The time between stages of development, however, is approximately 24 h (Lindner and Wright, 1983; Van Soom et al., 1997), which indicates
that the difference between observation of an 8-cell embryo or morula cannot be attributed
entirely to differences in ovulation time. Moreover, in a study in which similar unmated
cows were ultrasound scanned to determine the day of ovulation, the mean plasma concentration
of progesterone on day 4 following ovulation (approximately equivalent to day 5
following onset of oestrus) was 1.8 ng ml−1 with a standard deviation of 0.8 ng ml−1 (G.E.
Mann and M.D. Fray, unpublished observation).