SKIM MILK PROGESTERONE IN PREGNANT COWS FROM INSEMINATION THROUGHOUT LACTATION

The skim milk progesterone profile was assessed by radioimmunoassay, w ithout 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 c ollected 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 lut eum after insemination. Subsequent determinations were made every 2 we eks from day 46 until lactation ceased. On the day of artificial insem ination 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 insemina tion. The progesterone values then increased significantly (p<0.001) u ntil 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 1 20 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/m l) around day 180. The progesterone concentration then remained at the higher level until the animals were dried off.