Toxicology is a vast subject even when narrowed down to just plants that are toxic to horses, so this article identifies some key points to keep in mind. First, know the horse(s) under your care well. This includes their regular appetite, activity level, daily habits and so on. Early recognition that a horse doesn’t feel well or isn’t acting like himself goes a long way toward his chances of recovery, whether the issue is a poisonous plant or some other emergency. Second, know which toxic plants your horse is likely to come into contact with based on where you live and how your horse is managed and fed. This means becoming familiar with the poisonous plants that are found in your area of the country as well as any areas you might travel to. Setting aside topics like fescue toxicity in pregnant mares and young horses, laminitis from contact with black walnut shavings and “slobbers” or slaframine poisoning from fungus-infected clover, there are still plenty of plants to be concerned about in horses either because they are highly toxic or because horses are highly likely to encounter them. Every region in the United States has its own list of plants that are poisonous to horses. Every veterinarian, veterinary school and toxicologist has their own list, too. Lots of factors determine whether a plant makes it onto someone’s list, such as:
1. Where does the plant naturally grow? Locoweed is a highly toxic plant but it grows only in the West and Southwest, so horses who live on the East Coast and eat local hay have little chance of encountering it. On the other hand, some of the pyrrolizidine-alkaloid-containing plants such as ragwort, groundsel, rattlebox, fiddleneck, tarweed and others, can be found throughout the U.S.
2. What is the plant used for? Yew, rhododendron and oleander are all common ornamental shrubs, and horses can be exposed to them if they are planted too close to their enclosures or if clippings are inadvertently tossed to them.
3. How palatable is the plant? That is, will the horse eat it even when he has good-quality hay and pasture to choose from or will he ingest it only as a last resort because he’s hungry? Water hemlock and poison hemlock are considered very toxic, but, fortunately, most horses will avoid these plants because of their taste. But some horses actually develop a taste for certain plants like bracken fern.
4. Is there a seasonal component to the plant’s toxicity? Red maple trees are commonly used in landscaping and do not necessarily need to be removed from a farm as long as horses cannot access the wiltedleaves of the plant in the fall that may be blown into a pasture by the wind. Likewise, healthy and mature Johnson Grass or Sudan Grass—two sorghum species—may be unlikely to cause harm. But young, wilted, trampled or frostbitten plants are dangerous for horses to graze on.
5. How toxic is the plant? Some plants, like the cardiac-glycoside-containing milkweeds, need only be eaten once in small amounts for a severe effect to occur. Other plants, like yellow starthistle or Russian knapweed, must be eaten in larger quantities over time before the horse develops clinical signs.
These reasons and others make it challenging to come up with one list of toxic plants for the entire country. The best you can do for any horse in your care is to know what plants he may be exposed to in his pasture, paddock or even hay, remove or prevent as many of these exposures as possible and be on the lookout for the first sign that he’s in discomfort. In addition, many veterinary schools, state colleges, universities and county extension offices will be able to provide detailed information specific to your location.
Lydia F. Gray, DVM, MA is the staff veterinarian/medical director for SmartPak. She is a USDF bronze medalist and an “L” Education Program graduate with distinction. She competes her Trakehner, “Newman,” in both dressage and combined driving.