Red Maple Trees Toxic to Horses


by Erin Pittman

This article expired on February 27, 2011 and its contents should be considered out-of-date. It is maintained here for archival purposes only.

February 27, 2010

Red Maple Leaves

Some of the most beautiful trees around are red maples.  They offer vibrant red color in the fall as well as great shade from the hot sun.  But don't be fooled!  Red Maple trees, Acer rubrum, are extremely toxic to horses.  Horses seem to be able to safely munch on fresh green leaves right off the tree, but when they fall, either due to storm damage or autumn, it's a much different story.

Red maple trees contain an unknown toxin that is lethal to horses after ingestion of only 4-5 pounds of wilted or dried leaves (it is important to note that fresh leaves do not appear to be harmful).  In a 1999 study by Beasley and others, ponies fed 3 g dried maple leaves per kilogram of body weight (equivalent to approximately 2 pounds of leaves for a 660-pound pony) got sick and died within just 5 days. When dried leaves were given after September 15th as opposed to earlier, the time to death was shortened suggesting that the toxicity of the leaves is higher in the fall than at other times. Not all horses that ingest the leaves die, though the approximately 50 to 75% of horses suffering from toxicity die or are euthanized.  If a horse eats as little as a pound and a half of leaves, signs of toxicity can occur, namely massive destruction of red blood cells.  This destruction causes difficulty in breathing, lethargy, colic-like symptoms, jaundice and dark brown urine. If you notice any of these signs in your horse, you need to get immediate veterinary attention to offer supportive therapy for the suffering horse.  Reports of toxicity due to red maple are relatively common in Maryland, sometimes due to accidental ingestion of leaves along with hay placed on the ground under a red maple tree in the fall, consumption of windfall by horses after a storm, or even from leaves blowing in to a pasture from a neighbor's property! 

So what should you do?  Certainly, don't intentionally plant red maple trees or hybrids within reach of horses or in horse paddocks.  While toxicities due to other varieties of maples have not been firmly established, there is some evidence that sugar maples may also be toxic, and it is known that red maple can hybridize with silver maples so those hybrids aren't safe either.  You don't necessarily have to cut down your red maple trees, but you should be very careful to clean up downed limbs and fallen leaves before horses have any access to them, especially in the fall.  Another thing to keep in mind is that a well-fed horse with plenty of good quality hay or pasture to eat is unlikely to browse around for tree leaves, so make sure your horses have access to forage at all times.  One thing you might consider doing if you don't want horses to have access to the trees is to build a tree fence around any trees in the pasture (see photo below).  This keeps bored horses from gnawing on potentially dangerous trees.

Fenced tree in horse pasture

If you are unsure as to the type of maple trees already existing in or near a horse paddock, one distinctive characteristic of a red maple is the leaf itself.  The edges of red maple leaves are serrated or jagged, while those of sugar maples are smooth.  Silver maples also have a serrated edge, but the silver maple leaf is much more heavily lobed than that of a red maple.  The underside of the red maple leaf is silver in color and the petiole or stem is a bright red even in the summer months.  See the photo at the top of this article to help you in your identification.

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For more information, contact: Erin Pittman