A comparison of plasma LH concentrations in milked and suckling post-partum cows.

Plasma samples were taken every 10 min for periods of 8 h on 3 occasions during the post-partum period from 8 cows milked twice daily (Groups M1 and M2) and from 4 cows each suckling 4 calves (Group S). All samples were assayed for LH, and ovarian activity was monitored by measurement of milk progesterone. Three of the Group M1 cows had a delayed return to ovarian activity after calving (greater than 70 days) whereas all 4 Group M2 cows resumed ovarian cycles by 25.25 +/- 3.35 days post partum. Three Group S cows resumed cycles by a mean of 47.7 days post partum while the fourth had not done so by Day 100. Mean plasma LH concentrations were low pre partum (0.67 +/- 0.05 ng/ml) and up to day 5 post partum (0.80 +/- 0.03 ng/ml, Group S; 0.90 +/- 0.12 ng/ml, Group M1; 0.80 +/- 0.25 ng/ml, Group M2). Concentrations then increased up to Day 12 (1.23 +/- 0.20, 1.66 +/- 0.16 and 1.67 +/- 0.21 ng/ml in Groups S, M1 and M2 respectively). Between Days 13 and 20 a distinct pulsatile pattern was seen in the profiles of milked but not of suckling cows. The pulse frequency was lower (approximately 2/8 h) in those milked cows in which ovarian activity was delayed and varied from 4 to 10 episodes/8 h in milked cows that commenced ovarian activity early in the post-partum period. It is suggested that the pulsatile LH pattern is a prerequisite for the onset of ovarian cycles.