There are two major classes of mineral sources: inorganic and organic. Organic complexes, however, have been shown to be more effectively absorbed by the animal as compared to inorganic sources. Organic complexes are created when trace elements are linked to a protein or an amino acid. Many studies investigating organic complexes have involved disease situations and the pharmaceutical factor of trace elements. In stress situations, the organic complexes may be beneficial compared to inorganic sources, and a therapeutic response related to immune function has been noted. Consistent advantages of organic sources compared to inorganic sources in healthy cows with proper protein and energy nutrition have not been shown.