Effects of changing milk replacer feedings from twice to once daily on Holstein calf innate immune responses before and after weaning1

The objectives of this study were to determine the effects of switching Holstein calves to once-daily feeding during the fourth week of life (24±2.3 d of age; once-fed n=22; twice-fed n=22) on innate immune responses, and to evaluate whether carry-over effects occurred when the calves were weaned during the seventh week of life. Peripheral blood samples were taken immediately before the change in feeding strategy (24 d of age) and at 27, 31, 45, 48, 52, and 66 d of age and were analyzed for circulating cortisol, haptoglobin, total leukocyte counts, neutrophil:mononuclear cells, and hematocrit percentage. Heparinized whole blood was also stimulated with lipopolysaccharide (LPS) for 24h and the concentration of tumor necrosis factor-α (TNF-α) in the supernatant was analyzed. Neutrophil L-selectin and β₂-integrin expression were analyzed by flow cytometry. Simultaneous neutrophil phagocytic and oxidative burst responses to a heat-killed Escherichia coli were quantified by dual-color flow-cytometry. Treatment (once-daily or twice daily feeding) had no effect on pre- or postweaning performance. Once-fed calves tended to have more circulating neutrophils at 27 d of age, greater expression of L-selectin on neutrophils at 31 and 45 d of age, and greater intensity of phagocytosis at 45 d of age. Once-fed calves secreted less TNF-α in LPS-stimulated whole blood cultures at 45 d of age compared with twice-fed calves and this tended to persist through the immediate postweaning period. None of the other immune parameters differed after weaning between the preweaning feeding strategies. Consolidating calf milk replacer into one feeding during the fourth week of life was likely a mild and acute stressor, as evidenced by transient neutrophilia in the absence of suppressed functional capacities of neutrophils. Future research should address the mechanism and immunological significance of the persistent decreased TNF-α response in once-fed calves.