Behavioural, endocrine and immune consequences of mixing in weaned piglets

Abstract

Mixing piglets at weaning increases plasma cortisol concentrations and agonistic behaviour. In contrast to what is observed in older pigs, studies failed to show any effect of social environment on other behavioural variables or on immune function. The lack of effect of mixing may not reflect an absence of stress, but rather the fact that the physiological effects of social reorganisation are masked by the much more important effects of diet change. The aim of this experiment was to evaluate the reactivity of piglets to mixing by dissociating social reorganisation from weaning in itself. For this purpose, the influence of mixing was investigated 5 days after weaning (day 0) in eight control (C) and eight mixed (M) female pigs. Salivary cortisol and behavioural activity were measured from day −1 to day 3. Blood lymphocyte proliferation was measured on days 0 and 3. Cortisol levels were increased after mixing and returned to basal values within 24 h. Blood lymphocyte proliferation was not affected. Mixing increased resting behaviour. Cortisol and behavioural responses were influenced by the social position of individuals in their new group. Piglets seemed to avoid conflicting encounters by diminishing the synchronisation of their activities with their new group. These results suggest that social reorganisation could be stressful for weaned pigs. However, piglets seem to develop behavioural strategies, which could explain the absence of long-term endocrine and immune consequences of mixing.