Functional view on udder massage after milk let-down in pigs

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It has been suggested that the function of final udder massage during nursing in pigs is to increase and/or regulate milk transfer. Using the analogy between udder massage and the begging behaviour of nestling birds, we restate this suggestion as six alternative hypotheses. Final massage may smooth the nursing-to-nursing milk output fluctuations in individual teats (Hypothesis 1) or in the whole udder (Hypothesis 4). Massage may also equalize the differences in long-term milk intake between piglets (Hypothesis 2) or between whole litters (Hypothesis 5). It is also possible that the massage acts to amplify the differences in milk intake between piglets (Hypothesis 3) or between litters (Hypothesis 6). These hypotheses lead to different predictions about the dependence of massaging on milk intake or body weight gain, and the influence of massage on subsequent milk production. The predictions were tested using data on milk intake and massaging behaviour of 12 litters during the first 3 days of life. On Day 3, significant negative correlations were found between the massaging intensity of individual piglets in a given nursing episode and their weight gain during the two preceding nursings (which supports Hypothesis 1), between litter massaging duration in a given nursing episode and litter weight gain over the two preceding nursings (Hypothesis 4), between a piglet's gain since Day 1 and its massage intensity on Day 3 (Hypothesis 2) and between the growth of a litter until Day 3 and its massaging duration on Day 3 (Hypothesis 5). With one exception, none of the predicted influences of massage on production were substantiated. The results suggest that both short-term and long-term energy balance provide a negative feedback on the motivation to massage. Further research is needed on relationships between massage and subsequent milk production and on the impact of artificial selection regimes.