CONTRIBUTION OF DIETARY SULFUR TO THE INTERACTION BETWEEN SELENIUM AND COPPER IN SHEEP

The effect of concentration of sulphur (S) in a diet on the interrelat ionship between copper (Cu) and selenium (Se) metabolism was investiga ted in sheep. The animals received a sheep finishing diet in individua l feeding pens for a period of 74 days. The minerals were included in the diets according to a 3 x 2 x 2 factorial experimental design. The final mixtures contained three concentrations of Se; 0.35, 0.88 and 1. 34 mg Se (Se added as Na2SeO3); two concentrations of Cu; 6.7 and 17.0 mg Cu (Cu added as CuSO4) and two concentrations of S; 2.15 and 3.97 g S (S added as Na2SO4)/kg dry matter (DM). An increase in dietary Cu resulted in a significant (P < 0.01)increase in liver Se concentration ; 2.30 v. 3.43 mg Se/kg DM for the low v. the high Cu treatments. The addition of S reduced the hepatic Se concentration significantly (P < 0.01). This was most pronounced at the high Cu intake, resulting in a significant (P < 0.05) three-way interaction between the three mineral s, i.e. the hepatic Se concentrations were 2.53, 2.08, 4.07 and 2.79 m g/kg DM for the low Cu-low S, low Cu-high S, high Cu-low S and high Cu -high S treatments, respectively. Dietary S reduced (P < 0.01) the con centration of Cu in the liver from 678 mg/kg DM at the low S to 305 mg /kg DM at the high S intakes. Dietary Se did not change the concentrat ion of Cu in the liver significantly. Adding S to the diet caused a re duction (P < 0.01) in the Se concentration of rumen bacteria, i.e. fro m 3.48 mg/kg DM in the low S to 1.96 mg/kg DM in the high S treatments . Selenium concentrations in rumen bacteria and the liver were the mos t sensitive to changes in Se intake, followed by the heart, muscle, pl asma and whole blood. It was concluded that an increase in S intakes w ould decrease the accumulation of both Cu and Se in the liver of sheep , resulting in a diminishing degree of interaction between Cu and Se.