A means to assess nursing efficiency in the pig: the study of the milk ejection reflex

We have recently devised a method for recording the intramammary pressure in conscious lactating sows. This technique allows for unambiguous detection of milk ejection, and permits correlation with hormone release and the behaviour of the sows and their piglets. Each milk ejection is characterized by an abrupt rise in intramammary pressure lasting 8-40 sec only. This is preceded 15-30 before by a rise in plasma concentrations of the hormone oxytocin. Milk ejections only occurred when the sow was nursing her litter, lying on her side and making her teats available. She usually grunted in a rhythmic manner, with a sharp increase in the frequency of grunts about 23 sec before milk ejection. Grunting ceased within 1 min thereafter. The whole litter had to be suckling to obtain a milk ejection, which occurred about 2-4 min after the onset of the initial phase of massage of the udders, and was coincident with a brief period of quiet suckling. Milk ejections occurred about every 45 min, i.e. only once per suckling period, eventhough the period could last over 15 min. They never occurred if the mother was not nursing, or if the piglets attempted to suckle too soon after a milk ejection. Out of 144 periods of suckling, over 20% of failures to eject milk were observed, in which case there was no oxytocin release in blood. Some "incomplete" sucklings happened with no apparent reasons, but their incidence was very high when the sow appeared to be disturbed.