Relationships between immunoreactive estrone and estradiol in milk, blood, and urine of dairy cows.

Quantities of immunoreactive estrone and estradiol in blood plasma, urine, and milk were measured during the estrous cycle and pregnancy of cows. The objectives were to develop a radioimmunoassay procedure for quantifying estrogen in milk and urine and to compare changes in milk estrogen with those in blood plasma and urine. Concentrations of estrone and estradiol in milk varied during the estrous cycle. Relative concentrations of estradiol in blood plasma and milk were not different, but average estrone concentrations in milk were four times greater than those in blood plasma. Concentration of total estrogen (estradiol plus estrone) exceeded 1 ng/ml in colostrum and milk from cows milked prepartum, and was correlated with total estrogen in blood plasma and urine before and after calving. Blood plasma estrone was correlated only with milk estrone whereas blood plasma estradiol was correlated with urinary estradiol, milk estrone, and milk estradiol during the estrous cycle. These results raise possibilities that mammary gland of the lactating cow may concentrate preferentially estrone or convert estradiol to estrone. However, estimated excretion of estrogen through the milk represents no more than a fraction of 1% of the total excreted during the estrous cycle, and the proportion becomes less as gestation progresses up to at least 7 mo.