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.

Introduction

Several practices occurring in modern pig husbandry can affect the animal’s health and welfare. In growing or adult pigs, mixing of unfamiliar animals induces fighting (Rushen and Pajor, 1987, Moore et al., 1994, Otten et al., 1997), elevates heart rate (Otten et al., 1997, de Jong et al., 2001), plasma catecholamines (Otten et al., 1997, Ruis et al., 2001), body temperature (de Jong et al., 1999) and plasma cortisol (Moore et al., 1994, Deguchi and Akuzawa, 1998, Ruis et al., 2001) and reduces growth rate (Rundgren and Löfquist, 1989). Social reorganisation can also have immune consequences. It affects skin response to antigens (Moore et al., 1994), increases neutrophils/lymphocytes blood ratio (Moore et al., 1994, Ruis et al., 2001) and suppresses peripheral blood mononuclear cell proliferation (Hessing et al., 1995, Deguchi and Akuzawa, 1998, de Groot et al., 2001). Physiological consequences are usually transient, but there could be long-lasting behavioural alterations in defeated animals (Ruis et al., 2001).

In commercial piggeries, weaning often combines several stressful events such as a sudden change of diet, a move to a new housing environment, the mother-young link disruption and mixing with unfamiliar piglets. These sudden changes induce behavioural, endocrine and immune alterations (Blecha et al., 1985, Puppe et al., 1997, Carroll et al., 1998, Worobec et al., 1999) and are followed by a transient reduction of growth and digestive disorders (Madec et al., 1998, Carroll et al., 1998). Different stress factors involved in weaning including new housing environment (Metz and Gonyou, 1990, Puppe et al., 1997, Patience et al., 2000) and social reorganisation (Friend et al., 1983, Blecha et al., 1985, Puppe et al., 1997) have been studied. Studies showed that mixing piglets at weaning increases plasma cortisol concentrations and agonistic behaviour (Friend et al., 1983, Blecha et al., 1985, Puppe et al., 1997), but they failed to show any effect of social environment on other behavioural variables or on immune function (Blecha et al., 1985, Puppe et al., 1997). At weaning, the physiological effects of social reorganisation could be masked by the much more important effects of the diet change. In other words, the lack of effect of mixing would not mean that it is not perceived as a stressful event. 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, we investigated the influence of mixing 5 days after weaning, when most of the acute physiological adjustments of weaning have disappeared (Metz and Gonyou, 1990, Puppe et al., 1997, Carroll et al., 1998). The response to mixing was assessed by measuring salivary cortisol, blood lymphocyte proliferation, interferon-γ production and behavioural activity.