SUBCLINICAL SELENIUM INSUFFICIENCY .1. SELENIUM STATUS AND THE RESPONSE IN LIVEWEIGHT AND WOOL PRODUCTION OF GRAZING EWES SUPPLEMENTED WITH SELENIUM

Fine wool Merino and Border Leicester x Merino ewes grazed improved pastures low in selenium (Se) for 4 years at 6.3 and 12.5 sheep/ha; half of the ewes were supplemented with Se given as an intra-ruminal pellet. Selenium status of the ewes was monitored from Se concentrations in whole blood and plasma, and wool production, fibre diameter and liveweight were recorded. The unsupplemented ewes showed no overt signs of Se insufficiency; Se status was consistently low; and Se supplementation increased wool production and fibre diameter. Selenium status was lower in unsupplemented ewes grazed at the higher stocking rate and in ewes supporting a lamb; it showed seasonal trends and was negatively correlated with rainfall. Selenium concentrations in whole blood but not in plasma were lower in Merino than in Border Leicester x Merino ewes. Data relating to reproductive performance and the productivity of the lambs are presented in accompanying reports.