Regrouping of unfamiliar pigs is a common management practice, which usually occurs at several stages of the pig’s life. Every time pigs are regrouped, a new social structure must be formed (Meese and Ewbank, 1973). This results in aggression in which pigs will fight intensely for approximately 2 h. Less intense fighting will continue to occur over the next couple of days, until a relatively stable structure or organization is formed.
The aggression that results is a serious animal welfare concern. This concern has prompted many studies to investigate methods of reducing the regrouping aggression.
Some examples that have not been very successful include masking odours, using sedatives (Luescher et al., 1990), providing hide areas (McGlone and Curtis, 1985; Luescher et al., 1990) and environmental enrichment (Schaefer et al., 1990; Arey and Franklin, 1995). Behavioural differences among pigs can be reliably determined using the human approach test. This study investigated the theory of ‘pre-forming’ a stable hierarchy based on behavioural composition. While in the nursery with littermates, 222 pigs were individually tested in two human approach tests and classified as either being slow, medium, or fast to approach a novel person. Pigs were then regrouped at eight weeks of age. Groups of 12 pigs, consisting of either all slow (uniform slow), all fast (uniform fast), or slow, medium, and fast (diverse) were formed. Intact litters were used as a control treatment. Subsequent aggression, average daily gain, and approach times were assessed. Regrouped pigs took longer to approach than pigs in intact litters (28.6 versus 9.25 ± 3.03 s; P = 0.02) during the final human approach test. There was less fighting per pig in the intact litters (0.45) compared with regrouped pigs (greater than 3.61 ± 0.45 s; P < 0.01). Pigs in the intact litters tended to grow faster than pigs in the uniform fast treatment (863 versus 805 ± 21.6 g/day; P = 0.09). The slow pigs in the intact litters, diverse and uniform-slow treatments all became faster to approach over time (P < 0.05), and did not differ in average daily gain. Whereas, fast pigs in the intact litters became slower to approach than fast pigs in the diverse treatment (49.4 ± 8.61 versus 3.4 ± 7.04 s; P = 0.02) and tended to take longer than the uniform-fast pigs (49.4 ± 8.61 versus 26.7 ± 7.04 s; P = 0.09). There were no differences in average daily gain among fast pigs in any of the treatments (P > 0.1). The human approach test is useful for distinguishing among individuals. Regrouping affects the ability to habituate over time. Furthermore, the type of regrouping strategy influences the ease of habituation. Pigs that are initially classified as slow, medium, or fast may habituate differently to the test. The pigs also have different growth patterns depending on whether they have experienced regrouping. Behavioural uniformity tended to result in more aggression and less weight gain, when compared to intact litters.