On Oct. 14, the FDA announced that it would exercise “enforcement discretion” with regard to the inclusion of selenium yeast in horse feeds. What this basically means is that, although it’s still technically illegal to feed selenium yeast to horses, the FDA likely won’t do anything about it.
Selenium yeast is naturally chelated, produced by adding selenium salts to yeast cultures during their growth stages. The selenium is then taken in during fermentation and incorporated into the yeast cells in association with their proteins. While studies in cattle found that selenium yeast is better absorbed than selenium selenite (the currently approved form of selenium for horses), it remains to be proven for horses. There are also products that mix inorganic selenium salts with yeasts during the manufacturing process, resulting in a selenium-coated yeast. These seleniums are not truly chelated, although it’s possible they also enhance absorption. However, until studies are available with specific reference to horses, selenium yeast should be supplemented in the same dosages as sodium selenite.