Influence of molybdenum and sulfur on copper metabolism in sheep: comparison of elemental sulfur and sulfate.

French forages usually have low levels of molybdenum, and sulfur contents are often in excess in ruminant feeds. The minimum level of molybdenum able to trigger the copper sulfur molybdenum interference in sheep was measured with either elemental sulfur or sulfate (diets enriched to 5.2 g S/kg DM). Four groups of 5 lambs weighing 35 kg received elemental sulfur and 4 increasing doses of molybdenum in the first experiment. In a second experiment, 6 groups of 5 lambs received sulfate (one group was a control group; the second received sulfate only) and 4 increasing molybdenum doses. The sulfur-molybdenum-copper interference was quantified by the plasma copper fraction insoluble in 5% trichloracetic acid (TCA insoluble copper). Whatever the form of sulfur, the interference appears with a molybdenum level of over 2.4 mg Mo/kg DM. Sulfate interferes less than elemental sulfur with the molybdenum on copper metabolism.