Aggression in group housed sows and fattening pigs

High levels or prolonged aggression continue to be a major welfare issue for commercial pig farming, although it is predominantly seen over the first few hours to few days after mixing unfamiliar pigs. Aggression is influenced by numerous animal, management and housing factors. This chapter covers the aggression of group housed sows during gestation and of pigs from the weaner to the finisher stages. After providing an overview of the basis of aggression and dominance hierarchy in pigs, the scientific knowledge and relative importance of various factors on the prevalence of aggression are reviewed: time after mixing, age and previous experience of the animal, group composition (familiarity, size), feeding regimen, feed delivery, and housing design (space, stocking density, environmental enrichment). The conclusions offer avenue for future research.