The American Association of Feed Control Officials (referred to as AAFCO) dictates that certain ingredients are required to be in pet foods, with different rules for dog and cat food. Selenium is a mineral that has health-promoting properties, and it must be included in dog and cat foods.
Selenium is available in two forms: Sodium Selenite and Selenium Yeast. We use Selenium Yeast in all our premium dog foods because it is thought to be safest for their systems, even though it's more costly. Sodium Selenite is the only Selenium approved for use in cat foods at this time.