CONCENTRATIONS OF OESTRADIOL-17β, OESTRONE AND PROGESTERONE IN JUGULAR VENOUS PLASMA OF COWS DURING THE OESTROUS CYCLE AND IN EARLY PREGNANCY

Abstract

Plasma oestradiol-17β concentrations rise slowly in the late progestational phase of the bovine oestrous cycle before plasma progesterone levels fall and then rise rapidly to a pre-oestrous peak of about 6 pg/ml as progesterone levels fall rapidly. The oestradiol-17β level (on average) falls during oestrus and both steroids show minimum levels 1–2 days after oestrus. Both then rise again, oestradiol-17β to a second peak 6–7 days after the pre-oestrous peak as progesterone level continues to rise. In a cow in the first 30 days of pregnancy, when plasma progesterone levels remained generally high after the 15th day, the pattern of plasma oestradiol-17β levels was very similar to that expected had it not conceived. Thus in the cycle and the first 30 days of pregnancy the main oestradiol-17β peaks are found at about 21 day intervals, each being followed by a smaller peak 6–7 days later. Either oestradiol-17β peak may assume a pre-oestrous and pre-ovulatory role in the absence of high plasma progesterone levels.