Peripheral plasma progesterone levels during the bovine estrus cycle

Summary. Progesterone was determined daily in the peripheral plasma of six cows for a total of seven complete oestrous cycles. Progesterone levels ranged from less than 0\m=.\5ng/ml plasma during the follicular phase to 6\m=.\6ng/ml plasma (6\m=.\1to 10\m=.\2ng) at peak luteal phase. Progesterone levels in cows with 21-day cycles increased rapidly from Day 3 to Day 8(oestrus = 1) with a much slower rate of increase from Day 8 to Day 17. These cows showed a progesterone decrease of more than 50 % from the previous day on Days 18 (two cows), 19 (one cow) and 21 (two cows). Two other cows with cycles of 22 and 23 days ' duration both had a similar decline on Day 20. A variable time interval of 1 to 5 days was observed between the decline ofprogesterone and the occurrence ofoestrus. These data indicate that considerable variation may exist among cows as to time require-ments for follicle development and maturation. Monitoring ofperipheral levels ofprogesterone is suggested as a means ofstudying corpus luteum function