Pig's responses to repeated social regrouping and relocation during the growing-finishing period

In animal husbandry, pigs have to cope with social challenges when regrouped with unfamiliar conspecifics. Regrouping pigs may result in an acute stress and physical injuries thus reducing animal welfare. The aim of the present experiment was to evaluate the behavioural, cortisol and growth responses of pigs to a repeated social stress. Thirty-two growing-finishing pigs were housed in pairs. Animals of half of the pairs were placed in a new pen with a new partner three times a week during 4 weeks (R pigs, n = 8 pairs). Other animals stayed in the same pen with the same partner (C pigs, n = 8 pairs). During the first 90 min following each regrouping, vigorous fights were frequent in R pigs. Those pigs exhibited more agonistic behaviour than C pigs throughout the experiment (P < 0.05). The number of offensive acts realized by R pigs decreased throughout the experiment (P < 0.05) whereas the number of defensive acts did not vary (P > 0.1). The time-budget determined during 24 h 2 days after the third, sixth, ninth and 12th regrouping was similar in R and C pigs except the percentage of standing that was higher in R pigs after the last regrouping. R pigs tended to have a lower daily liveweight gain at the fifth regrouping (P = 0.07). Salivary cortisol concentrations were higher in R than in C pigs 4–5 h after the second and fifth regrouping (P < 0.05) but were similar in both groups after the ninth and 11th regrouping. Basal cortisol measured 2 days after the last regrouping in plasma and saliva was higher in R than C pigs. In response to a dexamethasone/ACTH challenge, cortisol in saliva and plasma was similar in both groups (P > 0.05). The behavioural and cortisol responses to regrouping and relocation in pigs decreased over time but were still obvious after 12 mixings. Repeated regroupings induce a chronic stress in pigs without altering severely growth rate.