Question

I recently moved my horses to a new farm, complete with one pasture described as a “failed orchard." I would like to turn out my horses in this orchard, as there is a lot of grass around the trees, but I am nervous. I am told that some grafting studies may have been done with the trees, possibly pistachio and maple trees, several decades ago. How would I know what tree species I am looking at, and whether or not they may be poisonous to my horses? Can you help?

Answer

Based on the information you provided, I would not recommend turning out your horses into this orchard. Fatalities have been reported in Californian horses that consumed pistachio leaves. Whether they are true pistachios or grafted onto maple rootstock probably would not make a difference, as both trees are known to produce tannins and other toxins in the leaves that can be harmful to horses, particularly when dried or wilted.

Unfortunately, those same toxins also make the leaf more flavorful and appealing to horses, so horses will eat them even though there is plenty of good-quality forage available around the trees. While it is true that horses will have to eat more than just a few leaves to be poisoned, the fact that they are tasty will make it difficult to monitor or control consumption.

Some of the signs of poisoning include colic, dark colored urine, anemia, depression, lethargy, weakness, jaundice, and muscular incoordination.

A professional arborist would be able to help identify the types of trees in the pasture. If you are interested in finding out more about the risk of toxicity, I recommend contacting your local Cooperative Extension office. Extension personnel will have much more information on the health problems associated with pistachio consumption in horses. Local veterinarians with experience in toxicity might also be a valuable resource.