Wilted or dried red maple leaves are extremely dangerous to horses. They contain concentrated amounts of gallic acid and other identified toxins that cause Heinz body anemia, a breakdown of the red blood cells that prevents them from carrying oxygen. The kidneys, liver and other organs may also be damaged. As little as a pound or two of leaves can be fatal.

Red maples are so common in the United States that removing them from your property is not practical or even advisable. But as plentiful as the trees are, red maple poisoning is relatively rare–mainly because most healthy, well fed horses will not eat wilted tree leaves in sufficient quantities to cause a problem.

Nevertheless, it is wise to take precautions to keep dangerous leaves away from your horses:

• Identify all red maples on your property, especially those in or near turnout areas. You don’t need to remove them, but be aware of the risky areas. 
• Check for and immediately remove fallen limbs after summer storms. It’s a common misconception that only leaves that wilt and fall in autumn are toxic, but green leaves that wilt on fallen branches present a very serious threat. Move horses from pastures with fallen branches until they can be taken away. Also remove fallen autumn leaves from the turnout areas. If the fallen leaves are especially thick, move horses to a different pasture for the season. 
• Do not allow your horse to browse on leaves while on the trail, and avoid tying horses near red maples when camping. 
• Monitor horses in dry lots. Horses with ample gracing or free-choice hay are less likely to browse on leaves, but those being kept on limited rations for health reasons may be more likely to reach over fences or eat anything that blows or falls into their turnouts. 
• Avoid planting new maple trees, of any variety, in or near turnout areas. Silver and sugar maples as well as various crosses and hybrids may also contain toxins but are rarely if ever involved in reported toxicities. Norway maples may be safer around horses but are considered an invasive species in the United States.
• Tell neighbors not to throw yard waste onto your property. Well-intentioned people may think they are giving your horses treats. 
• Be sure horses do not have access to areas where tree trimming or logging is going on.