Four farms that group-housed sows from about 2 weeks of lactation until weaning (G-farms) and 3 farms, used as controls, that kept the sows individually penned throughout the 5 to 6-week-long lactation period (C-farms) were compared in terms of pre- and post-weaning piglet growth rate and mortality, sow food intake and change in backfat thickness. Piglets from 169 G-farm sows and 136 C-farm sows were individually weighed at the time of grouping and weaning. In addition, some of the piglets were weighed 2 weeks post weaning. Piglet mortality was recorded during the pre- and post- weaning periods. Sow backfat thickness was measured at the time of grouping (at a corresponding time in the C-farms) and weaning, and sow food consumption was determined during the group-housing period. Piglet weight, growth rate and within-litter variation in growth rate did not differ significantly between the two groups during the group-housing and postweaning periods. However, the pre- weaning growth rate varied considerably between farms. For multiparous sows during the group-housing period, piglet mortality was higher (p = 0.002) in the G-farm group (6.5%) than in the C-farm group (1.4%). However, for primiparous sows the corresponding piglet mortality was similar (p = 0.21) in the two groups. Significant between-batch variation in mortality during the group-housing period was noted within the G-farms but not within the C-farms. At the time of weaning, backfat thickness tended (p = 0.09) to be higher in the G-farm group than in the C-farm group. For primiparous sows the decrease in backfat thickness was similar (p = 0.37) in the two groups. By contrast, multiparous G-farm sows gained backfat during the group-housing period, whereas multiparous C-farm sows lost some backfat (p = 0.02). G-farm sows consumed 23% more food than C-farm sows during the group-housing period. These results indicate that productivity is lower in the group-housing system, mainly owing to the poor performance of the older sows.
