A dose-response trial was conducted with dairy cows on six farms in the Ixopo district of the Midlands of KwaZulu-Natal. Up to 30 cows per farm received a single subcutaneous injection of 5 mL of a long-acting/slow-releasing selenium (Se) supplement containing 50 g Se as barium selenate/mL (Deposel). The same number of matching cows per farm was used as controls. The cows grazed cultivated pastures and received concentrates containing no added Se. Whole blood Se concentrations were determined on day 1 and at ca. three month intervals thereafter. The investigation was conducted for periods ranging from between 300 and 400 days on the different farms. Incidences of retained placenta, metritis, cystic ovaries, mastitis and calf mortality were recorded. Whole blood Se concentrations of the unsupplemented cows ranged from 17 to 90 ng/mL. Accepting that a concentration of >200 ng/mL blood indicates an adequate Se intake, the recorded concentrations suggest that the cows were deficient to marginally deficient in Se. The parenteral Se supplementation at the prescribed dose increased blood Se concentrations by an average of 32.8 ng/mL, but this was insufficient to raise blood Se concentrations to adequate or recommended levels for dairy cows. No significant differences were recorded on the incidence of retained placenta, metritis, cystic ovaries, mastitis and calf mortality. It was concluded that, if not supplemented, dairy cows grazing cultivated pastures in the Ixopo district of KwaZulu-Natal are likely to be deficient to marginally deficient in Se. However, the prescribed dose of 5 mL of the slow-releasing Se supplement was too low to overcome the deficiency.