Skim milk progesterone in pregnant cows from insemination throughout lactation.

The skim milk progesterone profile was assessed by radioimmunoassay, without extraction, from the day of insemination (day 0) until the cows were dried off on day 225 of gestation. A total of 418 samples were collected from 154 pregnant Holstein cows. The daily variation in skim milk progesterone was recorded from day 1 until day 45 of pregnancy to detect the commencement of progesterone secretion from the corpus luteum after insemination. Subsequent determinations were made every 2 weeks from day 46 until lactation ceased. On the day of artificial insemination and for the first 2 days after insemination, all the cows had a basal progesterone concentration < 0.1 ng/ml. A rise in progesterone (0.2 +/- 0.1 ng/ml) was first detected on the third day after insemination. The progesterone values then increased significantly (p < 0.001) until day 15. The values then remained nearly constant (2.5-3.5 ng/ml) until day 106 of pregnancy, when they began to decline. Between days 120 and 180 of gestation, progesterone was significantly decreased (2.2-2.9 ng/ml) before it rose again to the previous plateau (3.5-3.9 ng/ml) around day 180. The progesterone concentration then remained at the higher level until the animals were dried off.