Influence of Group Size on Performance and Tail Damage Caused by Tail Biting in Undocked Growing-Finishing Pigs

Abstract Tail biting is both an economic and welfare problem of grower-finishing pigs. This destructive chewing behavior of tails of pen-mates causes challenges for farmers who raise pigs with intact tails, resulting in the need to find management solutions. Our objective was to determine if group size influences growth performance and tail damage caused by biting behavior of undocked growing-finishing pigs. Pigs (n = 432; initial BW = 22 ± 3.8 kg) with intact tails were allocated to small pens or large pens on totally slatted floors, with equal ratio of barrows to gilts in each pen and balanced for average initial BW across pens. Small pens (SM; n = 24) housed 9 pigs/pen and large pens (LG; n = 12) held 18 pigs/pen. Floor space allowance (0.76 m2/pig) and feeder-space allowance (4 feeder spaces/9 pigs) were consistent across group sizes. Pigs were weighed individually and feed intake was recorded on a pen basis every 4 weeks for 16 weeks until market weight (120 kg ± 3.8 kg), from which ADG, ADFI, and Gain:Feed were calculated. Number of pigs that were dead (mortality) and removed (morbidity) from the study due to tail biting, lameness, and other health issues were recorded throughout the study period. Tail damage was evaluated using a 0 to 4 scale scoring system: 0 = no evidence of tail biting; 1 = evidence of chewing without visible blood; 2 = evidence of tail bitten with visible blood and no signs of infection; 3 = open wounds or abscesses with signs of infection; 4 = partial or total loss of the tail. Tail damage was assessed once weekly during the study period, and the maximum tail score (MTS) that each pig received over the entire 16 weeks was recorded. Data were analyzed using SAS software, with a mixed procedure for performance variables and a frequency procedure with chi-square test for mortality, morbidity, and MTS. Group size did not affect growth performance (all P > 0.70; Table 1), mortality, or morbidity (Chi-square = 2.33, df = 2; P = 0.31). However, group size affected MTS (chi-square = 10.78, df = 4; P = 0.03). Pigs in SM experienced a marginally lower prevalence of tail biting compared with LG (16% vs. 9% of pigs receiving an MTS of 0); but greater severity with 13.0% of pigs in SM vs. 7.4% of pigs in LG receiving an MTS of 4. These results suggest group size did not influence growth performance, mortality, or morbidity; but affect severity of tail damage caused by tail biting. Compared with LG, SM had more pigs that lost their tails due to tail biting, which seriously compromised pig welfare.