Prolactin and insulin levels in lactating sows in relation to nursing frequency

It has been established that sows up- or down-regulate their milk productio n as the frequency of nursings is changed. The amount of udder massage by p iglets might also influence milk production. To investigate whether these e ffects are associated with changes in prolactin or insulin levels, we enfor ced five sows each to nurse either every 35 min (MIN35) or every 70 min (MI N70) over a 26- to 28-hr period. Milk production was measured during the fi rst 24 hr of this period. During the last three to four nursings, blood was collected every 5 min. Plasma prolactin levels increased after milk ejecti on (P < 0.05), whereas insulin levels increased only briefly in MIN70 sows. Sows nursing every 35 min had lower basal (P < 0.001) and maximal (P < 0.0 5) concentrations of insulin than MIN70 sows. There were no differences bet ween the two groups in prolactin levels. Nursings with a postejection udder massage longer than 90 s tended to induce a higher increase in prolactin ( P < 0.1) than nursings with a shorter massage. When the effects of imposed nursing frequency were removed, there was an across-sows positive residual correlation between average prolactin levels (P < 0.05) and the duration of post-ejection udder massage during the preceding 24 hr. We conclude that w hen milk production of a sow is changed by altering the nursing frequency w ithin natural limits, the necessary alteration in catabolic state of energy metabolism may be associated with altered insulin levels. The duration of udder massage in a single nursing might have only a slight immediate impact on prolactin levels, but: may influence prolactin levels more substantiall y if increased for a period of 24 hr. (C) 1999 Elsevier Science Inc. All ri ghts reserved.