Effects of group size on social behavior following regrouping of growing-finishing pigs

A total of 480 growing-finishing pigs in three blocks were used in a 12-week study to quantify the effects of group size (10, 20, 40, and 80 pigs per pen) on aggression, aggression-related injuries, behavioral patterns, and the relationships among aggression, injuries, and growth rate. Pigs were regrouped at 8 weeks of age (BW=23.2±4.8 kg). Aggression of pigs during the initial 8 h post-regrouping was observed by live observation in two blocks. Injury scores were assessed on individual pigs 48 h after regrouping in three blocks. Total duration of aggression for the 8 h post-regrouping was similar among group sizes and averaged approximately 14 min per pig. The total number of fights per pig in the groups of 10 pigs was lower (14.9±0.94, P<0.05) than for groups of 20, 40, and 80 pigs (18.9, 18.5, and 19.4±1.33, respectively). When group size increased to 40 pigs, the average number of naïve fighting pairs (neither pig had previously fought) increased significantly (P<0.05). However, the ratio of total pigs in the pen to the number of naïve fighting pairs was similar among group sizes. The longest duration of fights and most number of fights occurred in the first 2 h post-regrouping. The proportion of dyads that were not observed to fight during the 8 h post-regrouping was higher in groups of 40 and 80 pigs (70.0 and 80.6%, respectively) than in groups of 20 pigs (51.3±4.0%, P<0.05) and groups of 10 pigs (35.0±2.9%, P<0.05). Across all group sizes, dyads that were observed to fight for the longest time had a lower percentage BW difference than dyads that fought for short durations (11.3 versus 15.9±1.02%, P<0.05). Total injury scores, which were positively correlated with aggression, were similar among group sizes. Although initial BW was positively correlated with the number of fights, the duration of fights and total injury scores, the overall average daily gain (ADG) was not correlated with aggression and injury scores post-regrouping. In conclusion, pigs in large groups did not evidence a higher level of aggression, which would have been required to establish a dominance hierarchy based on dyadic interactions. The commonly held fears of increased aggression, its consequent injuries, and decreased productivity in large groups were not found in this study.