The effects of pregnancy and parity number on behavioural and physiological responses related to the welfare status of individual and group-housed pigs

Abstract Plasma free-corticosteroid concentrations, aggressive behaviour and motivation to socially interact and explore a novel environment were observed in 39 female pigs housed in either groups of 4–7 animals or tether stalls over 2 parities. Gilts in tethers in their first pregnancy had significantly higher free-corticosteroid concentrations compared to non-pregnant gilts housed in tethers and gilts housed in a group of 7 (4.1, 2.9 and 3.0, ng ml −1 , respectively). A significant difference was also found between free-corticosteroid concentrations of group- and tether-housed animals at the second pregnancy (2.6 and 4.7, ng ml −1 , respectively), but there were no effects of previous experience of tethers or parity number. The chronically elevated free-corticosteroid concentrations were associated with reduced immunological reactivity (72 and 48% reductions in IgM and IgG response at 8 days after challenge, respectively) in tether- compared to group-housed pigs. In standardized motivation tests, social interaction, but not exploration, was higher in tether-compared to group-housed sows in both first and second pregnancies. In tethers, a higher proportion of social interactions between adjacent animals were aggressive and a higher proportion of these involved retaliation (25 vs. 0%) compared to the social behaviour of group-housed animals. It was concluded that unresolved aggression in tether-housed pigs may result in a chromic stress response with adverse effects on their welfare status. The effect of the design of stall divisions on the nature and level of aggressive behaviour warrants investigation.