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Boxelder seeds cause seasonal pasture myopathy in horses

Quick facts

  • Horses that eat enough boxelder seeds can develop seasonal pasture myopathy (SPM).
  • SPM is a muscle disease usually fatal in horses.
  • Signs of SPM include stiffness, dark urine and difficulty walking.
  • You can prevent SPM by not overgrazing pastures, trimming or removing boxelder trees and limiting grazing during high risk periods.

Identifying boxelder trees

Boxelder seeds in the fall
Boxelder seeds in the fall


  • Usually consists of three to seven leaflets attached to a common stem.

  • Oval-shaped that comes to a pointed end.

  • Teethed.

  • Leaves drop in the fall.


  • Winged similar to other maples.

  • Grouped in clusters.

  • Seeds drop between September and March.

boxelder tree with seed pods
Boxelder tree with seed pods



Author: Anna Renier, DVM, formerly with University of Minnesota 

Reviewed in 2021

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