Question: Can you clarify exactly when red maple trees are most toxic to horses? Is it in 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 kinds of maple trees on their land. As pretty as the fall colors are, I always worry about red maple leaves blowing into our pasture at this time of year.
Answer: 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.
Green leaves
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 die.
Autumn leaves
Likewise, leaves that naturally fall from red maple trees during the autumn can be very toxic to horses. Therefore they 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
Our expert
Anthony P. Knight, BVSc, MS, DACVIM, was born and raised in Kenya and received his veterinary degree at the University of Nairobi in 1968 and an MS from Colorado State University (CSU) in 1971 before accepting a faculty position in large animal medicine in CSU’s College of Veterinary Medicine & Biomedical Sciences. After 40 years as a CSU faculty member, he retired and moved to Tucson, Arizona. His interests lie in disease investigation, foreign animal disease, zoonotic diseases and plant toxicology. He has published two books on plant poisoning: A Guide to Plant Poisoning of Animals in North America and A Guide to Poisonous House and Garden Plants.
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