The aim of the study was to investigate agonistic behaviour of pigs after regrouping pigs under commercial sow farm conditions. The behavioural patterns were observed over a 48-h period, directly after weaning and 40 days later. Agonistic interactions were analysed by noting the times (start and finish), the aggressor/receiver and the winner/loser of a fight. Differences in agonistic behaviour within and between the two age groups were recorded. The median number of fights per pig was 40.0 and 6.5, respectively, and varied between 0 and 139 fights. A circadian rhythm for the number of fights per pig and hour was shown for both age groups. A dominance index (DI) was calculated to examine possible relations between the agonistic behaviour shown and the dominance of an individual pig. The DI was defined as the ratio of wins minus defeats divided by the sum of wins, defeats and stand-off outcomes. Dominant pigs (DI > 0) were engaged in more agonistic interactions, had a longer total fight time and initiated more fights than subordinate pigs. Significant correlations were found between the overall fight time and the dominance index. Potential coherences between agonistic behaviour and the number of skin lesions were investigated with a lesion score (LS). The LS ranked from 0 to 4 and was determined at the beginning and at the end of the observation period. The analyses of the LS showed significant preferences for the front third of the body. However, unambiguous findings were only made concerning the older age group. Growing pigs with an increased LS after 48 h had more fights per pig, a longer overall fight time and initiated more fights (p < 0.05) compared to pigs showing a lower or unchanged LS. In conclusion, the applied dominance index and lesion score are feasible methods to analyse fighting behaviour under commercial sow farm conditions. Potential relationships between agonistic behaviour and growth or reproductive performance should be analysed in further investigations. It might be beneficial to consider agonistic behaviour in pig breeding. In this context, the presented approaches are simple tools to measure agonistic pig behaviour.

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