Poisonous Plant Risk Increases as Pasture Grass Goes Dormant

As autumn rolls around, pastures become depleted as grass and plant growth slows prior to going dormant for winter. So, it's not surprising that this time of year can also bring an increase in reports of horses becoming ill from eating something they shouldn't have.

Karyn Bischoff, DVM, MS, Dipl. ABVT, a diagnostic toxicologist at Cornell University's New York State Animal Health Diagnostic Center, said it does appear there are more suspected cases of poison plant ingestion in late summer and autumn, but also says these are difficult to diagnose.

"Plant ingestions over time can cause chronic disease, so by the time the horse is noticeably sick, the plant is gone because it was all eaten, it's out of season, or the horses were moved to another stable or pasture after exposure," Bischoff explained.

She added that plant toxins can be difficult to find in the body as the particular toxin are sometimes not known, there are no good tests for it, or the toxin has metabolized and gone before the horse presents clinical signs of serious illness.

Happily, most toxic plants are not very palatable, and horses tend to avoid them.

Jack van Roestel, MSc, a longtime horse owner and forage consultant with AgraPoint in Kentville, Nova Scotia, stressed that prevention of poisoning begins with good pasture management.

"When it comes to grazing, it's good to have several paddocks that are rotationally grazed, with proper entrance and exit heights, rest periods, stocking numbers, and applied fertility," van Roestel said. "By having a fairly competitive grass stand, weed problems will be reduced."

Bischoff agreed that good pasture management goes a long way in preventing poisoning, but also stressed that horse owners should contact their veterinarian whenever they have concerns about health issues. "If they can't answer your questions right away, they are usually going to know the best places to find trustworthy information for you," she said.

Some wild plants to watch out for

Toxic plants will vary by the region you live in. Contact your local extension office to find out more information on noxious plants for your area. The botanical name is given first: the abbreviation spp. indicates there can be more than one species involved.

  • Senecio spp. Tansy ragwort, groundsel, or stinking willy
  • Prunus spp. Cherries, apricots, peaches and plums
  • Cicuta maculata. Water hemlock is one of the most poisonous plants found in North America.
  • Taxus spp. Yew (often used as an ornamental)
  • Sorghum halepense. Johnsongrass
  • Robinia pseudoacacia. Black locust.
  • Asclepias syriaca. Common milkweed
  • Phytolacca americana. Common pokeweed.
  • Pteridium aquilinum. Bracken fern.
  • Crotalaria spp. Rattlebox or rattlepod
  • Solanum spp. Bittersweet, nightshade. *Other members of the Solanum (nightshade) family including Physalis (Japanese lanterns, an ornamental) Datura (Jimsonweed) and the tomato (Lycopersicon) are also highly toxic.
  • Acer rubrum. Red maple. *Wilted leaves of red maple are especially toxic, but avoid feeding leaves of any maple species to horses.
  • Juglans nigra: Black walnut. *Avoid bedding made from sawdust or shavings of black walnut as these can cause laminitis in horses.
  • Astragalus and Oxytropis spp. Locoweeds. *These plants are not only toxic but addictive. Bischoff said once a horse has started eating the plant it must never again be in a pasture where locoweed is present, because it will seek the plant out.

For more on toxic plants and substances see "Poisoning in Horses: Common Toxic Substances."

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Jodi DeLong

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