Selenium is an essential trace element, which is used for the synthesis of selenoproteins, such as glutathione peroxidase (GPx), which can function as antioxidants. The selenium levels in raw materials depend on the amount of selenium in the soil where the crops are grown. Therefore, the selenium levels in feed are often variable and rather low, highlighting the need to add selenium to the diet. Such supplemented selenium can be added in different forms, that can be distinguished as inorganic and organic. Inorganic selenium, usually in the form of sodium selenite, is often supplied as it is cheap, and it will be used for the synthesis of selenoenzymes in the animal. Organic selenium, as L-selenomethionine, can also be used for the synthesis of selenoproteins. However, it also has an important benefit compared to inorganic forms, since organic selenium in the form of L-selenomethionine can be stored into animal proteins. This way it provides a safe deposit of selenium in the body, that can be used during times of stress when selenium demand is increased or feed intake is low. Therefore, L-selenomethionine is considered to be the most effective form of selenium.
There are different sources of organic selenium products on the market. One of such products are selenized yeasts. These are yeasts that have grown on a medium rich in selenium, which caused the yeast to incorporate the selenium inside the yeast protein. On average, selenized yeasts contain approximately 63% selenium in the form of L-selenomethionine, the rest of the selenium is present in other forms such as selenocysteine. These other forms of selenium cannot be stored in animal proteins. The selenomethionine content in selenized yeast is variable and depends on different producers but also on different batches of the same producer. When incorporating selenized yeast in the feed, it is important to consider that the digestibility of yeast protein, and therefore also for the selenomethionine in the yeast, is only approximately 70-80%. In practice, this means that when you have a selenomethionine content of 63%, the total digestible selenomethionine content is approximately 50%, with variation between batches.