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

It has been established that sows up- or down-regulate their milk production as the frequency of nursings is changed. The amount of udder massage by piglets might also influence milk production. To investigate whether these effects are associated with changes in prolactin or insulin levels, we enforced five sows each to nurse either every 35 min (MIN35) or every 70 min (MIN70) over a 26- to 28-hr period. Milk production was measured during the first 24 hr of this period. During the last three to four nursings, blood was collected every 5 min. Plasma prolactin levels increased after milk ejection (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 < 0.05) concentrations of insulin than MIN70 sows. There were no differences between 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 when milk production of a sow is changed by altering the nursing frequency within 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 substantially if increased for a period of 24 hr.

The role of nursing frequency in milk production in domestic pigs.

We conducted three experiments to assess how nursing frequency affects milk output in early pig lactation. In Exp. 1, nursing behavior of 12 individually penned sows was recorded on d 1 through 3 postpartum. The milk output was measured using the weigh-suckle-weigh method. The spontaneous changes in the nursing frequency between d 1 and 2 were positively correlated with the changes in milk output (rS = .64), and the same was true for changes between d 2 and 3 (rS = .77). In Exp. 2, we forced sows, 7 to 8 d after parturition, to nurse every 35 min (10 sows) or every 70 min (eight sows) for 24 h. Sows nursing at short intervals had more nursings without milk ejection, but they gave 27% more milk and their litters gained 44% more weight during the experimental 24 h than sows nursing at long intervals. Litters nursed at long intervals massaged the udder longer after milk ejection. In Exp. 3, milk output was measured after intervals of 35, 50, and 70 min imposed in varied order on 11 experimental sows in the second week of lactation. In a separate recording of two 50-min nursings, we measured milk output from teats that were, or were not, sucked at the first nursing (i.e., giving within-sow comparison of 50 or 100 min). The milk output after 35, 70, and 100 min did not differ from that after 50 min, although there was a tendency for a slight increase with longer intervals. The results suggest that glands are refilled early after milk ejection and that the increase in milk available with prolonged intervals is only slight. As a result, it is the nursing frequency that plays a crucial role in adjusting the milk output.