PS6-3.Social Network Analysis of Tail-biting Interactions Among Growing-finishing Pigs with Intact Tails Housed in Two Group Sizes

Tail-biting compromises the welfare and performance of growing-finishing pigs. Understanding the dynamics of tail-biting in pigs can help us develop management strategies to minimize outbreaks. We evaluated the effect of group size on tail-biting and the differences in social positions between biters and non-biters, and between barrows and gilts using social network analysis. Pigs (n = 315, initial weight=22.1±3.76 kg) with intact tails of mixed sexes were housed in small (9 pigs/pen) or large (18 pigs/pen) pens on slatted floors for 14 weeks until market weight (120.2±11.47 kg). Floor and feeder space allowances were identical across group sizes. Growth performance was recorded throughout the study. Tail injury was evaluated weekly to monitor tail-biting outbreaks (TBO). We declared a TBO when at least one pig in a small pen or two pigs in a large pen with blood on the tail. Behavior of pigs was recorded continuously using the NUtrack Livestock Monitoring System. Videos recorded the day before the first TBO were reviewed manually to document tail-biting events and the roles of pigs involved. The top 25% of pigs responsible for the most tail-biting incidents were classified as tail-biters and others as non-biters. Network metrics were calculated using RStudio. Data were analyzed using the Glimmix, NPAR1WAY, and Correlation procedures of SAS. Pigs in small pens had higher density, reciprocity, out- and in-degree centrality, betweenness centrality, and closeness centrality (all P < 0.05), suggesting that a higher percentage of pigs were involved in tail-biting both directly and indirectly than pigs in large pens. Gilts were victimized less frequently (P = 0.02) and had higher out-degree centrality (P < 0.01), suggesting that gilts initiated more tail-biting than barrows. Gilts and barrows had similar in-degree centrality, indicating they were bitten equally. In gilts, but not in barrows, biters tended to have lighter final weight (115.1 vs. 118.1 kg; SE = 1.53; P = 0.08) and lower ADG (0.92 vs. 0.95 kg, SE = 0.02; P = 0.10) and were less injured by tail biting than non-biters. In both sexes, biters had higher out-degree centrality and betweenness centrality (all P < 0.05) than non-biters. There was no difference in in-degree centrality between biters and non-biters, suggesting that biters were bitten equally as non-biters. Density and out-degree centralization tended to be associated with the percentage of pigs victimized by tail-biting (r ≥ 0.32; P ≤ 0.10). Weighted in-degree and binary closeness centrality were correlated weakly with victimization frequency (r = 0.15; P < 0.01) and latency to victimization (r ≥ 0.12, P < 0.01). These results suggest that pigs housed in different group sizes had different social structures and positions in tail-biting networks, which consequently can affect tail damage. Further research is needed to evaluate how tail-biting network metrics may change in pigs housed across a wider range of group sizes and during multiple TBO.