Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Some Pasture Plants are Problematic for Horses

Pet Column for the week of May 3, 2010

Related information:

Related site - ASPCA Animal Poison Control Center

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Ashley Mitek
Information Specialist

Source - Eric Dunayer, VMD, MS, DABT
There are countless poisonous plants in the world. Unless you are a specialist who can identify all the varieties, it is often difficult for owners to roam their pastures and know what to watch out for. But two of the most common poisonings in horses that owners should definitely look out for are from white snake root and maple leaf ingestion.

Dr. Eric Dunayer is a veterinary toxicologist at the ASPCA Animal Poison Control Center and an adjunct instructor at the University of Illinois College of Veterinary Medicine in Urbana. He explains that, "White snake root is very common in this part of the country."

This plant specifically causes heart damage, an organ that can be scarred if the disease is not caught early. Depending on the severity of the scarring, a veterinarian may declare the horse unsafe to ride.

"Horses can show symptoms after eating the plant in just a few days or weeks, depending upon the amount ingested," notes Dr. Dunayer. Symptoms owners would see are: incoordination, weakness, muscle tremors, swelling, and exercise intolerance.

Another plant poisoning in horses is red maple leaf ingestion. Fresh leaves are not a problem, but "the wilting leaves are toxic," says Dr. Dunayer. If you do have a red maple in your pasture, be sure to remove fallen branches before the leaves on them start to wilt.

Clinical signs of red maple poisoning are: red urine (because the horse’s red blood cells rupture and hemoglobin passes in the urine), weakness due to anemia (a decreased amount of oxygen carrying red blood cells), and icterus that can be seen on the gums.

To prevent any type of plant poisoning on your farm, Dr. Dunayer recommends that owners walk the pasture and send any suspicious plants to your local Extension office. In general, many horses know not to eat certain plants, but that is not always true.

For example, if you spray herbicide on the field, that in and of itself may not be toxic, but it does make many poisonous plants more palatable. This is because as the plant starts to die it becomes sweeter, and many horses enjoy that taste.

In the end, there are over 2,000 toxic plant species, but the growing patterns are very regional. For example, on the west coast yellow star thistle is a huge problem unlike in the Midwest. If you suspect your horse has eaten a poisonous plant, Dr. Dunayer recommends that you, "call your veterinarian as soon as possible and try to identify the plant."

Even if you cannot classify the plant, a veterinarian may be able to treat your horse symptomatically depending on the animal's clinical signs. But the sooner you can treat, the better the prognosis.

For more information about toxic plants, please contact your local veterinarian.