Because of welfare concerns and increased labor efficiency, calves are incr
easingly housed in groups. To reduce variability in live weight within grou
ps, farmers frequently regroup calves according to growth rate. We assessed
the consequences of repeated regrouping and relocation on the welfare of 3
2 male Holstein calves housed in pairs. Animals of half of the pairs (regro
uped 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 functio
ning of the hypothalamic-pituitary-adrenal axis and of the sympathetic nerv
ous system were assessed. Calves were weighed once a week, their health was
assessed daily, and abomasa were inspected when the calves were slaughtere
d. Calves reacted to the first mixing by interacting with the new partner a
nd increasing their general activity (sniffing the partner in regrouped cal
ves 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 reloca
tions were completed, regrouped calves were more active at the end of the d
ay and less active at night (P < 0.05). Cortisol responses to exogenous ACT
H were higher in regrouped calves (integrated response: 6,688 vs 5,508 ng.m
in/mL, P < 0.01). Basal cortisol levels, ACTH responses to corticotropin-re
leasing hormone, activities of catecholamine-synthesizing enzymes (tyrosine
hydroxylase and phenylethanolamine N-methyl transferase), and the incidenc
e of health problems and growth rates did not differ between the two groups
. Regrouped calves had fewer abomasal ulcers. Apart from the increased sens
itivity of the adrenal cortex of regrouped calves to ACTH and the modificat
ion in the daily rhythm of activity, there was no clear evidence that repea
ted regrouping and relocation stresses calves. Aggression between calves wa
s rare, and calves seemed to habituate to repeated mixing.