Written by Walt Friedrich
Most of us horse-owners are quite well aware that our horses can pretty much take care of themselves. They may be prey animals, but so are field mice, along with thousands of other species in between, and of them all, horses are probably one of the best equipped for self-protection. Weighing in typically at a thousand-pounds plus or minus a few hundred helps, along with immense strength, and maybe the quickest reflexes in the animal kingdom; he can break your leg or even kill you with a well-placed kick, and he has enough speed to outrun a hungry mountain lion. He tends to keep himself quite well self-protected.
But even with all his natural capabilities and protection, he's subject to some significant dangers from which he can't easily escape, such as infections from ticks and flies and other insects, as well as bites from snakes, critters he can't do much about. He's even subject to potentially fatal attacks from plants and trees, which we will address here. These threaten him from several directions: with some it's what he may eat, with others it's what he lies on or even just walks on! As for plants, the full list of threats is indeed long, and they can grow almost anywhere. And as for toxic trees, fortunately, the list is actually rather short, with only a few species being severely dangerous – which are:
Red Maple Tree (and its hybrids and variants) – a.k.a. Acer rubrum
Black Walnut Tree – a.k.a. Juglans nigra
Oak Tree – a.k.a. Quercus spp.
Wild Cherry Tree – a.k.a. Prunus
Let's have a look at these four; what follows is a compilation taken from the writings of experts in the fields involved. Augmentation of these findings is readily available online:
Red Maple Trees
It, and the Black Walnut, are probably the most dangerous of poisonous trees; they're the Bonnie and Clyde of the forest. A Red Maple tree is one of the most spectacular trees to behold in the fall. The deep crimson leaves are beautiful, but they are also toxic, particularly to horses. The leaves while alive and on the trees are not poisonous, but once they fall off the tree and wilt, they become deadly. Horses most often become exposed as the leaves fall from the trees in autumn, when a branch is blown off of a tree and into a pasture by a storm, and the leaves wilt on the broken branch. The toxin present in these wilted leaves is unidentified at this point in time; despite that, we know very well the damage it can cause. As little as a pound or two of leaves can be fatal.
Once ingested by the horse, the toxin begins to destroy the horse's red blood cells. Mass loss of red blood cells results in severe anemia developing. This destruction of red blood cells causes other problems as well; once a red blood cell is destroyed, the hemoglobin that had been carried by the cell is free in the bloodstream and the kidneys filter it out – but the kidneys are damaged in the process. Thus, horses poisoned by the Red Maple toxin are battling severe anemia and kidney disease at the same time!
Depending on how many leaves were eaten, signs can appear within a few hours or as long as four or five days after consumption. Signs include lethargy; refusal to eat; dark red-brown or black urine; pale yellowish gums and mucous membranes at first, advancing to dark brown. In addition, as his body begins to starve for oxygen molecules, his heart and respiratory rates rise significantly.
This horse must be promptly treated, but has a poor prognosis for survival. The only treatment is the administration of large amounts of intravenous fluids to flush his kidneys in an effort to keep them working; he might require oxygen, and if his anemia is severe he might even require blood transfusions. Thus, recovery depends on how many leaves were consumed and how promptly the horse receives care.
Of course, the best treatment is prevention. Once a Red Maple is identified near the horse's normal living area, take steps to ensure that he is not exposed to the wilted leaves; remove storm-blown branches from paddocks and pastures immediately. Given the serious danger of these poisonous leaves becoming available to the horse, it is prudent to actually remove the tree.
The Red Maple is found throughout the eastern United States and southeastern Canada. It is a tall hardwood with green leaves that have three large "fingers" or points and five prominent veins in the leaves. The leaves of this tree turn a brilliant crimson and sometimes yellow in the fall.
Special note: Research indicates that the leaves of at least two related species--the Silver and Sugar Maples--may contain the same toxic elements as Red Maples, but in less toxic amounts.
Black Walnut Tree
Toxins are usually concentrated in leaves, barks and nuts of poisonous trees, but in the case of the Black Walnut, the wood itself is believed to contain the toxin. Exposure is mainly through wood shavings used as bedding, and chips laid outside the stable to protect against the mud in wet weather. Two main paths of infection; a double whammy, and exposure to either can result in laminitis.
After exposure to Black Walnut shavings or chips, a horse can begin to show the hallmark signs of laminitis within a matter of days: the lower legs begin to swell, and the horse will be reluctant to move; other signs include shifting his weight from one front foot to the other, and rocking his weight backward – he will try to bear most of his weight on his heels. Look for a strong digital pulse and heat in his hoof. Call your vet if you see these signs of laminitis. If you suspect that your bedding is contaminated with Black Walnut wood shavings, get your horse out of there, and remove the bedding promptly. Laminitis is painful and serious, but with prompt proper treatment horses usually recover, though it takes time.
To be safe, horses should never be allowed direct access to Black Walnut trees. Remove any that grow in the horse's pasture and outside the fence.
The Black Walnut tree is tall, reaching as much as 100 feet. It's a common tree, ranging across the eastern U.S. and west to the Rockies. Its bark is dark brown, and its dark green leaves are compound and long, with serrated edges; the ground around the tree is usually covered with the tree's walnuts, still wrapped in their green husks.
Acorns, buds, leaves, and blossoms of Oak Trees are toxic to all livestock, although Oak poisoning is not very common in horses. The acorns, leaves, and blossoms contain tannins, which when digested are converted into toxic products of metabolism inside the animal's gastrointestinal tract. An indication of Oak poisoning in a horse is the onset of colic and bloody diarrhea. The horse's kidneys are also damaged by the toxic metabolites of Oak poisoning.
Treatment of Oak poisoning is aimed at maintaining hydration with intravenous fluids and protecting kidney function, much as with Black Walnut infection. It takes a large quantity of acorns to make a horse become ill. As with all poisonous trees, prevention is much preferred over treatment--remove Oak trees from your pastures or paddocks.
Wild Cherry Trees
Wild Cherry Trees' leaves and twigs contain prunasin, a cyanide known as prussic acid that when ingested, can be fatal. The poison becomes a threat when the leaves are exposed to stress that causes them to wilt.
Horses are poisoned by eating the leaves or seed pits. Once the plant material is chewed and exposed to the horse's stomach acid, hydrogen cyanide is released and rapidly absorbed into the horse's bloodstream. Cyanide works as a poison by preventing normal cellular uptake of oxygen. As a result, an affected horse's blood is bright cherry red because it is overloaded with oxygen that cannot be utilized by the horse's cells. Symptoms include difficulty breathing, weakness, excitement, dilated pupils, convulsions, coma, respiratory failure.
Horses with cyanide poisoning are usually found breathing heavily with flared nostrils. Their respiratory rates and heart rates are elevated. These two clinical signs along with the presence of bright red blood often lead to the diagnosis of cyanide poisoning. Tissue samples should be taken immediately and tested for the presence of cyanide. If found in time, the affected horse can receive treatment that will remove cyanide that is already bound to the red blood cells, as well as replenish the affected horse's body's store of compounds that naturally bind cyanide and render it harmless.
Wild Cherry trees are the largest type of cherry tree, capable of growing to 100 feet with trunks three feet thick. Valued for landscape appeal and strong, decorative wood used in manufacturing fine furniture, the trees bear fruit that's inedible to humans except when it's used in cooking to provide cherry flavorings. Their bright green leaves turn yellow to red in the fall. Wild Cherry trees are found throughout North America.
Your horse needs you to look out for sneaky dangers like poisonous trees and plants. Those we've covered above represent by far the greatest poisonous-tree dangers in the U.S., but your horse won't hesitate to eat some tasty-looking leaves no matter what tree contributed them – it falls to you, his owner, to look out for him. But just spotting such a danger isn't enough – it must be eliminated for your horse's safety. So please don't hesitate – when you see a tree we've discussed above and it's near enough to your horses that they can reach the leaves, go break out your trusty chain saw and make yourself some firewood. Stack up the results so they're out of reach of your horse, and clean up the work area when you're done so there's no dangerous “junk” laying about. Once the bad stuff is gone, it's obviously forever, and your horse is so much safer.
While the trees discussed here are the most common poisonous trees in the United States, there are other, less common, trees, such as Black Locust, that can cause problems in horses. However, there are far more poisonous plants in our environment than poisonous trees. If you are unfamiliar with the plants in your area or are unfamiliar with your horse's habitat, check with your veterinarian and your local cooperative extension agent to see if any plants might pose a problem. Also, you can find a rather lengthy list of toxic plants and trees at