We had a conversation on this a few weeks back. Especially realated to loss
of mane and sore feet. Subsequently we tested horses. They were DEFICIENT!!!
Not excessive. This was a surprise as horses are in eastern Colorado. So
don't make assumptions. A vet lab can test hair (for long term effects)
and blood (for short term effects). Feed tests are important too.
Vets and extension equine specialist confirm that hair loss and sore footedness
can be from deficiency of selenium.