Selenium concentration in the blood of ruminants grazing in northern New South Wales. 4. Relationship with tissue concentrations and wool production of Merino sheep

Relationships between selenium (Se) concentrations in blood and tissues, and between Se in blood or plasma and wool production were examined in fine-wool Merino ewes. Flock 1 contained 136 ewes, none of which were mated, and flock 2 contained 93 ewes which reared a lamb to weaning and 27 ewes which failed to lamb. Both flocks grazed Se deficient pastures, and were supplemented with varying quantities of sodium selenate given by mouth as aqueous solutions at 14-day intervals. Ewes in flock 1 were allocated to 17 and in flock 2 to 7 supplementation treatments, the range being from 0 to 8 mg Se/l4 days in both flocks. Wool production was measured before treatments were imposed, and for 2 years for flock 1 and 1 year for flock 2 after treatments were imposed. Samples of blood and plasma were taken at 3-monthly intervals from all sheep during the period of supplementation, and samples of liver, kidney and diaphragm were taken at the end of the experiment from sheep in flock 1; all samples were analysed for Se. Relationships between Se concentrations in blood, plasma or tissues and the quantity of supplementary Se given approached an asymptote at higher levels of supplementation, indicating that absorption or ultilization of Se were impaired at these levels; there was no evidence of clinical or subclinical toxicity. The asymptote was reduced in ewes which reared a lamb. Wool production generally increased when small quantities of supplementary Se were given, but there was no further response at higher levels, the greatest response of approximately 10% was observed in ewes which reared a lamb to weaning. Relationships between wool production, and blood, plasma and tissue Se concentrations were derived using a bent stick model which predicted that ewes would respond to supplementary Se when blood Se concentration was <O.022 to <0.076 8g Se/mL in the different groups; the higher values were observed in ewes rearing a lamb to weaning. The corresponding range in plasma was <0.014 to <0.044 8g Se/mL. It is suggested that Se concentrations <0.070 and <0.040 8g Se/mL in blood and plasma respectively indicate breeding ewes may respond to supplementation and could be used as guidelines for extension purposes; corresponding values for non-breeding sheep were <0.040 and <0.020 8g Se/mL respectively.