Selenium supplementation of grazing sheep. I. Effects of selenium drenching and other factors on plasma and erythrocyte glutathione peroxidase activities and blood selenium concentrations of lambs and ewes

Lambs or ewes grazing pastures in the low selenium region of New England in New South Wales were used to study the effects of selenium supplementation on blood and plasma levels of selenium and glutathione peroxidase (GSH-px), and to assess the possibility of using GSH-px activity as an indicator of selenium availability or selenium status in grazing sheep. When lambs were drenched with selenium at 14 days of age there were large and rapid increases in the initially low levels of plasma and erythrocyte GSH-px. The increase in plasma GSH-px was transitory, and a second selenium drench administered to half the treated lambs at 42 days did not evoke any further increase. Erythrocyte GSH-px activities of treated lambs remained elevated for a longer period than plasma GSH-px, and the second selenium drench extended the period of elevation. Alterations in plasma GSH-px of adult ewes drenched with selenium were similar to those observed in the lambs, but erythrocyte GSH-px activities increased much more slowly. The changes in plasma and erythrocyte GSH-px were accompanied by significant alterations in plasma and whole blood concentrations of selenium. Increases in whole blood selenium, because of the plasma component, were much more rapid than those in erythrocyte GSH-px. Transfer of ewes to a new grazing area with a different soil and pasture type led to similar changes in plasma and erythrocyte GSH-px to those produced by selenium drenching. Small increases with time were also observed in the erythrocyte GSH-px activity of untreated ewes grazing the same pasture continuously. These changes were presumably the result of changes in the selenium content and/or availability in the pasture. It was concluded that regular estimations of erythrocyte or whole blood GSH-px activities could be used as an indication of selenium availability and the selenium status of grazing sheep. Changes in pasture and/or grazing area, and the fact that erythrocyte GSH-px activity of adult animals does not immediately attain a new equilibrium value when selenium intake alters must however be taken into account.