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Horse Pasture is No Place for Poisonous Plants


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Pet Column for the week of July 24, 2000

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

By Carrie Gustavson
Information Specialist
University of Illinois
College of Veterinary Medicine


Fortunately, horses don't find most poisonous plants very palatable. However, there are
exceptions and sometimes horses will eat highly toxic or lethal doses of poisonous plants --
even when forage is adequate. That's why summer is a good time to explore your horse
pasture to be sure toxic plants are kept out of your horse's diet.

It's important to know what plants to look for in each season. "Different plants grow and
become toxic at different times of the year," says Dr. Val Beasley, veterinary toxicologist at
the University of Illinois College of Veterinary Medicine in Urbana. "Though red maples are
toxic when green during the growing season, the wilted or recently shed red leaves in the fall
are most potent in causing anemia and kidney damage in horses. Other poisonous plants are
most toxic as young sprouts in the early spring or when toxic seeds are produced late in
summer."

Oftentimes, what is safe for a person to eat can be toxic to a horse. Cabbage, broccoli,
onions, chives, kale, elderberry, pokeweed, and rhubarb can be toxic for herbivores such
as horses. "At the end of the garden season, be careful what you toss over the fence to the
horses," says Dr. Beasley.

The Solanaceae family of plants provides a good example. Though it includes the infamous
deadly nightshade, we put several of nightshade's relatives on our dinner plate. Luckily,
tomato's fruits and mature potatoes aren't toxic to us. But, the green parts of these and
many related plants of this family contain atropine or atropine-like substances. "For a horse,
atropine is bad news," says Dr. Beasley. "Atropine will slow the gut. That's the last thing a
horse wants because it will cause colic, which can kill a horse." Other toxic members of this
family include ground cherry, henbane, and jimsonweed.

Some plants contain saponins, which are detergents, or soaps, and present another danger
for horses. "Soapy substances can irritate the digestive system and cause diarrhea, cramps,
and colic," says Dr. Beasley. Plants that contain saponins include pokeweed, bouncing bet,
corn cockle, and English ivy.

Horses don't normally get sunburn, but if you notice blistering and cracking on any white
areas of your horse, he may have been munching on large amounts of prairie groundsel,
rattle box (Crotalaria), and kochia (fireweed). Though members of different plant families,
they all kill liver cells. When plant toxins damage the liver, photoactive substances derived
from chlorophyll build up in the blood. These substances are exposed to the sun only in the
white areas, so that's where you'll notice blistering, cracking, or loss of outer layers of the
skin. Other photoactive substances, such as wild parsnip or St.-John's-wort, cause the
same skin lesions without first damaging the liver.

Milkweed, dogbane, oleander, yew, lily-of-the-valley, white snakeroot, azalea, and other
Rhododendron species and purple foxglove all contain substances that may affect a horse's
heart. "These plants can hit the heart really hard and cause heart failure," says Dr. Beasley.
"Japanese yew can stop the heart suddenly. One or two leaves of oleander, a plant found
mainly in California, Arizona, and Florida, can kill a horse. White snakeroot causes scarring
in the horse's heart and may decrease performance and cause severe heart failure."

Horse owners should learn the list of plants, including many popular ornamentals, that can
cause the biggest problems and even death in horses. A few castor beans or one little
potted hydrangea can kill a horse. Larkspur, a plant sometimes found as an ornamental in
the Midwest, can cause paralysis and respiratory failure. Wild black cherry, chokecherry,
plum, and peach contain cyanide poison in the leaves, and especially in the seeds, which
can be rapidly lethal. Sorghum can also be a source of cyanide. Socrates demonstrated the
deadliness of poison hemlock, a plant that is ubiquitous in the Midwest.

If a poisonous plant is consumed by your horse, another animal, or a human, Dr. Beasley
recommends you take a sample of the plant, estimate the quantity eaten, note the time of
onset of signs, and call a veterinarian or physician immediately.

If you are not sure what certain poisonous plants look like, the University of Illinois
Poisonous Plants Garden on St. Mary's Drive near Lincoln Avenue in Urbana, just
northwest of the Veterinary Medicine Basic Sciences Building, is a good place to start.
When you visit the garden, pick up a copy of the brochure "Poisonous Plants Garden,"
which contains information about the plants you'll see. A quick stroll through the garden will
show you what poisonous plants common to the Midwest look like, and you'll learn what
effects they can have on your horse.

For a virtual tour of the garden, visit the Plants Toxic to Animals Web site:
http://www.library.uiuc.edu/vex/vetdocs/toxic.htm. There are also several good books
available on horses and toxic plants. Your local equine veterinarian can provide you with
more information on how to maintain a safe horse pasture.