The wilted or dry leaves of the red maple (Acer rubrum) are toxic to horses — an unidentified toxin with oxidant properties is present in wilted or dried leaves. Only the red maple, and possibly closely related hybrids, are known to be toxic. The toxin in red maples oxidizes hemoglobin with the formation of Heinz bodies, methemoglobinemia, and subsequent hemolytic anemia.
Poisoning is especially likely in the fall or following a storm when leaves of fallen branches become accessible to horses. The fresh green leaves apparently are not toxic, but once dried, they may remain toxic for up to 30 days. The bark from red maple trees is also toxic. Fatal poisoning of ponies that were fed 3.0 kilograms of dried red maple leaves occurred in one to five days. Doses of half this amount will induce formation of Heinz bodies.
For more plants that are toxic to horses, you can visit our eXtension Pasture Management Learning Lesson at http://campus.extension.org/mod/book/view.php?id=131, or for an even more complete list, visit Cornell’s poisonous plant website at http://www.ansci.cornell.edu/plants/php/plants.php?action=indiv&byname=c....
