THE RELATIONSHIP BETWEEN TEAT STIMULATION, OXYTOCIN RELEASE AND GRUNTING RATE IN THE SOW DURING NURSING

Before milk let-down, the teats of the sow are stimulated by the piglets for ∼ 2 min. Thereafter oxytocin is released, causing a contraction of the alveolar myoepithelium and ejection of the milk. The typical grunting of the sow starts at a low rate, whereafter it suddenly increases and reaches its maximum rate during milk let-down. This experiment was carried out to study the relationships between the amount of teat stimulation given by the piglets, plasma oxytocin levels and the grunt rate of the sow. The external jugular vein of three primiparous individually housed Swedish Landrace sows was chronically catheterized. After farrowing, the litters were standardized to 8 piglets. On Days 1, 3, 7 and 14 after the day of farrowing, blood samples were taken during and between three consecutive successful sucklings. The blood samples were analysed for oxytocin by radioimmunoassay. The gruntings of the sow, and the massaging and sucking behaviour of the piglets, were also registered. As more piglets were stimulating the udder by massaging the teats, the time of stimulation before the grunt rate peak decreased. The shorter the pre-let-down stimulation time (more piglets massaging), the higher the grunt rate peak. In contrast, the integrated amount of grunting was not related to the pre-let-down stimulation time. Maximum oxytocin levels and grunt rate peak were reached at the same time. The amount of oxytocin released during sucklings was not dependent on either the massaging time or the number of piglets massaging. The integrated amount of grunting, but not the size of the grunt rate peak, was positively correlated with both the size of the oxytocin peak and the integrated amount of oxytocin released. The present data indicate that a certain amount of udder stimulation is needed to trigger oxytocin release and that oxytocin release and grunting may be controlled by a common central mechanism. © 1990.