The role of organic sulphur in the copper-molybdenum-S interrelationship in ruminant nutrition

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Abstract
1. The ability of organic and inorganic sulphur to influence the copper and molybdenum metabolism of sheep was compared in a series of three 2 × 2 factorial design experiments. In each experiment, four groups of five to seven hypocupraemic ewes were repleted with a basal diet supplemented with 6 mg Cu/kg and containing S and Mo at one of two concentrations, 1 or 4 g S and 0.5 or 4.5 mg Mo/kg respectively. Sodium sulphate (Expt 1), methionine (Expt 2) or cysteine (Expt 3) were used as the S sources. Cu and Mo concentrations in plasma were estimated in each experiment and in Expt 3 the concentrations of Cu in liver and Mo in urine were also estimated.2. The effects of the three S sources on Cu and Mo metabolism were similar. Repletion of the plasma Cu pool was unaffected by Mo alone, reduced by S alone and totally inhibited by Mo + S. Plasma Mo was greatly increased by Mo supplements, slightly decreased by S supplements and unaffected by Mo and S supplements given together.3. In Expt 3 the treatments were found to affect urinary Mo and plasma Mo in a similar manner; S prevented dietary Mo from increasing Mo excretion. The only group to show a significant repletion of the liver Cu pool was that given Mo alone.4. Supplementation of the diet with organic S significantly reduced the within-treatment variation in plasma Cu and Mo, liver Cu and urinary Mo.5. It is suggested that variations in dietary S and Mo within the normal range for herbage affect the Cu and Mo metabolism of the grazing animal, and that total S rather than inorganic S is the more useful measurement in the context of the Cu–Mo–S interrelationship.