Some trees can kill horses, and horses can kill trees; make sure the equine-arbor relationship on your farm is a positive one.

Carol Elster McCleary’s 4-year-old gelding, Pico, remained in the barn one Friday evening in October 2011 while his three pasturemates—his dam and another mare and gelding—wandered out to graze. The pinto fox trotter was eating and drinking normally, but McCleary thought it was odd that he did not join the others. The next morning, with a sinking feeling in her stomach, she found Pico lying in the pasture, but he got up when she approached.

McCleary, who lives in Shorewood, Minnesota, contacted her veterinarian, who treated Pico for colic that morning and again in the evening when his condition did not improve. Sunday morning, McCleary again called the veterinarian, who this time suggested Pico had developed laminitis. But when McCleary saw blood in Pico’s urine, she immediately knew something else was going on and took the horse to the University of Minnesota veterinary hospital, 90 minutes away. 

Shortly after arrival and admission, her beloved Pico, whom she had owned since he was born, went down in the stall. Try as he might, he never rose again. The clinicians tested and diagnosed him with pasture myopathy. Something in his turnout area at home had poisoned him.

McCleary took her other horses off the pasture, had them tested for pasture myopathy with negative results, and asked Stephanie Valberg, PhD, DVM, director of the University of Minnesota Equine Center, to investigate her property to find the toxic culprit. Several months later, Valberg discovered the cause: box elder seeds.  

“I removed all box elder trees in and near my pasture to the tune of $10,000,” says McCleary. She also had the pasture vacuumed to remove any seeds. It was not until the following June that she put her horses back in the pasture.  

McCleary says fellow horse owners need to be aware of the dangers of toxic trees, warning that poisoning from box elder seeds “is a terrible death.”

Here our sources will describe toxic trees and shrubs to avoid so none of your horses end up like Pico.

Different Toxins, Different Signs

Toxic trees and shrubs cause a variety of clinical signs when horses ingest them. Some are respiratory, some cardiac, some neurologic, and some gastrointestinal, says Carey Williams, PhD, associate professor at Rutgers’ Equine Science Center, in New Brunswick, New Jersey. If an owner sees any signs of trouble, he or she needs to call a veterinarian immediately.

“Some toxicities are severe enough that, even if caught early, it might be too late by the time the veterinarian arrives,” says Williams. Other toxicities cause problems that don’t lead to death.

The degree of danger a poisonous plant poses is a function of the plant’s prevalence, toxicity, and palatability. Palatability is the key, though, says Williams. Some horses might find one plant more palatable than others, consume more of it, and then get sick. 

If good-quality forage is plentiful, horses will avoid most poisonous plants because they are unpalatable and have a bitter taste and/or smell. But during periods of drought or when pastures are overgrazed, animals might begin to investigate the undesirable plants. Other horses will simply nibble on anything. 

Some of the Nastier Ones

So, what trees and shrubs are toxic, and what do they do to a horse? Let’s look at 10, starting with the most poisonous.

Japanese yew “It is an ornamental shrub that can kill a full-grown horse with just a mouthful,” says Williams. “Only (consuming) 0.1% of their body weight is necessary (for poisoning). This will cause respiratory and cardiac collapse typically too quickly for the owner to even call a veterinarian.”

Red maple “Any of the species of maple trees is toxic (box elders are in the maple family, for instance), but the red maple is the most toxic,” says Williams. “When those three-lobed leaves wilt, they become very sweet but also very toxic, so horses will crave them even if they have other grass available.

“The most important time to watch for problems is after a storm that might have caused a limb of the tree … to fall into the pasture,” she continues. “Those wilting leaves are most toxic then. Only 1.5 pounds of leaves are necessary to cause symptoms of depression, lethargy, increased respiratory rate and heart rate, which will progress into coma and death.”

Leaves still are slightly toxic in the fall, she says, but most of them are already wilted and dead when they fall off the trees. Once the leaves die, they are neither as toxic nor as palatable to horses.  

Wild cherry This tree’s leaves, seeds, and bark are all toxic. Williams says only about 2.5 pounds of leaves are necessary to cause toxicity in horses. Cherry leaves contain a precursor to cyanide that binds and interferes with the cellular use of oxygen. If an animal consumes a large enough amount of this toxin, he will basically suffocate. Poisoning is difficult to catch in time to treat because signs come on so rapidly.  

Black locust Michigan State University Extension reports that this tree grows throughout the United States and Canada, and consumption of certain tree parts can be deadly to horses. Black locust seeds, leaves, bark, and twigs—whether fresh or dried—contain toxic proteins that affect the gastrointestinal tract as well as the nervous system. Just eight ounces of the leaves or bark can kill a 1,000-pound horse. Clinical signs begin as soon as one hour after consumption and include paralysis, abdominal pain, diarrhea, and abnormal heart rate and/or rhythm. Death can occur within a few days. 

Black walnut Horses suffer this tree’s toxic effects not by consuming parts of the tree itself, but when bedded or standing on black walnut shavings or sawdust. That dangerous contact with the tree’s toxin can cause depression, lethargy, and laminitis. Clinical signs usually appear in a matter of hours, and it only takes a few shavings and a short exposure time to cause the toxicity.

Oak These trees are moderately toxic. New leaves and young acorns, especially when green, are the most dangerous parts. The acorns’ tannic acid can cause portions of the intestinal lining to slough, says Williams. Clinical signs of oak toxicity include poor appetite, weight loss, and impaction, followed by diarrhea, kidney failure, and edema. In severe cases, oak poisoning can be fatal.  

Boxwood shrubs This dense evergreen plant comes in several varieties, but all types and parts are toxic. Horses that consume boxwood can suffer neurologic signs, colic, and potentially even respiratory failure, says Williams.

Horse chestnuts and buckeyes The horse chestnut tree and the buckeye appear similar and are in the same genus, but the horse chestnut is not as hardy or tall and features leaves divided into seven smaller leaflets. Their seeds are said to resemble a buck’s eye, and their leaves are green in summer, turning golden and orange in the fall.  

These trees are moderately to highly toxic in horses, says Williams. Poisonous parts include leaves, seeds, and young sprouts. Poisoning is more common in spring due to early sprouting, and clinical signs of poisoning include gastrointestinal irritation and neurologic signs.  

Holly tree or shrub The American holly, recognized by its evergreen prickly leaves and red berries, is moderately toxic to horses. Clinical signs after ingestion can include digestive upset or colic, leading to tremors or seizures in severe cases, says Williams. Toxic parts include but are not limited to the berries.  

Other potentially lethal trees and shrubs include the golden chain tree (Laburnum anagyroides), chokecherry (Prunus spp), Kentucky coffee tree (Gymnocladus dioicus), Russian olive (Elaeagnus angustifolia), and persimmon (Diospyros virginiana).

Incorporating Trees on Your Farm Safely

Before you go chopping down every tree in your horse’s field, know that having a horse farm with plenty of shade-giving, aesthetically pleasing, safe trees is possible. There are many nontoxic trees for farms—too many to list, says Williams.

First, eliminate any toxic trees and shrubs, or at least the most lethal ones, like seed-bearing female box elders. If this is not possible, avoid turning horses out in fields containing these trees (or with them nearby) during the fall or early spring when seed burdens are high. After you have cut down toxic trees, dispose of them properly (e.g., not within horses’ reach). 

Next, realize that just because the trees were cut down does not mean they are gone for good. All hardwood trees, including oak and maple, will sprout from the stump. Some, such as black locust, will sprout from the lateral roots (which leave the tree at the ground line), says Jeffrey Stringer, Extension specialist in the University of Kentucky’s forestry department. 

So what are you do? Grind out the stumps or use herbicides, he says. Typically, the latter involves applying 80-100% strength herbicides to the freshly cut stump. For large stumps, you only need to spray the 3 to 5 inches from the inside edge of the bark toward the center. 

Fence Off and Monitor

If you want to keep the problem trees on your property, use strategic fencing and monitor trees to ensure branches and leaves do not fall in the pastures. The distance you fence around the tree is primarily based on what part of the plant is problematic, says Stringer. If the bark is the problem and not the leaves, place the fence four to five feet from the tree trunk.

However, the problematic components of most toxic trees are the leaves and seeds. From that standpoint, horses should be kept—at a minimum—from under the tree’s drip line (the outermost circumference of its canopy, which inevitably gets larger as the tree grows). 

While windblown leaves that fall to the ground might float into pastures and paddocks, these dried leaves often are not problematic. It’s avoiding the green or wilting stages of leaves that is critical. By keeping horses from under the drip line, you help ensure that they can’t easily access broken limbs that hold wilting leaves. 

Educate Yourself

Many Extension offices offer tree and shrub guides. If you need help identifying a tree, bring samples to your local Extension office, or check out this online tree identification tool from Virginia Tech: dendro.cnre.vt.edu/dendrology/idit.htm. Further, the University of Idaho has provided a database of plants toxic to equids, authored by Genyce (Gen) Ahmann Hanson,  that you can access here: webpages.uidaho.edu/range/toxicplants_horses/Toxic%20Plant%20Database.html.