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

Abstract

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 1st estrus was 59.8 .+-. 3.7 days for 5 of the cows. In cows returning to estrus, plasma progesterone [P] concentrations 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 estrus preceded the initial P rise in 1 cow and followed it in 4. Ovarian palpation revealed considerable follicular development before the initial rise in P, but no clearly discernable corpus luteum until normal luteal-phase P levels were detected. Plasma estradiol-17.beta. concentrations fell after parturition and, although very variable, showed no apparent trend thereafter. Plasma concentrations of LH [luteinizing hormone, lutropin] varied in an episodic manner, with an apparent increase in frequency and magnitude of peaks up to 10-33 days before the 1st plasma P elevation. Subsequently there was little change, except for a decline in peak LH concentrations after the initial plasma P elevation.