Trees in Horse Pastures

include lethargy lack of appetite colic neurological dysfunction lack of coordination respiratory issues and elevated breathing andor heart rate. Always evaluate your horses 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. Fastgrowing trees are also at risk of having weak wood. Some trees with these characteristics include calleryBradford pear silver and red maple green

ash American sycamoreplane tree red mulberry SiberianChinese 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 treeofheaven mimosa Japanese angelica tree paper mulberry white mulberry princesstree cork tree calleryBradford pear beebee 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 welladapted andor 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 arent 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.