Abstract
Summary
Jugular blood was collected from 34 Holstein heifers from 26 days before parturition until 26 days postpartum or until first estrus. Serum prolactin varied between 80 and 110 ng/ml until Day 2 prepartum when it increased markedly to peak of 285 ng/ml at 1 day prepartum. Prolactin decreased linearly (p < 0.01) from the peak until 2 days after calving, stabilized at about 90 ng/ml until Day 9, and gradually declined until day 26 (36 ng/ml). Postpartum (Day 5-26) prolactin was lower (p < 0.05) than prepartum (Day 26-5) prolactin. Growth hormone followed a pattern similar to prolactin but peaked 24 hr later, at parturition, and decreased from the peak to concentrations characteristic of prepartum values by Day 4 postpartum. Serum growth hormone during late pregnancy (Day 26-5) was lower (p < 0.05) than during early lactation (Day 5-26). Serum LH was consistently low (0.4-0.8 ng/ml) until Day 9 postpartum when it increased to about 1.5 ng/ml until first estrus.
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