Does repeated regrouping alter the social behaviour of heifers?

Regrouping of cattle according to age, live weight or production stage is a common practice on farms. Our study aimed at determining how repeated regrouping modifies the social behaviour of dairy heifers, and more specifically if dominance relationships are formed more rapidly when animals become more familiar with regrouping. Thirty-two Holstein heifers were housed in pairs. From the age of 11 months, half of the heifers were placed in a new pen with a new penmate once or twice weekly for a total of 16 times over 11 weeks (regrouped heifers), while the other half remained in the same pen with the same penmate (controls). The heifers were monitored (a) for 3 h immediately after the 2nd, 7th, 13th and 16th regrouping, (b) for 24 h before the 1st regrouping, and after the 5th, 12th and 16th regrouping and (c) during a social confrontation test (with one animal for each treatment placed together in an arena) just before the 16th regrouping. Regrouping consistently induced agonistic interactions. Heifers exchanged the fewest agonistic interactions on the 7th regrouping and the greatest on the 16th. Dominance relationships were established most rapidly on the 7th regrouping (84 ± 20 min), but developed over the longest period on the 16th regrouping (158 ± 56 min). Observations for 24 h after the 16th regrouping showed that regrouped heifers moved and changed their activity more often than the controls. No treatment differences were observed in the social confrontation test. In conclusion, heifers do not habituate to regrouping and repeated regroupings have little effect on their subsequent social behaviour. An optimum of around seven regroupings appear to produce the fewest agonistic interactions and allow dominance relationships to be established more rapidly.