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Contaminants that are Potentially Toxic to Horses

Buckeye Nutrition

There are many contaminants that are potentially toxic to horses. This technical bulletin provided by Buckeye Nutrition lists and describes some of the more common ones found in forages, toxic plants, bedding and water. The goal at Buckeye Nutrition is to provide feeding programs based on these principles of nutrition, along with good management, to allow horses to grow and perform to their genetic potential and improve their overall longevity.

Contaminants in Forages (hay or pasture) - Mold and Fungus

Tall fescue (grown on approximately 40 million acres) is the most widely grown forage in the United States. It can be a very safe inexpensive source of forage for horses. However, it can become infested with a fungus that can cause many problems primarily related to foaling. The fungus is an endophyte (grows inside the plant) that does not adversely affect plant growth. The primary problems associated with infected fescue are abortions, prolonged gestations, dystocia, agalactia, and thick or retained placenta. The end result is increased foal and mare deaths. The endophyte can be found both in pasture and hay. There is a fungus-free seed available, but the only way to determine if the fescue presently being fed is fungus-free is by laboratory analysis.

Sweet clover is an important forage crop in some areas of the country. Sweet clover poisoning, caused by dicumerol, is produced from coumarin in plants that are infected with various species of Penicillim. Mold-free sweet clover hay is difficult to produce because the thick stems are hard to dry. Dicumerol interferes with Vitamin K needed for normal blood clotting thus the horses can hemorrhage internally.

Red clover can also be a high quality forage for horses. However, if harvested too wet, red clover hay can be infected with Rhizoctonia leguminicola, which may produce a mycotoxin called slaframine. This causes animals to slobber profusely; the condition is commonly called “slobbering disease” or “blackpatch”. This condition is not life threatening but does cause alarm. The slobbering usually subsides within 48 – 96 hours after removal of the affected hay.

Other Potentially Toxic Substances in Forages

Alfalfa hay grown in the southwestern United States may be infested with blister beetles. Blister beetles are attracted to the bloom of alfalfa plants and are crushed during the processing. The beetles contain cantharidin, which irritates the mucous membranes of the digestive tract. The horses develop a fever, colic, the sweats and a very severe, watery diarrhea. Ingesting five to six beetles can cause death in less than 48 hours. There is no antidote.

Blister beetles are about ¼“ wide and ¾” long. They can be of several colors, but have very distinctive heads and long narrow bodies. It is very important to remember that even though blister beetles are not likely to be a problem in the eastern United States the hay could have come from another area of the country.

Prussic acid (cyanide) poisoning can cause death in horses. Toxic levels often build up in the leaves of a number of plants, including sorghum, Sudan grass, and hybrids of the two, Johnson grass and wild cherry. Prussic acid is most likely to accumulate after a killing frost or during rapid growth after a drought. Wild cherry can be a problem after a windstorm when branches are broken off the tree and the leaves wilt. Death is caused by suffocation due to interference with the oxygen-transferring ability of red blood cells; the blood is cherry red in animals affected by Prussic acid.

Some plants accumulate nitrates during stress periods and/or heavy fertilization with nitrogen. The plants most likely to accumulate nitrates are Sudan grass, sorghum-Sudan hybrids, corn, wheat and oats. Some weeds that are found in hay such as nightshade, goldenrod, smartweed, ragweed and lambs quarters can also accumulate nitrates. Clinical signs of nitrate poisoning include labored breathing, staggering, muscle spasms and death. The blood is coffee-colored due to decreased oxygen-carrying ability of the hemoglobin.

Selenium levels in soils and plants vary greatly from one area of the United States to another. Selenium poisoning occurs primarily in the midwestern plains states. Most selenium toxicities in horses are due to consumption of secondary accumulator plants (those plants which accumulate selenium). Other plants that require high selenium soil to grow are called “indicator plants.” They can build up 100 times or more the selenium levels of other plants grown in the same soil. Locoweeds, woody aster, golden weed and prince’s plume are some plants that can produce selenium poisoning. They are toxic when eaten fresh or dried.

Foxtail can cause ulcerations of the lips and mouth due to its porcupine quill-like awns. The plant itself is not toxic but the horse will be unable to eat normally until the ulcers heal.

Toxic Plants

The yew and many other ornamental plants are toxic to horses. All parts of the yew are poisonous, whether fresh or dried. Death can occur within minutes of eating as little as one pound of the plant. The horse will quiver and then drop suddenly as if shot. Once signs appear, it is generally too late for treatment. Most horses will not preferentially eat yew, but problems occur when grass clippings containing yew trimmings are fed to horses. In the winter the green yews also appear to be more attractive to animals.

Bracken fern is also toxic because it causes a thiamine deficiency, but it is not nearly as poisonous as yew. Twenty to twenty-five percent of the total diet would have to be bracken fern to be fatal. However, bracken fern is very palatable; if available to the horse they will eat it.

Black locust, red maple, black walnut and oak trees can be toxic to horses. Some of these have toxic leaves while others like the black walnut are totally toxic.


If used for bedding, black walnut shavings can be a problem. Juglone is the toxic substance in these shavings. They will cause problems if ingested or if they contact with the skin. Within 12 to 24 hours after contact, the horse will develop signs of acute laminitis. They can recover and live with laminitis, but if severe death can occur.


Water contamination can be a problem in some areas. Runoff from strip mines and the dumping of brine water from gas and oil wells can cause mineral imbalances. Water that is extremely high in some elements can be toxic or interact with other elements affecting the animal’s health.


Contaminants that are potentially toxic to horses are numerous. Fortunately, horses are very selective eaters and will not usually consume toxic substances under normal circumstances. In areas where pastures are short or when severe weather has destroyed forages animals are sometimes forced to eat toxic plants. Horses that are kept in barns or dry lots may also be less selective when turned out to pasture.

If toxic plants or other toxic contaminates are harvested in hay it is more difficult for the horse to distinguish and separate the toxic portion. Under this circumstance, they are more likely to eat something that is toxic even though adequate feed is provided.

If a toxicity problem occurs with a horse it is important to obtain a complete, detailed history. This should include samples of feedstuffs, bedding and water. All pastures and turnout lots should be inspected for any sources of potential toxins. The combination of a complete history and samples should help provide an answer to the cause of the problem.

With good management taking into account the different seasons it is possible to greatly reduce the incidence of contaminants affecting animal health.

Reprinted with permission from Buckeye Nutrition