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Field horsetail and brackenfern: harmful plants to horses

  • Field horsetail grows in wet soils and is hard to control.

  • Brackenfern grows in open pastures and woodlands and is controlled by multiple herbicide treatments.

  • A 20 to 25 percent diet of field horsetail and brackenfern for three weeks can cause neurological problems in horses.

  • Neurologic signs include unsteady gait, twitching, depression and constipation.

  • An initial 0.5 to 1 gram dose of thiamine and decreasing daily doses can treat poisoning.

Field horsetail





Brackenfern and horsetail are toxic when horses eat it fresh (in pasture) or dried in hay. If horses eat a diet with 20 to 25 percent brackenfern or field horsetail for about three weeks, neurological signs may occur. Clinical signs may occur a week to ten days if horses eat a diet nearly 100 percent brackenfern or field horsetail.

Brackenfern contains an enzyme that causes neurological syndrome in horses. Field horsetail affects horses similarly.

Signs of poisoning

  • Depression

  • Constipation

  • Unsteady gait (usually in one to two days)

Clinical signs progress to:

  • Unsteady gait

  • Muscle twitching

  • Going down

  • Paddling

  • Seizing

These signs occur for a period of a week or more.


Horses will need an initial dose of 0.5 to 1 gram of thiamine. They will then need decreasing daily doses for three to five days.

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

Photos provided by University of Minnesota Strand Memorial Herbarium.

Reviewed in 2018

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