Because of welfare concerns and increased labor efficiency, calves are increasingly housed in groups. To reduce variability in live weight within groups, farmers frequently regroup calves according to growth rate. We assessed the consequences of repeated regrouping and relocation on the welfare of 32 male Holstein calves housed in pairs. Animals of half of the pairs (regrouped calves) were placed in a new pen with a new partner once a week for 14 wk. Animals of the other half of the pairs (control calves) stayed in the same pen with the same partner. Behavior was observed for the 3 h following four mixings and for 24 h after all relocations were finished. The functioning of the hypothalamic-pituitary-adrenal axis and of the sympathetic nervous system were assessed. Calves were weighed once a week, their health was assessed daily, and abomasa were inspected when the calves were slaughtered. Calves reacted to the first mixing by interacting with the new partner and increasing their general activity (sniffing the partner in regrouped calves vs controls: 5.5 vs 2.9, P < 0.01; percentage time stepping: 3.2 vs 1.3, P < 0.001). This effect disappeared by the ninth mixing. After all relocations were completed, regrouped calves were more active at the end of the day and less active at night (P < 0.05). Cortisol responses to exogenous ACTH were higher in regrouped calves (integrated response: 6,688 vs 5,508 ng x min/mL, P < 0.01). Basal cortisol levels, ACTH responses to corticotropin-releasing hormone, activities of catecholamine-synthesizing enzymes (tyrosine hydroxylase and phenylethanolamine N-methyl transferase), and the incidence of health problems and growth rates did not differ between the two groups. Regrouped calves had fewer abomasal ulcers. Apart from the increased sensitivity of the adrenal cortex of regrouped calves to ACTH and the modification in the daily rhythm of activity, there was no clear evidence that repeated regrouping and relocation stresses calves. Aggression between calves was rare, and calves seemed to habituate to repeated mixing.

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