Provision of active ventilation via solar-powered fans to outdoor hutches during summer improves immune responses of dairy heifers.

Despite the preweaning period being critical for immune system development, heat abatement for calves in summer is often overlooked during this early-life period. Herein, we evaluated selected markers of innate and adaptive immunity development in outdoor hutch-housed Holstein dairy heifers with 2 types of ventilation during a Midwestern summer. Heifer calves were housed in outdoor hutches with passive (PASS; 0.07 m/s natural air speed, n = 16) or active ventilation (ACT; 1.1 m/s air speed via solar-powered fans, n = 16) from birth to 28 d of life. At d 28, fans were turned off, and all heifers were naturally ventilated. Heifers were weaned at 49 d and monitored until 56 d of age. Blood samples were collected at various stages of the preweaning period (d 1-56) to assess IgG, glucose, blood hematology, gene expression, and functional assays. At birth, circulating IgG, glucose, and hematology parameters were not different between groups. Relative to heifers exposed to PASS, ACT heifers had reduced white blood cell, monocyte, neutrophil, and lymphocyte counts, particularly the first 28 d. The ACT heifers had a greater percentage of neutrophils with phagocytic activity on d 28 and 42 of life, and tended to have neutrophils with greater percentage of oxidative burst on d 42. The mRNA expression of haptoglobin, IL-6, IL-8, IFN-γ, and NFKβ1 was downregulated in peripheral blood leukocytes of PASS heifers, relative to ACT. Providing active ventilation to heifers during the first 4 wk of life in summer enhances neutrophil phagocytic capacity, reduces circulating neutrophil and lymphocyte counts, and downregulates the expression of key pro-inflammatory cytokine and interleukin genes in peripheral leukocytes.