Selenium is found naturally in plant feed ingredients, although concentrations vary greatly depending on the region in which the feedstuff originated. Selenium may be supplemented in either the organic or inorganic form.

Organic selenium is composed largely of selenomethionine, while inorganic selenium is simply a salt, usually sodium selenite or sodium selenate. Selenium-enriched yeast such as SelenoSource AF is an excellent source of organic selenium

The selenium status of layers may be assessed by measuring the level of selenium in the blood plasma or serum, whole blood, egg, as well as blood or liver glutathione peroxidase activity.

Selenium and vitamin E work together as antioxidants, and thus proper supplementation of both is essential.

SelenoSource AF was at least 50% more bioavailable than sodium selenite.

Organic selenium as selenomethionine may be incorporated into body proteins such as muscle tissue and eggs, whereas inorganic selenium cannot. Selenomethionine comprised the major source of selenium in SelenoSource AF.

It is suggested that all added selenium should be in the form of selenium yeast (SelenoSource AF). NRC recommendations vary with age and weight of the birds.