Effects of Cyclic Heat Stress on Blood Cell Profiles and Antibody Concentrations in Broilers Selected for Low Water Conversion Ratio

Heat stress is a major environmental challenge in poultry production that reduces growth, welfare, and immune function. As global temperatures rise, understanding how genetic selection influences responses to heat stress (HS) is critical for sustainable production. Broilers selected for low water conversion ratio (LWCR) represent a potential strategy for improving resource efficiency; however, their immune response capabilities under HS remain unclear. This study evaluated the effects of cyclic HS on hematological parameters and immune function in LWCR and modern random-bred (MRB) broilers. Birds were reared under thermoneutral conditions (24 °C) or exposed to cyclic HS (36 °C for 8 h daily from 29 to 42 days of age). At 42 days, blood samples were collected and analyzed for total white blood cell concentrations, concentrations of various leukocyte populations, and plasma immunoglobulin levels. Cyclic HS reduced white blood cell concentrations, including lymphocytes, heterophils, T cells, and B cells in both genetic lines, while monocyte concentrations were unaffected. Plasma IgG concentrations were decreased under cyclic HS, whereas IgM and IgA concentrations were not affected. These findings indicate suppression of both hematological and immune functions. Responses to cyclic HS were largely consistent between LWCR and MRB broilers, with minimal differences between lines. These results suggest that genetic selection for water-use efficiency does not impair immune responses under cyclic HS conditions. Overall, cyclic HS broadly suppresses physiological and immunological functions in broilers, and selection for improved water efficiency can be achieved without compromising immune competence.