Some horses, in fact, do more than nibble. A horse could strip an entire adult tree of bark within a day and even bite into the wood beneath, destroying the tree, our sources say.

The horse might have his reasons for chewing trees—possibly responding to physical or mental needs—but your trees shouldn’t have to pay the price, they said. Here are some reasons horses might chew trees, how they do it, and tips for protecting your trees while keeping your horse’s needs in mind.

His Salad Isn’t Satisfying Enough?

Scientists still don’t know why horses chew trees, but they have some theories. Trees—and wood in general—could complement the horse’s nutritional needs, especially for fiber when it’s lacking, said Mariette van den Berg, BAppSc (Hons), MSc, PhD (Equine Nutrition), of MB Equine Services consultancy, in Armidale, New South Wales, Australia.

“We see more tree-eating in feral populations here in Australia when there’s a drought and the grass is all dried up,” she said. “In those cases they’re certainly seeking fiber by browsing trees.”

They could also be seeking water in the trees, van den Berg added. Trees channel water up from the ground through a system of “capillaries” in the trunk, and horses could be aiming for that moisture—destroying the bark in the process. “Horses don’t always eat the bark,” she said. “Sometimes they just strip it and leave it to get to the center of the tree and bite at that.”

But even horses whose nutritional needs are being met could chew bark and branches because they want the varied texture, she said. In the wild, horses graze and browse a wide variety of plant types and species, including trees and bushes. Part of that could be similar to why humans put croutons in their salads—for the crunch. “Horses are very oral,” said van den Berg.

Another reason for tree chewing could be self-medicating, she added. Self-medication is an animal’s ability to use plant secondary compounds (PSC) or other non-nutritional substances to combat or control disease.

“There’s not much research into how horses self-medicate, but we know from other species—cattle, goats, and sheep, for example—that grazers select plants that seem to give some kind of relief for pain or a worm burden or even have a pleasure effect,” van den Berg explained. “It would make sense that horses would do that as well, but for the moment, we just don’t know.”

Although some people believe horses benefit from anti-inflammatory or pain-relieving effects of willow trees, for example, not enough science supports that idea at this time, said Paolo Baragli, DVM, PhD, researcher in the University of Pisa’s Department of Veterinary Sciences, in Italy. “It’s more likely that trees are a source of food fibers, because a lot of stable diets are poor in long fibers,” he said. “Or maybe they simply they like the smell or flavor of trees.”

The Dangers of Horse Teeth on Trees

A horse’s chewing can be disastrous for trees, said Anja Schmitz, PhD, of the Institute of Grassland Science at Georg-August University Goettingen in Germany, and Aida López-Sánchez, PhD, of the Department of Natural Systems and Resources at the Polytechnic University of Madrid in Spain. Schmitz, López-Sánchez, and their fellow researchers recently studied damage to fruit trees by grazing animals, including cattle, sheep, and horses.

Bark stripping—especially if the horse removes a full circle of bark around the trunk—can be deadly for trees. Horses can also do significant damage to tree cores with their sharp front teeth, Schmitz said.

“The most important difference between cattle and horses regarding their grazing anatomy is their dental structure,” she explained. “Cattle only have lower incisors, while horses have upper and lower incisors. Cattle use their tongues to graze grasses and forbs to a greater extent than horses. The horses’ two pairs of incisors enable them to graze selectively and bite plants very close to the ground, but it also allows them to harm and remove tree bark. So cattle aren’t able to bite the bark to the extent that horses do (although they might eventually harm the bark with their horns).”

“Horses have the dental structure and the body height to be able to expose trees to greater extent of damage than cattle (without horns), for example,” seconded López-Sánchez. “Once a horse has learned on the delicate bark of young trees, he might not forget that.”

Meeting Both Species’ Needs

The simplest and most effective way to protect trees from horse teeth is fencing them off out of horses’ reach, said van den Berg. “You can block individual trees with standard fencing or use electric fence ribbon around a row of trees,” she says. “Or you can even get creative with tree guards, using palettes or wire mesh (provided you’re watching out for entanglement risks or injury from stray wires).”

Fencing off trees only satisfies the needs of the trees, however. Preventing horses from eating trees could be denying them access to a resource they need, said van den Berg. “One nice method is what I call ‘cut and carry,’” she explained. “Take large and small branches from various trees and put those in the horses’ enclosure. That allows them to meet their tree-chewing needs—whatever those might be—on your terms and without stripping the main part of the tree.”

Some kinds of trees are toxic to horses, however, she added, so check science-based guidelines on suspected tree toxicities, specifically for horses, which are more sensitive to toxins than other grazers due partly to their single stomach. (The American Society for the Prevention of Cruelty to Animals has an extensive list of poisonous trees and plants.)

“As a general rule, I’d recommend mulberry, birch, or willow, but there are plenty more depending on where in the world you live,” van den Berg said.

Fruit trees might be a welcome treat, added Schmitz. “Fruit tree bark might be more palatable because it includes less bitter compounds or tannins, and the bark is thinner and less rough than that of oak trees, for example,” she said.

“There is some anecdotal knowledge that grazers prefer beech over oaks due to these reasons,” she added. “In Germany and elsewhere, several centuries ago, there was a tradition of feeding beech branches to stabled horses in winter when forage got sparse.”

Take-Home Message

Many horses seem to like chewing trees, although scientists still don’t have a clear understanding why. However, they’re capable of causing great destruction to trees, so handlers need to take caution in protecting trees from horse bites. To meet their tree-chewing needs, owners can ensure their horses are getting enough fiber and water and can supplement their horses with cut branches from nontoxic trees.