Abstract: 【Objective】 The effects of different doses of Bacillus subtilis on the growth performance,intestinal mucosa morphology,serum biochemistry and immune indexes of nursery pigs were studied to explore the optimal dose of Bacillus subtilis in nursery pig breeding,and to provide a reference for the efficient application of Bacillus subtilis. 【Method】 Two hundred and forty nursery pigs with similar body condition were selected and equally divided into four groups with six replicates per group and ten pigs per replicate.Piglets in control group were fed a basal diet,and in the three experimental groups were fed a basal diet supplemented with 200,400 and 800 mg/kg Bacillus subtilis (the viable counts were 2.0×10⁹,4.0×10⁹ and 8.0×10⁹ CFU/kg),respectively.The experiment lasted for 60 days.At the end of the experiment,one pig was randomly selected from each replicates,and serum,jejunal and ileal samples were collected.Growth performance,intestinal mucosal morphology,serum biochemical and immune indexes of nursery pigs were measured. 【Result】 Compared with the control group,the addition of 400 and 800 mg/kg of Bacillus subtilis to the diets significantly increased the final body weight and average daily gain of nursery pigs in the experimental period (P＜0.05),and significantly reduced the feed-to-gain ratio (P＜0.05),significantly increased the villus height and the ratio of villus height to crypt depth of the ileum and jejunum of nursery pigs (P＜0.05),and significantly decreased the crypt depth of the ileum and jejunum (P＜0.05),significantly increased albumin,total protein,immunoglobulin A,immunoglobulin G,and interleukin-2 contents in serum of nursery pigs (P＜0.05),and significantly decreased tumor necrosis factor-α content of serum (P＜0.05).The addition of 200 mg/kg Bacillus subtilis had no significant effect on growth performance,intestinal mucosa morphology,serum biochemistry and immunity indexes of nursery pigs (P＞0.05) 【Conclusion】 Adding Bacillus subtilis to nursery pig diets could improve their growth performance,optimize the intestinal mucosal structure,and improve the level of protein turnover metabolism and immune function of the body.Taking all factors into consideration,the appropriate dosage for addition was 400 mg/kg (viable cell count was 4.0×10⁹ CFU/kg).

Key words: Bacillus subtilis; nursery pigs; growth performance; intestinal mucosal morphology; serum biochemical indexes; immune function