2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
By Theresa A. Fuess, Ph.D.
University of Illinois
College of Veterinary Medicine
The most common problem with horse pasture is that there is not enough of it. People underestimate the needs of their horse and the size of their pasture. Then the pasture ends up overgrazed. The horse eats the grass down to the dirt, which allows weeds--things the horse would not normally eat--to take over.
"Diagnosis of pasture poisoning is difficult unless you know what the horse ate," says Dr. Gavin Meerdink, clinical professor of toxicology at the University of Illinois College of Veterinary Medicine at Urbana. "You can survey the potentially harmful plants in the pasture, but it always seems you can't discover the plant that the horse ate." Analytical procedures for plant agents from animal fluids or excreta are limited.
"Most plants contain some sort of toxic ingredient," says Dr. Meerdink. "Anything can cause colic if overconsumed. However, we have desirable forages that we plant and try to propagate. Any other plants in your pasture should be suspect."
Pasture management prevents weed poisoning. A good pasture has an adequate stand of grass, such as bluegrass, orchard grass, or bromegrass, and possibly a legume. It has to be kept fertilized and managed. A good stand of grass will keep the weeds out. "If the pasture is overgrazed, remove the horse. Promote desired regrowth with some fertilizer and manage weed control early. It is a lot cheaper than dealing with weeds and poisoning," says Dr. Meerdink.
The most commonly diagnosed, most toxic, and most catastrophic poisonous plant for horses is the yew, a shrub planted as an ornamental evergreen. Just a few sprigs can kill a horse. "Often, illness and death result when a horse eats clippings thrown to it after the shrubs were trimmed. We know of a case or two of this every spring," says Dr. Meerdink.
Oak, black locust, red maple, and black walnut trees can be toxic to horses. If these are in your pasture and your horse finds them attractive, then the trees should be fenced. Wild cherry trees, found in fence rows, are also toxic. Horses are less likely to consume toxic quantities if suitable forage is available.
Buckwheat, St. Johnswort, and alsike clover can cause extreme sensitivity to sunlight. Skin without pigment, such as a blaze or white nose, may become swollen and eventually slough. People often mistakenly blame something else, such as fly spray.
White snakeroot, another potentially poisonous plant for horses, grows in undisturbed shaded areas.
In the muggy, wet weather of mid-summer, clover may get black patch disease, which is caused by a mold that also produces a potent agent that causes sudden salivation in horses. Recovery is uneventful and rapid when the diseased clover is removed, but severe consequences are possible with prolonged exposure.
Dr. Meerdink cautions that a grass diet alone may not provide adequate mineral balance such as calcium, phosphorus, salt, and magnesium. Mineral block or supplement should be balanced with your horse's diet.
For information on pasture problems in your area, contact
you local equine veterinarian.