Selenium supplementation of sheep by topdressing pastures under high rainfall conditions

Two 4 hectare paddocks on a property in a high rainfall area of South Westland were topdressed with two different formulations of selenium (Se) prills at a rate of 1 kg/ha. Whole blood Se and glutathione peroxidase (GPx) concentrations were recorded at monthly intervals in four groups of adult Perendale ewes. These consisted of two groups of 25 set-stocked on each of these paddocks, the main mob of ewes which ranged over the rest of the farm and received routine oral administration of Se, and a group of 25 untreated control ewes running with the main mob. Blood Se and GPx concentrations in the two groups receiving prills showed a large increase over pre-treatment levels at the first monthly sampling after application of the prills, and remained significantly higher than the concentrations recorded for the main group and the untreated controls for 12 months. Pasture Se concentrations in the treated paddocks showed an initial peak at 1 month, but had declined to a level similar to control pastures by 6 months. At no stage did any of the blood values for Se iu the treatment groups approach toxic levels. In contrast, the main mob were given 5 mg of Se (orally combined with an anthelmintic drench twice per year), but in each case blood Se and GPx values only rose slightly above those of the untreated controls and quickly returned to similar concentrations.