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What determines the social rank of a sow in a group-housing system?
The welfare of a sow in a group housing system depends on her social rank, with low ranking sows sustaining more skin lesions than high ranking sows. This study was designed to investigate what makes a sow high or low ranking and how social rank affects reproductive performance of sows. Sows (n = 150, parity 1 to 9) were mixed in pens of 15 sows after weaning, with the frequency composition by parity remaining consistent across pens. Aggressive interactions were video-recorded during the first 4 h after mixing. Among the 15 sows in each pen, three groups of 5 were classified as high, middle and low ranking based on outcomes (won, lost, and unsolved) of aggressive interactions that each sow involved in. Individual weight, body condition, and backfat thickness of sows were recorded before mixing and before farrowing. Heart rate and fear response of sows were measured 5 to 6 wk after mixing. A fear score was given to each sow based on her response to approach of a human, with a score of 0 indicating the most fearful and 6 the lest fearful. Litter size and individual weight of piglets were recorded at farrowing and at weaning in a group-farrowing/lactation system. Data were analyzed using the Glimmix and Mixed procedures of SAS with sow ranking, parity, and their interaction as fixed effects. Compared to low ranking sows, higher-ranking sows were greater in parity (3.86 vs. 1.56, SE=0.22; P <0.01), heavier (257 vs. 205 kg, SE=6.1; P <0.01) at mixing, and less fearful (3.74 vs. 2.08, SE=0.52; P <0.01). High ranking sows had more total pigs born per litter (13.5 vs. 11.9 piglets, SE=0.58; P =0.04), more stillborn pigs (0.7 vs. 0.2 pigs/litter, SE=0.23; P <0.01), higher pre-weaning mortality of piglets (35.9 vs. 21.0%; Odds Ratio=2.10, CI=1.57 to 2.81), and smaller litter size at weaning (8.1 vs. 9.3 pigs/litter, SE=0.48; P =0.07). High and low ranking sows were not different in body condition, backfat thickness, heart rate, or body weight of piglets farrowed or weaned. These results suggest that parity, body weight, and fear response collectively determine the social rank of a sow. The effect of social rank on reproductive performance was confounded with sow parity, and the poor reproductive performance of high ranking sows as indicated by smaller litter size weaned was associated with greater parity than low ranking sows in the current study.
Keywords: group-housing, social rank, sows