Effect of regrouping on social behaviour and milk production of mid-lactation dairy cows, and individual variation in aggression

Dairy cows are often mixed into new social groups for management reasons, but this is recognized as a cause of social stress. The objectives of this study were to investigate the effect of regrouping on social behaviour, self-licking and milk production of mid-lactation cows introduced in pairs, and to see whether individual variation in aggression is consistent before and after regrouping. In 7 replications (n=7), 14 mid-lactation cows were introduced in pairs into an established group of filler (resident) cows. After regrouping, agonistic contacts and displacement of the introduced cows increased during the first 3 and 2 days, respectively. Compared to baseline (the day before regrouping), the number of social licking events between the introduced cows and the resident cows in the pen did not change after regrouping, but the proportion of social licking between the two introduced cows increased sharply after regrouping (38 ± 9 %) compared to baseline (10 ± 9 %). Duration of social licking decreased declined after regrouping; whereas, self-licking increased on the day of regrouping. Compared to the resident cows, milk production of the introduced cows significantly decreased on the first 2 days after regrouping, and showed a negative linear association with agonistic contacts received and with displacements lost. Two measures of aggressive behaviour (proportion of agonistic contact initiated and proportion of displacements won) were relatively consistent before and after regrouping (R² = 0.75 and 0.68 respectively), suggesting that the differences reflected individual differences in aggressiveness, rather than social status within a given group. In the present experiment, low-and high-aggressive individuals were not different in milk production, social licking and age at first calving, but low-aggressive cows had higher 305-day projected milk production (12,928.0 ± 580 kg) than high-aggressive cows (10,530.0 ± 530 kg). Individual variation in aggression was not associated with body weight, although the heaviest cow in the group won all encounters before and after regrouping. The findings of this study provide the first insights that introducing cows in pairs may mitigate the effects of social stress during mixing.Land and Food Systems, Faculty ofGraduat