Segmented regression modeling of a phytogenic blend on growth and immune function in broilers: a dose–response study

This study investigated the dose-dependent effects of a phytogenic feed additive, on growth performance, immune response, hematology, and organ development in broiler chickens. A total of 300-day-old broiler male chicks (Ross 308) with the average weight of 41.2 ± 2.32 g were randomly allocated into 4 treatment groups (0.0, 0.25, 0.50, and 1.00 mL/L phytogenic blend in drinking water) with 5 replications per groups (15 birds per pen) in a completely randomized design. Growth performance showed a clear bell-shaped response, with the 0.50 mL/L dose optimizing feed intake, body weight gain, and feed efficiency compared to the control and the highest dose. Humoral immunity was significantly enhanced at intermediate doses, where antibody titers against Newcastle and infectious bronchitis disease peaked. Similarly, key immunoglobulin concentrations were maximized at intermediate to high BMX levels. Hematological profiles also improved, with hemoglobin and leukocyte counts increasing at mid-to-high doses. Key stress indicators, such as the heterophil-to-lymphocyte ratio, were most favorable at intermediate doses. The development of crucial lymphoid organs (bursa and thymus) and concentrations of serum proteins also peaked within this intermediate-to-high dosage range. Multivariate analysis of all systemic variables confirmed that the 0.50 mL/L dose yielded the most beneficial immunological and physiological profile, while the 1.00 mL/L dose showed diminishing returns or adverse effects. In conclusion, BMX induced significant, dose-dependent benefits, with 0.50 mL/L emerging as the optimal inclusion level for enhancing performance, immunity, and systemic health in broilers.