Plasma progesterone in cattle. II. Levels during the oestrous cycle, pregnancy and parturition

Plasma progesterone was determined in cattle with a competitive protein binding assay during the oestrous cycle, pregnancy and parturition. Peak luteal phase levels of 4--7 ng/ml (12,7--22,2 nmol/l) were found 12--16 days after clinical oestrus followed by a rapid decline to less than 0,5 ng/ml (1,6 nmol/l) on the day of oestrus. Levels of 3--7 ng/ml (9,5--22,3 nmol/l) were found 3 weeks after artificial insemination while levels of 3,8--6,5 ng/ml (12,1--20,7 nmol/l) and 3--7 ng/ml (9,5--22,3 nmol/l) were found at 6 and 7--8 weeks respectively. Between the second and third month of pregnancy levels of 3,5--6,5 ng/ml (11,1--20,7 nmol/l) were measured increasing to 3--8 ng/ml (9,5--25,4 nmol/l) from the fifth month until shortly before parturition. Levels of approximately 3--4 ng/ml (9,5--12,7 nmol/l) were found until 3 d before parturition with a drop to 2 ng/ml (6,4 nmol/l) 1 d before calving. Less than 1 ng/ml (3,2 nmol/l) was measured on the day of parturition and during the first 3 d after calving.