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University of Minnesota Extension

Oak buds and green acorns can harm horses

Quick facts

  • 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.



  • 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


  • Nuts with tough leathery shells

  • Mature in one or two seasons

Oak leaves
Oak leaves
Green acorns
Green acorns


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.



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.

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

Reviewed in 2021

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