Innate immune responses and health of individually reared Holstein calves after placement into transition-pens 23 d after weaning.

Sixty-four Holstein dairy bull calves were all reared in individual calf-hutches and pens until they were randomly assigned to treatments of Grouped (pens of 3, n=36 calves) or Control (left in home hutch, n=8 calves) at age 68±2·3 d (body weight 74·9±1·5 kg). Blood was drawn at age 66, 70, 74 and 88 d for ex-vivo immunological and biochemical analyses. Calf starter intake was measured daily and individual body weights were measured at age 68, 78, and 89 (±2·3 sd) d. Grouped-calves consumed less starter (P<0·05), and weighed 6·4±1·99 kg less (P<0·05) than Control-calves by age 89 d. Group housing was a mild stressor, as evident by a transient suppression in neutrophil oxidative burst at age 70 d, but there was a lack of difference in the neutrophil:lymphocyte ratio of peripheral leucocytes and neutrophil expression of L-selectin at age 70 and 74 d. However, grouped-calves had elevated total peripheral leukocyte counts at age 70 d (P<0·05) and tended (P<0·10) to be greater at age 74 and 88 d. In addition, neutrophil phagocytosis of Escherichia coli increased (P<0·05) at age 74 d in Grouped-calves. These data indicate that moving calves into transition-pens with 3 calves per group decreases performance, but this may not be due primarily to extreme stress or disease. These data do indicate that it is important that calves have a competent immune system and any potential stressors are limited when they are moved into transition-pens because they are exposed to a wider diversity and (or) load of pathogens.