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SESK: Stuff Everyone Should Know · sesk.info/horse-bracken-fern-thiamine

Bracken fern harms horses by breaking down which vitamin?

Thiamine (vitamin B1), through the enzyme thiaminase.

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  1. Vitamin C
  2. Vitamin A
  3. Thiamine (B1) ✔

Why: Bracken fern contains thiaminase, an enzyme that destroys thiamine. A horse that eats it for a while can become weak and lose weight. Remove it from pastures and hay.

Huh, didn't know that: The ASPCA lists weakness, weight loss, staggers and tremors as signs.

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References

Answer: Thiamine (B1). Bracken fern contains thiaminase, an enzyme that destroys thiamine. A horse that eats it for a while can become weak and lose weight. Remove it from pastures and hay.

  1. Bracken Fern, ASPCA Toxic and Non-Toxic Plants www.aspca.org/pet-care/animal-poison-control/toxic-and-non-toxic-plants/bracken-fern
    Thiamine deficiency: weakness, weight loss, staggers, tremors, death
    Checked 2026-10-10.

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