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PERIPHERAL PLASMA PROGESTERONE LEVELS DURING THE BOVINE OESTROUS 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 0·5 ng/ml plasma during the follicular phase to 6·6 ng/ml plasma (6·1 to 10·2 ng) 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 of progesterone and the occurrence of oestrus. These data indicate that considerable variation may exist among cows as to time requirements for follicle development and maturation.

Monitoring of peripheral levels of progesterone is suggested as a means of studying corpus luteum function.

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