Is it true that the leaves of the red maple tree are poisonous to horses? If so, what are the signs? We have noticed several in our pasture now that it’s fall and are concerned. LL, Vermont

Dear LL,

Yes, the leaves and bark of the red maple (Acer rubrum) are toxic to horses, although the toxic principle itself is still unknown. What we do know is that horses that eat wilted or dried leaves or bark of this tree develop severe anemia due to damage to their red blood cells.

Signs of red maple poisoning include:
• Weakness
• Depression
• Feed refusal
• Abdominal discomfort (colic)
• Laminitis
• Pale or dark mucous membranes
• Dark brown urine
• Abortion in pregnant mares

Call your veterinarian immediately if you suspect your horse has eaten any part of this tree. Death can occur as quickly as 18 hours after ingestion or up to 7 – 10 days afterwards. Fluids, oxygen and blood transfusions may be helpful if administered early.

The red maple grows in the entire half of the eastern United States (and Canada), as far west as Minnesota and Texas. The best thing you can do to protect your horse from this toxic tree is to remove it from your pastures and make sure no leaves or branches fall into the pasture or are within reach of the horses. Because horses tend to nibble on potentially poisonous trees and other plants when they’re hungry, make sure your pastures are nutritious and you supplement with hay before the grass loses its nutritional value.

In researching this answer, I used two valuable resources every horse owner should have:

1. Horse Owner’s Field Guide to Toxic Plants, by Sandra Burger and Anthony Knight from Breakthrough Publications

2. ASPCA Animal Poison Control Center