Blood copper levels were found to be a poor indicator of copper status. Copper oxide was shown to have low bioavailability and to be better than no copper supplementation only after 90 days on the deficient diet. These data are interpreted to show that the bioavailability of copper oxide is much below copper sulfate, even when fed at levels near the NRC requirement. Several other recent studies have shown copper sulfate to be superior to copper oxide because of differences in bioavailability. Baker et al.
Copper carbonate is considered to have intermediate availability.
Copper metal is totally unavailable. If copper nutrition was as simple as determining the copper in the basal diet and adding a highly available copper source, copper deficiency would not be a problem. However, because copper absorption and metabolism can be affected by molybdenum, sulfur, calcium, zinc, iron, manganese, cobalt, lead, cadmium, and selenium, deciding how much supplemental copper is required is not always straightforward. For example, in sheep, dietary molybdenum levels can be the primary factor affecting copper requirements. However, if molybdenum intakes exceed 10 ppm, copper deficiency may occur on diets that would normally be adequate.