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

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Abstract
Plasma samples were taken every 10 min for periods of 8 h on 3 occasions during the postpartum 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, [luteinizing hormone], 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 (> 70 days); all 4 group M2 cows resumed ovarian cycles by 25.25 .+-. 3.35 days postpartum. Three group S cows resumed cycles by a mean of 47.7 days postpartum while the 4th had not done so by day 100. Mean plasma LH concentrations were low prepartum (0.67 .+-. 0.05 ng/ml) and up to day 5 postpartum (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 (.apprx. 2/8 h) in those milked cows in which ovarian activity was delayed and varied from 4-10 episodes/8 h in milked cows that commenced ovarian activity early in postpartum period. Evidently the pulsatile LH pattern is a prerequisite for the onset of ovarian cycles.