| The experiment was conducted using a total of 1392 one-day-old, Cobb commercial male broilers to investigate the effects of a commercial probiotic onperformance, intestinal physiology and the microflora in the intestine of broilers. Broilers were randomly allotted by BW to 1 of 6 replicate cages (29 chicks per cage) for each of 8 treatments in a randomized block design involving one factorial arrangement of treatments. Dietary treatments included the basal diet (including com, soybean, rapeseed, cottonseed meal) added with 100 mg/kg antibiotic, 0%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, or 0.3% .The experiment last 42 d and divided into the starter period (1-21d) and the finisher period (22-42d). The results as follows: 1) The treatments with probiotic and antibiotic produced a greater BW,ADG and ADFI (P<0.05) than the control only on day 21. The effect of probiotic was lower than the antibiotic, but no significant difference was observed between them except for treatment 5 (P=0.019) and 7 (P=0.018). From 1 to 21 d, the ADFI of the probiotic was lower than the antibiotic (but treatment 8), however, there was significant difference only in treatment 3. In the first three weeks, there was a tendency to improve the feed to gain ratio (F/G) of treatment with probiotic and antibiotic. No significant differerce was observed for the performance of the chickens in the last three weeks. 2) The addition of probiotics shortened significantly the small intestinal length of broiler chickens. Compared with the control and the antibiotic, the relative length of duodenum (P<0.001), jejunum (P=0.008), ileum (P<0.001) and the whole small intestine (P<0.001) of probiotics were extremely significantly decreased on day 21. Though, only the length of jejunum (P=0.007) was decreased on day 42. 3) No significant effect was observed on villus height, crypt depth and the ratios of villus height to crypt depth (V/C) in the three segments of small intestine (duodenum, jejunum and ileum). There was just a numerical tendency to improve these parameters. 4) The amylase and lipase activity in the duodenum of treatment with probiotic and the the amylase activity in the jejunum significantly enhanced (P≤0.001) . No significant difference was observed of the amylase and lipase activity in the jejunum for the treatment wihe probiotic relatively to the antibiotic (P>0.05) . The addition of probiotics enhanced the content of the acetic acid at 7d, propanoic acid at 20d,butyrate at 7d,20d and 41d and the relatively to the control and antibiotic. No significantl difference was observed on the content of acetic acid and propanoic acid at 41d.5) Compare with the control, there was no significant difference to be observed in the number of the bacteria with the addition of probiotic, but the populations of the lactobacilli and coliform were significantly (P<0.001) decreased with the addition of antibiotics. However, no significant difference was observed in the number of lactobacilli (P=0.180) and coliform (P=0.210) of broilers fed different diets at 42d. 6) The variety of the microbiota increased as the age increased, the addition of probiotics could increase the variety of the microbiota, while the addition of the antibiotic decreased the variety of the microbiota. And it was more obvious at 7d,20d than at 41d. In conclusion, the results indicated that the probiotic was able to improve the growth performance of broiler chickens and the microflora, with the increase of the amylase and lipase activity, the content of butyric. That was more obvious in the starter period than in the finisher period.
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