Selenium deficiency in horses is most often treated with supplements that contain this  essential trace mineral. Selenium supplements are available in different forms and concentrations, and they are designed to meet the needs of individual horses. Most horses require 1 mg of selenium per day in form of supplements. Supplements can be mixed with the feed or administered as an injection. The injectable form of selenium is commercially available, and it is quite safe to use in horses.
Acute selenium deficiency can be treated with a selenium supplement called selenomethionine, which is available in injectable form. This medication is effective immediately and does not cause any side effects.
Oral supplementation is used after the injectable treatment is finished. This is done by mixing a daily oral dose of selenomethionine with the horse’s regular feed.