I remember reading back in April 28, 2009 where 21 polo ponies had an overdose of selenium at the U.S. Open polo championship in Florida. I will never forget how tragic
Ryan is right, selenium is an antioxidiant! If a horse miss selenium, the repercussions can be a muscle degeneration, the fragility of red blood cells, muscle aches... If you give him too much selenium, he can have an intoxication or
serious health problems (deformation and loss of the hoof, cardiorespiratory issues, loss his eyesight...)!
Selenium is an antioxidant. It has an effect on muscle and tissue and cardiac tissues so it is rather important. The quantity of selenium needed varies according the horse's activity.
From what I've seen, Selenium is an antioxidant, a quite powerful one.. It helps oxygenate muscles and get rid of lactic acid, making it rather important for the health of our horses. When they do lack selenium, they can experience muscles soreness and be less flexible, especially in the hips and back, similar to a tying up.. Hope it helps a bit! :)