Prolactin in bovine milk near the time of calving and its relationship to premature induction of lactogenesis.

Twenty-three primiparous heifers and pluriparous cows were milked once daily beginning 3 to 10 days before parturition. Retrospectively, they were divided into groups representing either successful or unsuccessful premature induction of lactogenesis, depending on milk production prior to calving. Successfully induced animals had prepartum milk yields greater than 4 kg/milking and unsuccessful animals yielded less. Ten of 23 animals satisfied the criterion for successful premature induction at 2.6 +/- .4 days before calving and 3.6 +/- .8 days after first milking and had peak prepartum yields of 7.9 +/- .7 kg/milking. Total number of prepartum days milked did not differ between successful and unsuccessful groups. Greater milk yield prepartum was significantly associated with higher milk prolactin concentration on days -6 to -4. Heifers had less milk yield than cows but did not differ in milk prolactin concentration or in total prolactin per milking. Premature induction of lactogenesis did not confer significant lactational benefits for more than 1 day postpartum or over the first 35 days postpartum. Premature induction of lactogenesis as a result of prepartum milking did not, as hypothesized, decrease the ability of mammary secretions to accumulate prolactin in large amounts. All groups rapidly lost this ability after parturition, suggesting that the hormonal environment of late pregnancy may promote the transfer of prolactin from blood into milk when there is regular removal of prepartum milk.