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Effects of group size and social rank on welfare and performance of gestating sows in a group-housing system with floor feeding
This study was conducted to investigate the interactive effect of group size and social rank on welfare and performance of gestating sows group-housed in pens with floor feeding. Pregnant sows (n = 152, parity 1 to 6) were allocated to 4 large pens (26 sows/pen) and 8 small pens (6 sows/pen) at 35 d after mating. Both large and small pens provided the same floor space allowance (1.5 m2/sow). Aggressive interactions among sows during the initial 2 h and during the first meal after mixing were recorded. Rank indices were calculated for each sow based on outcomes of fights, and sows were categorized as high, middle, and low ranking within each pen. Sows were scored for skin lesions at 24 h and 5 wks after mixing. Salivary samples for cortisol analysis were collected from high ranking and low ranking sows during the same periods as assessment of skin lesions. Body weights before mixing, before the subsequent farrowing, and at weaning, litter size and weight at birth and weaning were registered for each sow. Data were analyzed using the Mixed and Glimmix procedures of SAS. There was no interaction of group size and social rank for any variable. Low ranking sows fought less frequently (9.3 vs. 20.7 fights/sow/2h, SE=3.17; P<0.001) than high ranking sows at mixing, but had similar skin lesions (P>0.10) at 24 h after mixing as high ranking sows. Low ranking sows sustained more skin lesions (P=0.01) than high ranking sows at 5 wks after mixing. Low ranking sows and high ranking sows entered gestation pens with similar body weights, but low ranking sows gained less weight (33 vs. 50 kg, SE=5.6; P<0.001), and had lower weights (250 vs. 268 kg, SE=5.9; P<0.001) before farrowing than high ranking sows. There was no difference in cortisol concentration between high and low ranking sows. Group size did not affect the number of fights per sow involved, but sows in large pens had more skin lesions (P<0.001) at 24 h and at 5 wks (P<0.001) after mixing than sows in small pens. Neither social rank of the sow nor group size affected litter size or litter weight at birth or at weaning. These results suggest that low ranking sows had poor welfare than high ranking sows in pens with the floor feeding system, as indicated by reduced weight gain and increased skin lesions.
Keywords:
Group size, Social rank, Sows