Abstract
Two trials were conducted to draw the phase-response curve of productive and immunological variables in heat-exposed layer chickens at different ages (71 to 130 d, and 211 to 270 d). Birds were acclimated to the following conditions for 60 d: constant optimal ambient temperature at 24°C and high ambient temperature at 34°C for 8 h/d (10:00-18:00). Data collection and biochemical measurements were performed every 10 d. In both age ranges, high temperature favored the innate immunity (P < 0.01) at the cost of performance (P < 0.05) during a given period, including the relative abundance of B and T-helper lymphocytes, lymphocyte proliferation ratio (B and T lymphocytes), and serum immunoglobulin contents (IgG and IgM) in the peripheral blood, as well as splenic expression of inflammation-related genes (iNOS, TLR-4, TNF-α, IL-6, and INF-γ). Compared with laying hens, growing pullets showed a time-delayed activation of immune response following heat challenge, and had no immunosuppression up to the end of exposure. Overall, the immune system of layer birds has a trade-off with production tissues in a hot environment, and exhibits distinct age-range-specific responses of acclimatization.