Research update: Selenium toxicity

Farm Forum

Horses are very susceptible to chronic selenium toxicity (selenosis) if grazed on forages grown on high selenium soils for prolonged periods. Selenium toxicity from soils is mostly an issue in the Western U.S.; however, horses on multiple supplements containing selenium can be affected by selenium toxicity. Clinical signs of selenium toxicity include dysplastic or corrugated hoof lesions (see photo), dull hair coat, mane and tail alopecia, and varying lameness with severe cases resulting in untreatable hoof necrosis and sloughing resulting in euthanasia. In this study, conducted by researchers in Utah, mane and tail samples from horses that exhibited classical hoof lesions of chronic selenosis were analyzed.

The horses had grazed for 6 months, from approximately May 15 until November 15, each year for three grazing seasons in a pasture containing forage and water sources with elevated selenium concentrations.

The segmented hair samples showed a cyclic pattern in selenium concentrations in the mane and tail, which corresponded to entering and exiting the contaminated pasture. The selenium concentration in the tail of one horse could be traced for three grazing seasons.

These results demonstrate that in some cases, hair samples can be used to determine selenium exposure in horses for up to 3 years post‐exposure. For more information, visit

Summarized by Krishona Martinson, PhD, Univ. of Minn.