Influences on progesterone concentration in bovine milk.

Factors affecting progesterone concentrations in milk of lactating dairy cows were studied and discussed as they pertained to pregnancy diagnosis. Adding a preservative to milk allowed storage at room temperature for 10 days with no effect on progesterone analysis. Progesterone content in first, composite, and last milk was associated with fat content, although correlation between progesterone concentration in composite milk and fat percentage was low. Variation was considerable in individual samples for each of the milk fractions. Day of the estrous cycle was the most important factor affecting progesterone concentration in milk from cycling cows. Samples for pregnancy diagnosis by milk progesterone should be collected near the time that nonpregnant cows are expected to return to estrus. Progesterone concentration in milk from pregnant cows remained high throughout gestation with progesterone concentrations similar to those in the luteal phase of the estrous cycle.