SELENIUM FERTILIZERS FOR PASTURES GRAZED BY SHEEP .1. SELENIUM CONCENTRATIONS IN WHOLE-BLOOD AND PLASMA

Two forms of prilled selenium fertilizer from Mintech N.Z., Na2 SeO4 ( Selcote(R)) and BaSeO4 incorporating a slow-release technology, were a pplied to pasture once at 10 and 20 g Se ha-1 in 1987 at Bakers Hill, Western Australia. The pasture was grazed at 10 sheep ha-1 for 3 years . In the first year, half the sheep were also treated with a selenium intra-ruminal pellet. The Selcote fertilizer produced a rapid rise in pasture selenium concentration, resulting in a similar response in the concentrations of selenium in the whole blood and plasma which reache d their maximum values within 7 weeks of commencing grazing. The selen ium concentration then declined until, after 15 months, there was insu fficient selenium in the pastures to increase the selenium status of s heep. In contrast, the slow-release formulation provided adequate sele nium in each of the 3 years with sheep reaching their maximum plasma s elenium of 80 mug Se L-1 in the second and third year. At relatively l ow values for whole blood selenium (less-than-or-equal-to 75 mug Se L- 1), supplying an additional selenium source had an additive effect on selenium concentration in the blood, but at higher concentrations (gre ater-than-or-equal-to 350 mug Se L-1) additional sources of selenium o nly marginally increased the whole blood concentration of selenium. Pl asma was a more sensitive indicator of the selenium content of pasture in the short term than whole blood. The results indicate that either Selcote or the BaSeO4 fertilizer provides an effective alternative to current strategies for supplying Se to grazing animals.