Effect of dietary sulfur and selenium concentrations on selenium balance of lactating Holstein cows

The effects of dietary sulfate and selenium concentrations on selenium bala nce in dairy cows were investigated. Midlactation Holstein cows (n = 30) we re fed diets containing either 0.1 or 0.3 mg of supplemental Se (from sodiu m selenate)/kg of dry matter and 0, 0.2, or 0.4% added S from a mix of calc ium and magnesium sulfate in a factorial arrangement. The experiment lasted 112 d. Dry matter intake was linearly reduced with increasing S, but the e ffect was greater when 0.3 mg/kg of Se was fed (significant interaction). T reatment effects for yields of milk, milk fat, and milk protein were simila r to those for dry matter intake. Increased dietary S linearly reduced plas ma Se concentrations. Increasing dietary S linearly reduced apparent (42.7, 33.1, and 30.1%) and estimated true (50.5, 46.0, and 42.3%) Se digestibili ty. Excretion of Se via feces (1.6 vs. 2.8 mg/d) and urine (0.5 vs. 1.3 mg/ d) was higher and output in milk (0.4 vs. 0.3 mg/d) was lower for cows fed 0.3 mg/kg of Se compared with 0.1 mg/kg, but no Se effect was found for est imated true Se digestibility. Dietary S from sulfate reduced Se balance esp ecially when cows were fed diets with less than 0.3 mg of Se/kg of diet dry matter.