The importance of prolactin for lactation in the ewe

The effect of 2-bromo-alpha-ergocryptine (bromocriptine) on the induction, initiation and maintenance of milk secretion was studied in post-parturient ewes and in ovariectomized ewes artificially induced to lactate by treatment with oestrogen plus progesterone and then dexamethasone. Treatment with bromocriptine (about 0.4 mg/kg every 3 days) lowered and maintained the plasma concentration of prolactin at less than 12 ng/ml. Ewes receiving bromocriptine concurrently with oestrogen plus progesterone during the priming phase had a significantly lower (P less than 0.05) mean cumulative milk yield than control ewes, although the milk of the treated ewes contained normal amounts of fat, protein and lactose. Administration of bromocriptine during dexamethasone-induced lactogenesis had no significant effect on the mean cumulative milk yield but significantly (P less than 0.05) increased the milk fat and protein content. In established lactation, bromocriptine markedly reduced the milk yield in both intact and ovariectomized ewes. The concentration of protein was not significantly affected although the milk fat content was higher in the bromocriptine-treated than in the control ewes. The effects of bromocriptine on milk yield and composition during galactopoiesis could be reversed by concurrent infusion of prolactin and the results suggest that prolatin is an important hormone during mammogenesis and galactopoiesis in the sheep.