistle, knapweed, Russian knapweed

Geographic Location: This reed-like weed with clusters of thorns is mainly in the northwest and west, from the Rockies to the West Coast. It generally grows along roadsides and in sandy areas where species competition is scarce.

The Hazard: Over a period of regular grazing, perhaps 30 days or more, a toxin called sesquiterpene builds up in the horse and targets the brain’s motor skills functions.

Symptoms: protruding tongue; difficulty swallowing, eating and drinking; trouble walking and moving

Prognosis: Because it takes many days of grazing on the plant before symptoms exhibit, the poison usually has caused irreversible brain damage and permanently compromised the horse’s motor skills by the time the situation is discovered.

Genus Species: Eupatorium rugosum

Common Names: white snakeroot, squaw weed, fall poison

Geographic Location: This plant grows on the eastern half of the continent. It has heart-shaped leaves and white flowers in the fall and it is typically found in partially shaded areas, such as under tree canopies or at the edge of a wooded pasture.

The Hazard: The plant contains treetop, which targets the muscular/skeletal system, including the heart. Horses usually will have grazed on it for one to three weeks before symptoms appear.

Symptoms: trembling, weakness, depression, loss of appetite and energy, irregular heartbeat

Prognosis: Horses can recover with good supportive care, but the heart tissue will be permanently scarred, affecting the animal’s cardiovascular strength.

Genus Species: Acer Rubrum

Common Names: Red Maple, Scarlet Maple, Swamp Maple, Water Maple

Geographic Location: This tree is found east of the Mississippi River to the East Coast and west of the Mississippi River through Louisiana, Arkansas and Minnesota. It’s generally found along waterways and in wooded areas.

The Hazard: Like the cherry tree, the maple tree is most toxic in the fall when the leaves are turning color and falling, yet the fresh leaves of maple trees can be toxic as well. The exact toxic agent is unknown, but clinical tests have shown that these trees can cause hemolytic anemia in horses.

Symptoms: difficulty breathing, depression, weakness, panting, dark red to brown urine

Prognosis: Horses can recover from this poisoning if promptly treated, but permanent liver damage may result.

What to Do if Poisoning is Suspected

If you suspect a horse has ingested a poisonous plant, your first line of defense is to call your veterinarian and isolate the horse from further exposure to the plant. If possible, collect a sampling of the plant (or plant remnants) to show to your vet. Also try to determine how much of the plant was ingested. Is there a visible “dent” in the shrubbery or grazed patch in the growth stand?

Unfortunately, it’s not really possible to keep “antidotes” on hand; treating plant poisoning usually involves two steps: a vet-administered activated charcoal and a cathartic to speed up the horse’s elimination process, and the individualized treatment of each exhibiting symptom. Horse owners are discouraged from administering cathartics or absorbents, such as mineral oil, themselves. A vet first should evaluate the horse to rule out other ailments that could cause similar symptoms.

You also might want to enlist the help of the Animal Poison Control Center, a nonprofit organization based in Urbana, Ill., that handles emergency phone calls and inquiries 24 hours a day, seven days a week. The call is toll free (1-888-426-4435), yet a $45 consultation fee is charged on a per-case basis. Medical Director Knight emphasizes that one “case” can involve numerous calls between the horse owner, the local veterinarian and the call center expert.

Knight says it’s ideal that the caller know the genus species of the ingested plant, because there’s some cross-over among the common names used for plants. However, a common name is better than no information at all. The poison control center can give detailed information on the toxins within the plant, how much has to be ingested to cause problems, and what symptomatic and supportive care is necessary.

Proactive Ways to Prevent Poisoning

There are several measures stable managers can take to minimize the risk of accidental poisoning. Dr. Dan Brown, an associate professor in the Animal Science Department at Cornell University in Ithaca, N.Y., recommends walking your property regularly to watch for new growth patterns of hazardous vegetation.

“Don’t depend on surveying your pasture from outside the fence line either,” he says. “You have to walk into the field to really know the composition of what’s out there.”

Arm yourself with knowledge before surveying your property. Refer to an illustrated field guide, search the Internet or ask a plant specialist to take the walk with you. (See sidebar for suggested books and websites about poisonous plants.)

If you’re unsure about a plant’s toxicity, make an appointment with an expert to bring in a sample for identification. County extension agents, veterinarian toxicologists, botanists and the employees at greenhouses and herbariums are all good resources. The day of your appointment, dig up one of the plants—root, stem and all, if possible—and wrap it in wet newspaper to keep it fresh. If you’re required to mail in a sample, press the plant between two pieces of paper and let it dry out before sending.

It’s easiest to survey your property for poisonous plants in the spring and summer when most species are in full bloom, but experts encourage horse owners to keep an eye on their pastures year-round.

“Walking your pasture several times in every season is advisable,” says Tejzes. “That way you get a good idea of what’s out there, what stage of growth it’s in and whether you need to take appropriate measures before something becomes a problem.”

Providing a variety of healthy feed sources and keeping your horses well exercised also deters your animals from nibbling on harmful vegetation. Oftentimes, horses will wander to less-palatable plants that are poisonous when they don’t receive ample feed or the pasture is overgrazed. However, don’t assume that feed bales are toxin free either. Carefully look over each one before tossing it to your horses. Sometimes clippings of poisonous plants, such as groundsel, oleander and ground ivy, inadvertently can end up in a bale.

Routine exercise and handling also curtails boredom, which can trigger not-so-healthy munching habits in horses.

Poor nutrition also can trigger an attraction to poisonous plants, according to Dr. Kip Panter, research toxicologist with the USDA Poisonous Plant Research Lab in Logan, Utah.

“For instance, if your horses are nitrogen deficient, they’ll move to high nitrogen plants, like lupines, which have upwards of 30 to 40 percent protein in their seed pods,” he explains. “The problem is that lupines also contain toxic alkaloids. To compensate for the deficiency, the horse will eat more of the poisonous plant in a short period of time. When that happens, the toxin overrides the animal’s ability to metabolize and excrete it, and toxicosis occurs.”

The use of pesticides and weed killers also can draw horses to otherwise unattractive plants. According to Dr. Michael Knight, medical director of the Animal Poison Control Center in Urbana, Ill., once a plant is treated, it becomes softer and more succulent, which makes it tastier to the horse. For this reason, Knight suggests strictly following the directions on pesticide labels and keeping your horses away from herbicide-treated areas until the targeted vegetation is completely dried out and remnants are removed.

Horses are inquisitive creatures; most anything with leaves, flowers or even thorns can attract their attention under the right circumstances. It’s up to us to manage our land so that it remains safe and toxin free season after season.


Frost-damaged or dying cherry trees release cyanide, but that’s not the only reason they are dangerous to horses. Cherry trees growing in Kentucky have served as the preferred breeding ground for the Eastern tent caterpillar, a pest strongly linked with Mare Reproductive Loss Syndrome (MRLS). The condition, which devastated the Kentucky thoroughbred industry in the spring of 2001 and continues to affect hundreds of horses in the state, causes pregnant mares to abort or have still births.

There has been much discussion about the role the Eastern tent caterpillar plays in MRLS, yet the latest research, conducted by the University of Kentucky, College of Agriculture, centers on two theories: a virus or bacterium often associated with the caterpillar is the trigger, or the caterpillar itself is the trigger.

Meanwhile, entomologists at the university have been studying treatment strategies in controlling Eastern tent caterpillar populations. To date, results have shown that Talstar is effective in controlling all sizes of caterpillar larvae, while Dipel works more effectively on small larvae than large larvae. Researchers also tested the effectiveness of injecting nest-infested trees with an insecticide called Inject-A-Cide B. Test results showed “excellent control” within one week of the injection treatment.

Research also indicates ineffective methods include applying horticultural oils and insecticide soaps, as well as spraying pastures to kill caterpillars migrating to other food sources.

While Eastern tent caterpillar populations soared in Central Kentucky in 2001 and 2002, studies last winter revealed a decrease in caterpillar egg masses. Entomologists hope this indicates a lower caterpillar population for spring 2003.

Source: University of Kentucky, College of Agriculture

Recommended Reading

1) “Toxic Plants of North America”

Authors: George E. Burrows and Ronald J. Tyrl

Publisher: Iowa State University Press

2) “Poisonous Plants of California

Authors: Thomas Fuller and Elizabeth McClintock

Publisher: University of California Press

3) “Natural Poisons in Horses”

Author: Jeffery O. Hall

Publisher: ASPCA Animal Poison Control Ctr.

4) “Natural Toxicants in Feeds, Forages and Poisonous Plants”

Author: Peter Cheeke

Publisher: Prentice Hall

5) “Field Guide to Venomous Animals and Poisonous Plants”

Author: Steven Foster

Publisher: Houghton Mifflin Company


1) ASPCA Animal Poison Control Center –

2) Poisoning of Horses by Plants, Ministry of Agriculture and Food, Ontario, Canada –

3) Poisonous Plant Extension, Animal Science Department, Cornell University –

4) Guide to Poisonous Plants, College of Veterinary Medicine and Biomedical Sciences, Colorado State University –

5) Poisonous Plants Home Page, School of Veterinary Medicine, University of Pennsylvania –






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