| Summary: | This study was conducted to investigate the effects of supplementing <i>Bacillus subtilis</i> and an antibiotic (Zinc bacitracin) in the diet of broilers on growth performance, organ weight, blood metabolites, and digestive enzymes of broiler chickens. A total of 600 1-d Arbor Acres broilers were randomly allotted to five treatments. Each treatment consisted of six replicates with four pens, and each pen had five birds. The chicks were fed (1) the basal diet (control), (2) the basal diet with 500 mg/kg Zinc bacitracin (APZ), (3) the basal diet with <i>B. subtilis</i> at 1 × 10<sup>8</sup> CFU/g (<i>B.Sut</i>-1), (4) the basal diet with <i>B. subtilis</i> at 3 × 10<sup>8</sup> CFU/g (<i>B.Sut</i>-3), and (5) the basal diet with <i>B. subtilis</i> at 5 × 10<sup>8</sup> CFU/g (<i>B.Sut</i>-5). The experiment lasted for 42 days. In this study, the supplementation of diets with <i>B. subtilis</i> (<i>B.Sut</i>-3 and <i>B.Sut</i>-5 groups) increased body weight gain from 1 to 21 days compared with control (<i>p</i> < 0.05). Additionally, the <i>B.Sut</i>-3 group had a significantly heavier bursa of Fabricius than control at 21 days (<i>p</i> < 0.05). Serum total protein, albumin, and high-density lipoprotein concentrations were increased in <i>B.Sut</i>-5 and APZ groups (<i>p</i> < 0.05) over the whole period. Serum low-density lipoprotein, very low-density lipoprotein, triglyceride, and total cholesterol concentrations were decreased in <i>B.Sut</i>-5 and APZ groups at 21 and 42 days (<i>p</i> < 0.05). Chicks in the <i>B.Sut</i>-5 and APZ groups had higher serum lipase, pepsin, and amylase activities (<i>p</i> < 0.05) at 21 and 42 days. From the results obtained from the study, it can be concluded that <i>Bacillus subtilis</i> ATCC19659 at 5 × 10<sup>8</sup> CFU/g could be applied as an alternative to antibiotics in poultry diets.
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