Abstract
1. The objective of the study was to evaluate the potential effects of dietary probiotic supplementation and stocking density and their interaction on growth performance, humoral and cellular immune responses of broiler chickens raised under hot climate conditions.2. A total of 1800 1-d-old unsexed broiler chicks (Ross 308) were allocated in a completely randomised 3x2 factorial design, with three concentrations of dietary probiotic (0, 200 and 400mg/kg) containing 4x10(9) cfu/g of Bacillus subtilis and two stocking densities (12 or 18 birds/m(2)) in a total of 6 treatment subgroups.3. Dietary probiotic and stocking density had no significant effect on live body weight, feed consumption and feed conversion ratio.4. Dietary probiotic supplementation had a positive effect on serum immunoglobulin M (IgM) and cell-mediated immunity when compared to the control at 6weeks of age, whereas serum immunoglobulin A (IgA) and immunoglobulin Y (IgY) were improved, but not significantly so. Dietary probiotic and stocking density had no effect on relative weights of the bursa of Fabricius and spleen.5. Dietary probiotic supplementation had a positive significant effect on serum IgM concentration and cell-mediated immunity without penalising growth performance in broilers raised under high ambient temperature.