Housing pigs in large social groups: a review of implications for performance and other economic traits

Housing pigs in large groups is common, but the economic and animal welfare disadvantages have not been adequately addressed. The implications of group size (range 3-120 pigs) for growth performance are quantified using regression analyses of data from 20 earlier studies and almost 22,000 animals. The relationships between group size and immunocompetence, clinical health status, within-group variation in growth rate, carcass characteristics, the occurrences of vices and reproductive performance are reviewed. A significant, negative, approximately linear relationship in average daily gain (ADG) with increases in group size was recorded during the weaner (weaning to 30 kg) and grower (31-68 kg) stages. Weaner stage pigs showed a reduction in food intake that was not present during the grower stage. Consequently the efficiency of growth was compromised during the grower stage. No influence of group size on performance during the finisher stage (greater than or equal to 69 kg) was detected. None of the other traits measured were consistently and significantly influenced by changes in group size. A large group size may compromise the growth performance of young pigs, but the long-term consequences for other economically important traits is likely to be slight. (C) 2003 Elsevier Science B.V. All rights reserved.