Factors Affecting Progesterone Concentration in Cow's Milk and Dairy Products

Milk and blood were collected (2 hr after morning milking), during estrus (day 0) and on days 1, 4, 7, 10, 13, 16, 19, 22, 25 and 30 from eight cows which were later confirmed to be pregnant during the collection period. In milk and blood, respectively, mean progesterone concentration (ng/ml) increased gradually from day 0 (.8, .1) to an apparent plateau at day 10 (range of means for days 10 to 30, 16.8 to 22.8, 4.2 to 6.1). Overall mean concentration of progesterone in milk (14.1) was higher (P<.005) than in blood (3.3). Milk (50 ml) and blood were collected from 10 cows immediately before morning milking, immediately after morning milking (milk only) and 3, 6, 9 and 12 hr post-milking. Fat (%) and progesterone concentration (ng/ml), respectively, of milk were lower (P<.05) immediately pre-milking (2.0, 8.0) than immediately post-milking (7.8, 23.2). Concentrations for both end points then decreased linearly with time to pre-milking levels at 9 to 12 hr post-milking, whereas progesterone concentration in plasma did not change significantly.