Concentrations of oestradiol-17β and progesterone in bovine plasma and defatted milk during the post-partum anovulatory period, during oestrous cycles and following ovariectomy

Abstract

Concentrations of oestradiol-17β and progesterone in plasma and defatted milk of British Friesian dairy cattle were measured by radioimmunoassay in samples taken daily for 23 to 35 days from five early post-partum cows (two of which were ovariectomized within 24 to 48 h of calving) and from three cows with regular oestrous cycles. During the first five to 10 days post partum, in both the intact and ovariectomized cows, progesterone levels in plasma and defatted milk were basal (< 0·3 ng/ml). Oestradiol-17β levels fell from > 15 pg/ml to < 1 pg/ml, presumably due to clearance of placental oestradiol-17β following cessation of production at calving. They then rose to a pre-ovulatory peak in the intact cows (about 13 days post partum) and a post-ovulatory peak six days later. Concentrations of progesterone were typical of those known to occur in plasma and defatted milk during the oestrous cycle. Similar oestradiol-17β and progesterone levels were found during later oestrous cycles. In the ovariectomized cows steroid levels remained basal after the first oestradiol-17β post-partum fall. Concentrations of oestradiol-17β and progesterone in plasma and defatted milk throughout these various periods were highly correlated ( r = 0·76 and 0·90 respectively). Mean levels of oestradiol-17β and progesterone in defatted milk were respectively 66% and 52% of those in plasma; they reflect changes in plasma during the post-partum, anovulatory period and oestrous cycles, and may be used in studies of bovine ovarian function.