Oak buds and green acorns can harm horses

  • Oaks are found in nearly all upland hardwood forests.

  • Horses must eat a lot of oak buds or green acorns before showing symptoms of poisoning.

  • Feed refusal, constipation and frequent urination are all signs of poisoning.

  • Keep oak branches out of reach of the horses and fence off areas plentiful with green acorns.

Identifying 

Leaves

  • Drop in the fall

  • Cluster at the ends of twigs

  • Dead leaves often remain on the tree over winter

  • Red oaks have pointed leaves with bristle-tipped lobes

  • White oaks have round lobes or large regular teeth

Oak leaves
Oak leaves

Acorns

  • Nuts with tough leathery shells

  • Mature in one or two seasons

Green acorns
Green acorns
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Toxicity

Newly emerging oak leaves or buds
Newly emerging oak leaves or buds

Oak buds in the spring and green acorn hulls in the fall cause oak poisoning in horses. Oak toxins are highest in:

  • Immature leaves

  • Members of the black and red oak species

  • Immature acorns

The chemical toxins tannin and gallotannin are likely responsible for oak poisoning in horses. But researchers haven’t identified all the toxins in oak.

Animals must eat large amounts of oak buds or green acorns for a few days to a week before showing signs of poisoning.

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Control

You don’t need to cut down oak trees in your horses pastures. Keep the branches trimmed and out of reach of horses. Fence around young or small oaks for protection. Fence horses out of areas where green acorns are plentiful.

Krishona Martinson, equine Extension specialist; Lynn Hovda, DVM, adjunct assistant professor, College of Veterinary Medicine; Mike Murphy, former professor, College of Veterinary Medicine; Patrick Weicherding, former Extension educator.

Reviewed in 2018

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