Relationship of dietary selenium to selenium in plasma and milk from dairy cows.

In a 13-wk trial Holstein cows in mid-lactation were fed four diets containing natural selenium alone or supplemented with .1, .2, or .5 mg of inorganic selenium per kilogram of diet. Dietary selenium concentration averaged .334, .385, .456, and .772 mg/kg. Selenium plasma in wk 7 averaged .112 mu/ml with no differences among treatments; milk selenium ranged from .040 to .046 mu/ml and was higher in the two higher selenium diets. In wk 13 selenium in plasma and milk averaged .119 and .054 with no treatment differences. The 7 and 13 wk concentrations were higher than pretrial .084 and .033 mu/ml for plasma and milk during which time dietary selenium concentration was .254 mg/kg. Selenium concentration increased linearly from about .08 to .120 mu/ml of plasma and about .030 to .055 mu/ml of milk as intake of selenium increased from about 2 to 6 mg/day. Increase in selenium intake from 6 to 12 mg/day resulted in little change in plasma and milk selenium. Moderate concentrations of dietary selenium (.3 to .7 mg/kg) do not result in toxic amounts of selenium in milk.