Effects of dietary supplements of thiomolybdates on copper and molybdenum metabolism in sheep.

When 4 groups of 3 lambs were reared on a milk substitute containing 10.5 mg of Cu per kg DM with or without supplements of Cu, 5 mg, and Mo, 3 mg per kg DM in a 2 X 2 factorial design, Mo, given as ammonium tetrathiomolybdate (MoS4) reduced hepatic Cu retention by one-third but had no effect on the amount or distribution of Cu in plasma or on plasma Mo concentrations. When 2 groups of 5 lambs were weaned on to a cereal-based diet containing 3.5 mg Cu per kg DM and one was given MoS4 in the amount used previously, concentrations of Mo and TCA-insoluble Cu in plasma and the rate of depletion of liver Cu were increased. When Cu, 12 mg per kg DM, was finally added to both diets the plasma changes were reversed but the hepatic retention of Cu was still reduced. In a second experiment, 6 groups of 5 hypocupraemic ewes were repleted with a dietary Cu supplement in the presence of one of 3 Mo sources, molybdate (MoO4), dithiomolybdate (MoO2S2) or tetrathiomolybdate (MoS4) providing 3.0 mg Mo per kg DM each in the presence of low or high dietary S. The effects of the S supplement were dominant, causing the MoO4 and MoO2S2 to behave like MoS4 in impairing Cu and Mo absorption and inducing TCA-insoluble Mo and Cu to accumulate in the plasma: S enhanced these responses to MoS4 even further. On the low S diet, the responses to MoO4 and MoO2S2 were similar but small. It is concluded that at high dietary Mo and S concentrations, sufficient MoS4 forms in the rumen and enters complexes with Cu to impair Cu absorption but that the formation of MoO2S2 does not play an important role in the Cu and Mo antagonism.