Milk progesterone in cows during oestrus cycle and postpartum period as a monitor of reproductive status

The mean oestrus cycle length measured by milk progesterone assay was found to be 20.66 +/- 0.22 days and the follicular phase was about 7.33 +/- 0.18 days; while the luteal phase was about 13.33 +/- 0.26 days. The progesterone concentration in the milk was significantly affected by day and phase of the cycle. The milk progesterone concentration did not exceed 2 ng/m1 during the first 15 days postpartum, but its level increased gradually to reach a peak as [17 ng/m1] on day 26 postpartum