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.
Original Article: http://www.cvm.uiuc.edu/petcolumns/showarticle.cfm?id=16