A wide variety of plants may be poisonous to horses. Toxic plants grow in most areas of the country. It is important for horse owners to develop the ability to recognize poisonous plants in their own area, because some horses will not avoid consuming them. Information concerning recognition and management of toxic plant growth can be obtained from local veterinarians and agricultural extension agents. (Click here to see some toxic plants)
Some of the more common poisonous plants in the western United States include the common groundsel, which can cause permanent liver damage, and yellow star thistle, which affects the brain. Oleander, a common ornamental in the west, is highly toxic to horses, causing convulsions, diarrhea, colic, irregular heartbeat, and death.
The yew, a common ornamental plant in many areas of the country, is sufficiently toxic that consumption of only 3 or 4 ounces of the plant may cause death by stopping the heart. Another common ornamental plant toxic to horses is the horsetail, which can produce a deficiency of vitamin B. Signs of horsetail intoxication include weakness, decreased appetite, and loss of coordination. The seeds of a number of legumes, such as some beans and peas, also can be toxic for horses. The clinical signs depend on the toxin in the particular seed and may involve many organ systems. A variety of sorghum plants, such as Sudan grass, can cause incoordination and staggering as well as bladder inflammation.
Ingestion of tall fescue by pregnant mares can result in a variety of reproductive disorders, including abortion, difficulties during birth, thickened or retained fetal membranes, depressed milk production, and an increased risk of foal death. The problem with fescue is actually not the plant itself but a fungus that grows on the plant. Identification and removal of infected plants are necessary to prevent the disease. Pastures should be replanted wit fungus-free fescue seed. Another disease-causing fungus associated with plants ergot, which infects the seeds of a variety of cereal grains and can cause abortion, gangrene (death and decay of tissue, usually caused by loss of blood supply and subsequent invasion of bacteria), and neurologic disorders. Another fungus, Aspergillums flavus, produces a toxin (alflatoxin) in grains and peanuts, causing decreased growth and severe liver disease.
In general, the signs of plant intoxication depend on the particular plant and the amount of toxin ingested. Preventing exposure to poisonous plants in the best defense against their toxic effects. If consumption of a toxic plant is suspected, a veterinarian should be consulted immediately.