Friends forever? Inter-individual distances in group housed lactating sows

Group housing of pregnant sows is common practice in the EU, but during lactation sows are usually housed individually. However, if sows would prefer to be around and interact with other adults during lactation, group housing would with respect to sow welfare constitute a more suitable practice. Therefore, as a first step, we studied the relative closeness between sows in a multi-suckling system (87 m2) for five sows and their litters that consisted of five individual farrowing pens (7 m2), and a communal lying (32 m2), dunging (13 m2) and feeding area (7 m2). In 8 batches (40 sows), the inter-sow distance (i.e. <1m apart, 1-5 m apart or >5m) was determined with 10-min scan sampling over 24 h on D-5 (one day after sows entered the system), D5, D12 (2 days before and, respectively, 5 days after the piglets gained access to the communal area) and on D24 of lactation. Sows were considered to be close if they were within 1m of each other. Association between each pair of sows was calculated by using the Index of Association (IA) of Martin & Bateson (2007), with an IA of 0 vs. 1 indicating no vs. complete association. The overall average IA was 0.08 (SD: 0.10) and the lowest and highest IA were 0 and 0.53, respectively. IA differed substantially between the 4 observation days and between the 8 batches. These results will be discussed in relation to day of lactation, and the health, behaviour and productivity of these sows.