Rural Heritage Vet Clinic

Toxic Plants in Your Horse Pasture
by Sandra M. Burger

Two big misconceptions are that horses know better than to eat a toxic plant and that all toxic plants taste bad. My best horse Hans died of plant poisoning. I have had many horses, but have never bonded with one as strongly as with Hans. One beautiful spring day I gave him a bath and tied him to a Chinaberry tree to dry. He was fine, ate his supper, and returned to his paddock as usual. The next morning he was suffering from what appeared to be colic. It was the beginning of my worst nightmare.

Perhaps Hans could have been saved, but my veterinarian mistakenly treated him for colic rather than poisoning‹a common occurrence, since little is known about toxic plants. As I discovered, little attention is given in vet school to poisonous plants. The few photocopied handouts distributed at one of the major universities show only line drawings and offer the barest essentials about treatments for poisoning. No wonder, then, that most veterinarians have scant knowledge about how to recognize and treat poisonings. I contacted our county Agriculture Extension office, but all they had were more line drawings and even less information.

The data I subsequently gathered from books, veterinarians, and herbalists needed to be published and made available to all equine owners so they would not suffer through the horror I experienced, watching a treasured horse suffer a slow and agonizing death. Hans' story is included in my book, Horse Owner's Field Guide to Toxic Plants. You may wish to read about Hans when you are alone; many readers have told me his story brings tears to their eyes, even though they never knew the horse. I wrote the majority of the book in solitude, in the middle of the night, just for that reason.

For the book I compiled a complete listing of poisonous plants found in the United States, plus many references for Canada. I included both a description and a full-page color photo of each plant, along with information on how to recognize a poisoning, what to do before the vet gets there, treatments, and proper plant disposal. Some of the most toxic plants are:

Golden chain. Ingesting seeds in the amount of .05% (per horse's body weight) causes convulsions, coma, and death.

Oleander. One ounce of leaves or ingesting .0005% (per body weight) causes death. Do not burn—the smoke is toxic, too.

Apple, Cherry, Peach. Believe it or not, one cup of seeds from these plants can kill a pony. (Remarkably, birds are not affected by the seeds). Keep animals away from trees so they do not stuff themselves on fallen fruit. The fleshy part of the fruit is fine for them to eat, but the seeds contain cyanide and the leaves contain cyanogenetic glycosides, which cause depression, colic, convulsions, and death. Wilted leaves are especially toxic.

Boxwood. Ingestion of one pound will cause respiratory failure and eventual death.

Castor bean. This weed is sometimes found in grain that is not well cleaned at the feed mill, and is also sometimes mixed in with hay. One mouthful for a horse equals stomach hemorrhages and a slow, painful death. Fortunately horses usually dislike the taste. Check all your feed for any questionable material it might contain.

Ground hemlock and water hemlock. Ingestion of 1% of the horse's body weight will cause death. This is a quick acting plant—the horse is usually found lying next to it.

Black walnut. Horses bedded in a 5% mixture of black walnut shavings will founder. The recovery rate is good if the horse is immediately removed, but the recovery period is lengthy.

The severity of any plant poisoning depends on a horse's age, height, weight, amount consumed, soil and water growth conditions of the plant, and time of year. Green sprouts are the most toxic part of many plants, and poisonings occur more frequently in the springtime.

Symptoms of poisoning may occur immediately, as with oleander or golden chain, or over time, as with salt brush, sudan hay, johnson grass, rye, fescue, or klein. Yes, you could be slowly poisoning your horse with your hay!

This information, and more, is in my book, which took me a year and a half to put together and is the only complete guide available today on plants that poison horses. Chances are good that you have some of the more than 100 toxic plants in your horses' field. Please make yourself aware of the harmful plants in your horses' environment and dispose of them properly.


Sandra M. Burger of Bandera, Texas, is the author of Horse Owner's Field Guide to Toxic Plants. This article appeared in the Spring 2001 issue of Rural Heritage.

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26 October 2011 last revision