What form does copper need to be in mineral blocks so the cows can utilize it?
Copper sulfate, carbonate, and oxide often are the inorganic forms of copper most often fed to cattle. Sulfates have the highest biological availability while carbonates and oxides have intermediate and very low (if any) bioavailability, respectively, according to Ammerman and Miller (J. Anim. Sci. 35:681; 1972). Copper chelated with organic materials (linked to amino acids or protein) usually has a higher bioavailability than sulfate because it is not as likely to complex with sulfide or selenide in the rumen. These bind with copper to make the copper insoluble and unavailable for animal use. Molybdenum also can complex with copper making it unavailable. Injectable forms of copper (glycinate and EDTA) are available but can prove toxic due to very rapid availability. Copper in glass beadlets or as copper needles release copper very slowly but remain in the reticulorumen and release copper over several months. These have been used primarily in Europe and Australia for grazing animals. Copper sulfate is used most widely and has higher bioavailability than the carbonate and might be preferable, but simply adding more of the carbonate to account for lower bioavailability also will work. In regions of high sulfate diets or water, use of the oxide or carbonate may be preferred. Because copper is a cumulative poison for sheep and possibly goats, trace mineralized salt for these species should NOT contain any or as much copper as trace mineralized salt for cattle.