Selenium is supplemented for dairy cattle using organic or inorganic selenium sources. In plants, Se is randomly incorporated also in proteins and its organic form is found as an analogue of sulphur amino acids, i.e. selenomethionine (SeMet) or selenocystein (SeCis). Inorganic forms are mostly found as selenite (SeO3-2) or selenate (SeO4-2). However, it is important to distinguish between the metabolism of the different Se sources. Whereas the organic form (SeMet and SeCis) can be metabolized as amino acid, the inorganic forms are readily incorporated through selenide (HSe-) into functional selenoproteins, like glutathione peroxidase. Thus, selenoamino acids are incorporated randomly to any body protein and are available as potential Se sources only after protein turnover. In addition, there is a risk for high yielding dairy cows to loose Se for functional selenoproteins when selenoamino acids, especially SeMet, are incorporated into milk proteins, considering that about 70% of Se in milk is incorporated to casein. Measurements of Se content in soil and feed samples have therefore only a complementary value, because the metabolism in dairy cows is difficult to be foreseen with only the Se content in soil and feeds. Se supplementation is important for all animals, but metabolism of the different Se sources needs to be taken into consideration. Whereas acute deficiencies are corrected with inorganic Se sources, organic sources seem to be more efficient to overcome temporarily deficiencies.
ruminants; selenomethionine; functional selenoproteins; glutathione peroxidase
