Farming Magazine - August, 2014


Working Horses: Reducing the Risk of Red Maple Poisoning

By Vicki Schmidt

Red maples are known for their red buds in the spring and vibrant red leaves in the fall. The palm-shaped leaves with distinctively serrated edges are easy to identify.
Photos by Vicki Schmidt.

Horse owners in the Northeast may not pay much attention to predictions of increased storm events from climate change. However, storms and high winds should be of major concern if your farm and pastures have red maple trees.

Red maples (Acer rubrum) are common in both dry and moist soils in the northeastern U.S. and Canada. Red maples, often called swamp or soft maple, burst with rounded scarlet buds and bright red flowers in the spring. The red maple also provides one of the most vibrant shades of red in the tapestry of fall colors.

In the wilted stage, the leaf of the red maple produces gallic acid, which is found in many other plants, including sumac, witch hazel, tea leaves and oak bark. Gallic acid is extremely toxic to horses. When a horse ingests wilted or dry red maple leaves, gallic acid is released into its system. In a horse, gallic acid acts as an oxidant and damages red blood cells, causing acute hemolysis. Hemolysis is the breakdown of red blood cells, which inhibits the transport of oxygen. With red maple poisoning, the blood cannot release oxygen to the body tissues. A few leaves can cause significant poisoning, while consuming more than a small branch will easily kill a horse.

Indications of red maple poisoning occur within one or two days after a horse consumes the leaves. Symptoms include abortion in pregnant mares, blue to brownish mucous membranes, depression, colic, rapid pulse, lethargy, deep and labored breathing, increased heart rate, discolored blood, jaundice, dark brown urine and laminitis.

Storms and high winds often blow red maple leaves and branches into areas grazed by horses. The browsing nature of horses makes these leaves a treat, but they must be avoided. Only a few small leaves could easily kill a foal.

Prognosis is always guarded to poor, with most horses dying within 18 to 24 hours of ingestion. Treatments are limited, costly and tragically delayed in rural or remote areas. Veterinarian Karilyn Bonney from the Annabessacook Veterinary Clinic in Monmouth, Maine, feels that education has made a difference when it comes to equine red maple poisoning deaths. Bonney reported that the clinic saw several cases every year decades ago, but continuous outreach to clients has helped to significantly reduce the number of suspected red maple poisonings.

Tips to reduce risk

Gallic acid has also been found in silver maple and sugar maple leaves, and it is suspected that sugar maple leaves will produce the toxin in their wilted stage. Red maple can also hybridize with other maples, including sugar and silver, creating types of trees that increase risk. For these reasons, all maples should be avoided near horses, pastures and hayfields.

The amount of gallic acid increases in leaves during the summer, with most poisonings occurring in the late summer and autumn, when leaves fall into pastures. Horses are browsers as much as grazers, and the leaves are palatable to them. This attraction increases the risk of poisoning. Fallen limbs and leaf clusters, which occur after high winds and storm events, should be removed immediately.

Take-home message

Do not allow horses access to maple trees, especially red maples. Work to eradicate red maples from your pastures, as well as areas adjacent to pastures and hayfields. Do not incorporate red maple leaves into hay bales, as the dried leaves remain toxic for well over a month.

Vicki Schmidt owns and operates Troika Drafts, a 100-acre working draft horse farm in western Maine. The farm features drafts and crosses for work, sport and show. Comment or question? Visit and join in the discussions.