As praiseworthy as trees are, there are a few situations where horses and trees don’t mix. In some cases, fruit- or nut-bearing trees contribute to colics when horses gorge on their produce. In others, falling branches or uprooted trees injure nearby horses. But the gravest dangers arise with the few tree species that are toxic enough to sicken or kill horses.

Of the non-ornamental native trees, the most deserving of the skull-and-crossbones warning are those that produce cyanide in their wilted leaves. Cyanide suffocates animals by blocking oxygen transport via the red blood cells. The red maple (Acer rubrum) is one such tree whose leaves are harmless most of the year until wind damage or seasonal change causes them to fall from the tree and wilt. Red maple leaves have serrated edges and can turn either red or yellow in the fall. “There are other trees that shed red leaves in the fall, but the red maple has some distinctive features,” says Anthony Knight, BVSc, MRCVS, who specializes in toxic trees and plants at Colorado State University. “The underside of the red maple leaf tends to be silvery in color.” Signs of poisoning, including lethargy, discolored urine and darkened gums, may not appear for four days.

Equally toxic are cherry (black cherry, chokecherry, and fire cherry) peach and plum trees, all members of the Prunus species. These leaves also produce cyanide when wilted, affecting horses within a few hours of ingestion.

To be safe, remove these deadly trees or relocate horses away from pastures or paddocks bordered by or containing them. In general, horses are not likely to eat leaves or any other tree parts unless they are quite hungry. However, when curiosity or boredom spurs exploratory bites, the horse may ingest enough of the deadlier species to do harm.

The following trees have no place in horsekeeping areas because of their toxicity or potential for causing digestive distress. They are listed in order of the risk they pose to horses, starting with the most hazardous:

Yew (taxus sp.)Oleander (nerium oleander)Red Maple (Acer rubrum)Cherry trees and relatives (prunus sp.)Red Maple (Acer rubrum)Cherry trees and relatives (prunus sp.)Black Walnut (juglans nigra)Black Locust (robinia pseudoacacia)Horse Chestnut, Buckeyes (aesculus hippocastanum)Oak trees, acorns(quercus sp.)Russian olive, also known as oleaster (elaegnus angustifolia)

For more information on toxic trees, including detailed descriptions and photographs, visit the Colorado State University website.

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