Free choice feeding of selenized-salt has been widely used for sheep and cattle because supplementing grazing animals is often difficult. Fewer studies have been conducted with horses, but salt appears to be a reasonable source of selenium, particularly for horses that graze. If you use a commercial grain mixture fortified with selenium, you probably don't need a supplement. If you are not sure whether or not your horses are getting enough selenium, and the horses have muscle problems, selenium may be involved. Your veterinarian can take a blood sample and have it analyzed for selenium or glutathione peroxidase, an enzyme that requires selenium.
Selenium is needed for a horse's normal muscle function and health.
Use moderation in all things, and good things come in small packages. The horse needs 1 to 3 mg of selenium per day, but 50 mg per day may cause toxicosis. One mg is 0.0000022 lb. Thus you must exercise care when using selenium supplements. The amount of supplement you add must of course be greater than one mg, because the selenium in the supplement is diluted with carrier.