Effects of group size on agonistic behaviors of commercially housed growing pigs

Objective: this study was conducted to understand the effects of group size on the agonistic behaviors of pigs reared at differing pen densities. Methods: thirty groups of pigs (a total of 350 individuals) housed at low, medium, and high group density (5, 10, or 20 individuals in 6.0 m x 6.0 m pens) were consecutively observed for 10 h on days 30, 90, and 180 with the aid of video technology. Results: the frequency of vocalizations was lower at low group density and higher at high group density on all investigated days. Pigs housed at high group density showed significantly more agonistic behaviors than those at low group density. Conclusions: the study reveals a higher level of aggression in older pigs and at high group density. It is concluded that group density is a major cause of the observed agonistic behaviors.