The Importance of Prolactin for Initiation. of Lactation in the Pregnant Ewe

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Abstract
A single injection of ergocryptine (0.5 mg/kg liveweight) given to ewes 0.5-20 days prepartum or 2 injections (0.5 mg/kg liveweight per injection) given .apprx. 30 and 10 days prepartum reduced concentrations of plasma prolactin to negligible (< 5 ng/ml) values for 4 wk after parturition, but did not affect concentrations of growth hormone and placental lactogen. Milking of treated ewes had no effect on concentrations of plasma prolactin during the 1st 4 wk of lactation, but concentrations of growth hormone were increased during the 10-20 min period after milking. The half-life of prolactin in plasma was estimated as 21 min. In spite of the dramatic effect of ergocryptine on plasma prolactin all treated ewes secreted copious quantities of milk of normal composition. Mean daily yields of ewes with ergocryptine were not significantly different (P < 0.05) from those of untreated control ewes, but the mean .+-. SEM [standard error of the mean] of total milk production over the first 3 wk of lactation for ergocryptine-treated ewes was significantly lower (P < 0.05) than that of control ewes (9.5 .+-. 1.11 vs. 14.1 .+-. 1.20 kg milk). Prolactin is not an essential component of the lactogenic and galactopoietic complexes of hormones in the ewes.