Abstract
Summary
Basal clearance rates (CR; ml/min/kg body wt) and secretion rates (SR; μg/hr/kg body wt) of prolactin (PRL) were calculated for Holstein cows. Among late lactating, pregnant cows (n = 8) tested in December, estimates of basal CR and SR were not different (P > 0.05) during infusion of either 4 or 8 mg PRL/cow/hr for 4 hr. In a second experiment, basal CR estimates were similar (P > 0.05) in four nonlactating and four early lactating cows but basal SR was 35% greater (P ~ 0.1) in early lactating than in nonlactating cows tested in April. In a third experiment designed to avoid possible seasonal effects, basal CR estimates of PRL were 75 and 25% greater and basal SR estimates 140 and 40% greater (P < 0.05) in early lactating and late lactating cows than in nonlactating cows, respectively. Differences in basal CR and SR of PRL among all lactating cows were correlated with mean daily milk production (r = 0.36; P < 0.05 and r = 0.52; P < 0.005, respectively) during the week prior to each experiment. We conclude that increased basal CR and SR of PRL are associated with increased milk yields in dairy cows.
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