Behavior and performance of pigs previously housed in large groups.

A study was conducted to evaluate the effects of social familiarity and initial BW variation at mixing on performance and welfare of pigs born to group-housed lactating sows. A total of 180 pigs from 24 litters were used in a random design with 4 treatments in a 2 x 2 (social familiarity x initial BW uniformity) factorial arrangement. Pigs were born in group-farrowing rooms where they mingled in large groups of 66 to 80 pigs from 10 d of age. At 8 wk of age (BW=23+/-3.1 kg), pigs were allocated to 20 pens of 9 pigs (5 castrated males and 4 females) in a grow-finish room, with 5 pens assigned to each of 4 treatment combinations without consideration of relatedness. Familiar groups consisted of pigs from 1 farrowing room, and unfamiliar groups consisted of 3 pigs from each of 3 different farrowing rooms. Uniform BW groups were formed by using the middle 2 quartiles, and variable BW groups by using the heaviest and lightest quartiles of pigs. Aggression and activity behavior were directly observed by scan or continuous sampling during a period of 4 h on the first 3 d, d 7, and d 14 after grouping. Injury scores were assessed on all pigs immediately before and 48 h after grouping. Body weight gain and apparent feed intake were measured every 2 wk for 14 wk. Aggression in familiar groups was minimal throughout the observation periods. Compared with that in familiar groups, total duration of fighting was greater in unfamiliar groups on d 0 (upon grouping, 48.5 vs. 0.5+/-10.88 s/pig(-1).4 h(-1); P<0.001) and on d 1 (10.8 vs. 0.4+/-3.24 s/pig(-1).4 h(-1); P<0.05) after grouping. Unfamiliar pigs had greater injury scores (6.6 vs. 1.8+/-0.28; P<0.001) and spent less time eating on d 0 (5.1 vs. 8.8+/-0.92% of total observation time; P<0.01) after grouping compared with familiar pigs. The ADG and ADFI were decreased in unfamiliar groups during the initial 6 wk, but not for the entire 14-wk period in comparison with familiar groups. Body weight variation did not affect behavior and performance in familiar groups but increased aggression-induced injuries in unfamiliar groups. The results indicated that grouping unacquainted pigs derived from large groups induced overt aggression, associated injuries and initial reduction in performance, which causes welfare concerns on mixing pigs born to group-housed lactating sows.