Q. I have a row of 40-foot black cherry trees near my horse pasture, about 25 feet back from the fence line. Now that it’s fall, a lot of the dead and dying leaves are blowing into the pasture. I have round bales out for the horses to eat and there’s still some grass, but the leaves are present over much of one side of the pasture. I’ve heard that black cherry leaves can be poisonous to horses—is this true?
A. Autumn, with trees shedding their leaves and seeds, can pose an interesting challenge to horse owners. Toxic leaves that otherwise would not be accessible to horses can fall into pastures, particularly when windy weather blows them in from a distance. Horse owners should therefore be aware of what leaves are in their pastures, as leaves from far-away trees might now be available for their horses to consume.
Black cherry trees (Prunus serotina) belong to the Rosaceae family and are native to North America, Mexico, and Central America. As with other members of the Prunus family (which includes choke cherry, fire cherry, peach, and plum trees, among others), black cherry tree leaves contain cyanogenic glycosides that can be converted to cyanide (prussic acid) when the leaves wilt. If an animal consumes wilted leaves, cyanide poisoning can occur.
Thankfully, when consumed, dead, dried, and crumbling cherry tree leaves do not pose as much risk to horses as wilted green leaves. Horses are unlikely to eat these leaves regardless of the time of year if they are well-fed, so provide ample forage (e.g., free-choice hay) in your pasture, especially if grass is sparse. Keep in mind that some horses might consume leaves out of curiosity regardless of how well-fed they are. Horses are less at risk of cyanide poisoning than ruminants. A lethal dose for a 1,200-pound dairy cow is estimated to be between 1.2 and 4.8 pounds of wilted green leaves.
A potentially greater concern associated with black cherry trees lies in their being a favorite food source for Eastern tent caterpillars. These caterpillars pose a very significant risk to pregnant mares and can cause late-term foal losses, early- and late-term fetal losses, and weak foals. If these trees exist anywhere near fields used for breeding mares, the safest thing to do is remove them.
Many other trees families outside of the Prunus family pose a risk in the fall, including maples, especially red maple. If maple branches blow down into pastures and horses have access to wilting and dying maple leaves, they’re at serious risk of intoxication. Consumption causes methemoglobinemia (elevated hemoglobin levels) and hemolytic anemia (red blood cell destruction). While you can find lists of trees that are dangerous to horses, it can be challenging to determine if they exist on your property.
If you are unsure whether plants on your property pose a risk to horses, contact a qualified botanist or extension agent, and have them come out to your property to identify all the trees near the horses. You can then send this information to a veterinary toxicologist to see if any of the identified plants are problematic. Most university veterinary diagnostic labs have a toxicologist on staff that should be able to help.