Q: Can you clarify exactly when red maple trees are most toxic to horses? Is it when the leaves turn red and naturally drop in the autumn, or when entire branches with green leaves fall and then wither? I’ve heard both and am a bit confused.

We don’t have any of these trees on our property, but our neighbors have several different kinds of maple trees on their land. As pretty as the fall colors are on their property, I always worry about red maple leaves blowing into our pasture this time of year.

A: You are right to be worried. Toxins in wilted and dried red maple (Acer rubrum) damage the hemoglobin in the horse’s red blood cells, leaving them unable to carry oxygen. The damaged blood cells may rupture, overwhelming the kidneys with waste products. If the horse loses red cells faster than they can be replaced, its tissues will be starved of oxygen, causing vital organs to fail. The onset, severity and duration of red maple toxicosis depend on how many wilted leaves the horse ingests, but once signs of poisoning develop, odds for survival are poor. Red maple poisoning is fatal in 60 to 70 percent of cases.

Most cases of this type of poisoning occur when horses eat dried green leaves from fallen red maple branches. After a limb or branch is blown down during a summer storm, for instance, its dried green leaves remain toxic for several weeks. Research has shown that horses ingesting dried red maple leaves at the rate of 3.0 grams/kg of their body weight—roughly one to two pounds for the average horse—will develop hemolysis and death.

Likewise, leaves that naturally fall from red maple trees during the autumn can also be very toxic to horses and, therefore, must be removed from the pasture—or the horses moved to another area free of maples. And although red maple (Acer rubrum) is most commonly associated with poisoning in horses, the wilted leaves of silver maple (Acer saccharinum) and sugar maple (Acer saccharum) are also potentially toxic to horses, so should be treated similarly. Incidentally, in a research setting, horses fed fresh green leaves from red maple trees did not become ill.

Anthony Knight, BVSc, MS, DACVIM
Professor Emeritus
Colorado State University
Fort Collins, Colorado