| Summary: | This meta-analysis determined the effect of <i>Bacillus</i> spp. on growth performance of growing–finishing pigs and then assessed causes for the heterogeneity of responses detected using meta-regression. A database of 22 articles published from 2000 to 2020 was identified, and 9 articles fitted the selection criteria and were integrated in the final database. Statistical analysis was performed to analyze the effect size for ADG, average daily feed intake (ADFI), and F:G ratio using a standardized means difference (SMD) at a 95% confidence interval. A meta-regression analysis was used to investigate the cause of heterogeneity, using the individual SMD for each study assessment as the outcome and the associated SE as the measure of variance. Dietary <i>Bacillus</i> spp. supplementation had no effect on ADFI (SMD: −0.052, <i>p</i> = 0.138) and numerically increased ADG (SMD: 0.113, <i>p</i> = 0.081) and reduced the F:G ratio SMD: −0.127, <i>p</i> < 0.001). Meta-regression outcomes suggested that the number of animals per group was an essential component promoting heterogeneity in ADG. Overall, the inclusion of <i>Bacillus</i> spp. (median 486 mg/d) in growing–finishing pigs can increase ADG and can decrease the F:G ratio.
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