Welfare and Performance of Finishing Pigs Kept at Two Group Sizes on Ad Libitum vs. Restricted Feeding

This 2 × 2 factorial study examined the welfare and performance of finishing pigs at two group sizes (9 or 18 pigs) over 12 weeks. For each set of groups of either 9 or 18 pigs, half of the pigs in each group size were fed ad libitum, while the others received a mildly restricted ration. Treatments were assigned to 16 partially slatted floor pens in a randomized block design, with a floor space of 1.15 m2/pig. Except in Week 1, there were proportionally fewer pigs with ear (p = 0.020) and tail (p < 0.0001) bite marks in groups of 18 than in groups of 9. Ear bite marks declined over time in both group sizes (p < 0.0001). There was also a significant interaction between group size and week regarding severe bite marks on the ears (p < 0.0002). Tail bite mark prevalence increased over time in the smaller groups but decreased in the larger groups (interaction: p < 0.001). A higher proportion of pigs in smaller groups sought human contact in Weeks 1 and 6, but this measurement equalized by Week 10 (interaction: p = 0.008). There were proportionally more pigs with tucked tails in the smaller groups in Week 1 but not in later weeks (interaction: p < 0.0001). Group size did not influence pig cleanliness or locomotion disorders. Ad libitum (vs. restricted) feeding increased average daily gain (p < 0.001), feed intake (p = 0.002), and slaughter weight (p = 0.030). Results suggest better welfare in the larger than in the smaller groups.