1. An experiment was conducted to study the effects of different Bacillus subtilis concentrations on productivity, carcase quality, immune response and the gut microflora of broiler chickens. 2. There were 5 treatment groups: control, with no added Bacillus subtilis supplementation; and 4 treatment groups receiving feed supplemented with different concentrations of B. subtilis. 3. Weight gain was significantly higher and the feed conversion rate was significantly better in all groups receiving feed supplemented with B. subtilis, regardless of its concentration. The groups given B. subtilis supplementation produced relatively bigger breasts, and smaller carcases and thighs, compared with the control group. 4. The appearance of increased diffuse lymphohistiocytic infiltration and solitary lymphoid follicles in the mucosa, and a stronger response to NDV vaccination, indicate increased immunological responses in chickens fed with the B. subtilis supplemented diet. 5. The higher inclusion rate of B. subtilis did not increase Lactobacillus concentrations in the ileum or in the caecum, but decreased the E. coli population significantly.

This trial was conducted to study the effects of different Bacillus subtilis concentrations on immune response of broiler chickens. There were 5 treatment groups: control, with no added B. subtilis supplementation, and 4 treatment groups receiving feed supplemented with different concentrations of B. subtilis. The trial was conducted with 225 broilers. The weight of broiler chickens in all groups receiving feed supplemented with B. subtilis was significantly higher and the feed conversion was better independently of the concentration than that of the control chickens. The degree of diffuse lymphohistiocytic infiltration and the number of solitary lymphoid follicles in the mucosa increased in accordance with the B. subtilis concentration of the feed. The birds from the groups fed B. subtilis-supplemented diets had significantly increased antibody responses to vaccination against Newcastle disease virus. The appearance of increased diffuse lymphohistiocytic infiltration and solitary lymphoid follicles in the mucosa and a stronger response to NDV indicate increased immunological response in chickens fed with a B. subtilis-supplemented diet.