Some endocrine changes associated with the post-partum period of the suckling beef cow

Seven Hereford cows with single calves were bled by jugular venepuncture, daily from parturition until 63-79 days post partum; 6 of the cows were also bled through jugular cannulae every 15 min for 8 h every 10 days. The average post-partum interval to first oestrus was 59.8 +/- 3.7 days for 5 of the cows. In cows returning to oestrus, plasma concentrations of progesterone were low until 55.5 +/- 3.0 days post partum, rose to exceed 0.5 ng/ml plasma for 4.0 +/- 0.4 days, declined for 5.0 +/- 0.5 days and then rose again to normal luteal-phase levels. First oestrus preceded the initial rise in progesterone in 1 cow and followed it is 4. Ovarian palpation revealed considerable follicular development before the initial rise in progesterone, but no clearly discernable corpus luteum until normal luteal-phase progesterone levels were detected. Plasma concentrations of oestradiol-17 beta fell after parturition and, although very variable, showed no apparent trend thereafter. Plasma concentrations of LH varied in an episodic manner, with an apparent increase in frequency and magnitude of peaks up to 10-33 days before the first elevation in plasma progesterone. Subsequently there was little change, except for a decline in peak LH concentrations after the initial elevation in plasma progesterone.