Trees in Horse Pastures

By Laura Kenny

Trees are a beautiful source of shade in horse pastures, providing a lovely, picturesque landscape. Most of the time, trees pose no risks to horses in pastures. However, trees can cause problems in pastures, including toxicity to horses, injury to trees, and broken fences from fallen branches.

Toxic Trees

There are numerous trees that can be toxic to horses. They each have different levels of toxicity, and some specific parts of the plant can be more toxic than others (i.e. leaves, acorns), depending on the species.

• Red maple (Acer rubrum). The wilted leaves of this tree are highly toxic, though the toxin is unknown. Even after they fall off the tree in autumn, the dried leaves can retain toxicity for up to a month. The worst-case scenario would be a branch falling into a pasture and horses eating the wilting leaves. This can result in rapid death. It is thought that hybrids of red maple (including sugar and silver maple) may also be toxic.
• Chokecherry (Prunus virginia). These trees, and other cherries like wild black cherry (Prunus serotina), contain cyanide compounds in the seeds, leaves, bark, and shoots. Toxicity is highest in both new growth and wilted leaves and branches (i.e. if a branch falls during a storm). Apricot and peach trees are in the same genus (Prunus) and are thought to contain the same toxins in the leaves and seeds.
• Horse chestnut/Buckeye (Aesculus hippocastanum/Aesculus glabra). These pretty trees, with their distinctive fruits and seeds, contain toxins in the leaves, sprouts, seeds, and seed husks. Horses may eat them because they typically leaf out earlier than other plants in the spring. Note that the horse chestnut is a different plant than the American chestnut (Castanea dentata) and other chestnuts.
• Black locust (Robinia pseudoacacia). The bark and seeds of this showy tree contain lectins that can cause gastrointestinal problems. Paired thorns at the base of each twig aid in identification of this tree. Note that honey locust (Gleditsia triacanthos) is a different plant than black locust.
• Oaks (Quercus spp.). It is not uncommon to see a large, old oak in or next to a horse pasture. Unfortunately, the leaves, bark and acorns, containing high levels of tannins, can be toxic to horses when consumed in quantity. Young leaves and flower buds and unripe green acorns are the most toxic. Many horses will leave them alone and never have a problem, but others may gorge themselves on green acorns and get sick.
• Black walnut (Juglans nigra). This tree is also common in horse pastures, and its primary danger is not from being consumed. Shavings from black walnut wood can cause laminitis when horses stand on them. As little as 20% black walnut shavings in a bedding mix can be enough to cause laminitis. If you remove a black walnut tree from a pasture, be aware of the danger from the sawdust that remains on the ground. It has also been suggested that pollen and leaves from black walnut can be toxic to horses, causing mild respiratory signs. The walnuts that fall on the ground may become moldy, which can cause health problems as well.

Other toxic trees

• Kentucky coffee tree (Gymnocladus dioicus)
• Chinaberry tree (Melia azadarach)
• Crab apple tree (Malus sylvestris) 
• Cherrylaurel (Prunus laurocerasus)
• Golden chain tree (Laburnum spp.)
• Tung nut tree (Aleurites fordii)
• Mesquite (Prosopsis spp.)
• American persimmon (Diospyros virginiana)

This is not an exhaustive list. Many ornamental trees and plants can be toxic. When researching trees to plant, make sure to look up whether they are toxic to horses. Use the sources cited at the end of this article or contact your local Extension office.

There are many clinical signs of toxicity in horses. These signs can include lethargy, lack of appetite, colic, neurological dysfunction, lack of coordination, respiratory issues, and elevated breathing and/or heart rate. Always evaluate your horse's health daily and be watchful for signs of poisoning. Contact a veterinarian immediately if you suspect your horse has eaten a toxic plant or tree.

Trees and Fencing

When planting a tree in or near a pasture, it is best not to give horses access to the tree trunk. Regardless of whether the tree is toxic, horses may eat or chew the bark and damage the tree. If the bark is removed in an entire circle around the trunk, this is called girdling and the tree may die. 

To prevent horses from girdling trees, it is best to separate them with a fence. If the tree is already in the pasture, you can circle the trunk with fencing just far enough that the horses cannot access the bark with an outstretched neck. Do not wrap trees in chicken wire or snow fencing. Horses may still eat through the wrap, causing it to loosen which could then become a hazard to their eyes, teeth, and legs.

When it comes to pasture perimeter fencing, it may seem like a good idea to place the fence close to a tree line to benefit from the shade while keeping horses from chewing the bark. However, remember that strong winds, ice storms, and other severe weather events can cause tree limbs to fall and damage the fence. No one wants their horses to get loose because a tree knocked down a section of the fence.

Some tree species are notorious for having weak wood or weak structure which leads to branches dropping easily. Fast-growing trees are also at risk of having weak wood. Some trees with these characteristics include callery/Bradford pear, silver and red maple, green ash, American sycamore/plane tree, red mulberry, Siberian/Chinese elm, boxelder, white willow, mimosa, tulip poplar, and weeping willow. Several of these trees are also invasive and are not recommended for planting.

Invasive trees are not native to an area, spread quickly, and can take over native, ecologically diverse areas rapidly. Invasive trees in Pennsylvania include: Amur maple, Norway maple, Sycamore maple, European black alder, tree-of-heaven, mimosa, Japanese angelica tree, paper mulberry, white mulberry, princess-tree, cork tree, callery/Bradford pear, bee-bee tree, and Siberian elm.

Lastly, in addition to storms breaking off branches, there is a phenomenon known as "Sudden Summer Branch Drop" in which a seemingly healthy older tree drops a large limb during calm weather. This has been reported on oak, sweet chestnut, beech, ash, poplar, elm, sycamore, willow, and horse chestnut trees, among others. It is unknown what causes this, but it appears that trees that lose a branch from sudden summer branch drop are likely to lose another. It is recommended to have large trees inspected and pruned by a professional arborist regularly. Rot diseases within the tree may also cause limbs to fall.

Conclusion

Between the potential for poisoning from toxic trees, tree death from girdling, and fence destruction from falling limbs, some may choose to place fence lines clear of trees and provide shade for your horses using a shed or other shelter. That being said, many horse farms have successfully pastured horses in fields with trees for many years. When choosing trees for horse farms, avoid species of concern listed in this article. To find an appropriate tree, check with a local nursery and inquire about trees that are well-adapted and/or native to your area and the characteristics of the spot it will be planted (wet soil, dry soil, etc.). You may also want to consider trees that aren’t considered "messy," i.e., they drop a lot of bark or seeds. Once you have chosen a few options, do some research to ensure that they will be safe for horses.