FIELD TRIAL TO EVALUATE THE EFFECTS OF DIFFERENT FORMS OF ORAL SELENIUM SUPPLEMENTATION ON PRODUCTION IN LACTATING HOLSTEIN COWS

Two 13-week trials were conducted to compare responses in lactating da iry cows to a sustained-release selenium bolus and oral sodium selenit e as supplemental selenium sources for cows fed a total mix ration. Gr oup 1 cows were fed selenium mixed into their ration at the rate of 5. 5 mg/cow/day in Trial 1 and 3 mg/cow/day in Trial 2. Group 2 cows in b oth trials each received a sustained-release selenium bolus designed t o provide 3 mg selenium/day, administered orally with a balling gun. G roup 3 in each trial was the control group and received no supplementa l selenium. Serum selenium concentrations in supplemented groups in ea ch trial were higher (p < 0.05) than in controls by Week 2. These conc entrations remained significantly elevated over the control group cows for the length of each trial. The intrareticular sustained-release se lenium bolus was as effective at maintaining serum selenium concentrat ions as feeding 3.0 mg or 5.5 mg selenium selenite daily. Both seleniu m sources maintained serum selenium concentrations close to appropriat e target levels for high-producing dairy cattle (> 70 ng/ml).