Effect of direct-fed microbials on intestinal villus height in broiler chickens: A systematic review and meta-analysis of controlled trials

Abstract

Objective: This systematic review and meta-analysis was conducted to evaluate the overall effect of direct-fed microbials (DFM) or probiotics on intestinal villus height in broiler chickens. Materials and Methods: Relevant studies were collected by searching PubMed, SCOPUS, Poultry Science Journal and Google Scholar. Studies of randomized or non-randomized controlled trials were included using DFM in broiler chickens and reporting intestinal villus height (VH) (the primary outcome), crypt depth (CD) or villus height-to-crypt depth (VH/CD) ratio. The overall effect on intestinal VH, CD or VH/CD was evaluated using the standardized mean difference (SMD) with a random effects meta-analysis. Subgroup analysis for the VH was planned a priori for 6 characteristics (broiler breed, sex, DFM product, microbial species, duration of treatment and application route). In addition, study quality (using SYRCLE’s risk of bias tool) and publication bias were evaluated. Results: A total of 25 studies were identified (with a total of 296 comparisons) that met the inclusion criteria. Supplementation of DFM for broiler chickens was associated with increased VH in the small intestine (SMD = 3.38, 95% confidence interval (CI) (2.69, 4.06), p<0.001, I2 = 92, n = 113 comparisons) and with greater VH/CD ratio in the small intestine (SMD = 2.73, 95% CI (2.09, 3.36, p<0.001, I2 = 91, n = 87 comparisons). In subgroup analysis for the VH, significant differences among subgroups were found in 5 characteristics (broiler breed, DFM product, microbial species, duration of treatment and application route). Most studies were presented with unclear risk of bias. Publication bias was also observed. Conclusion: Supplementation with DFM for broiler chickens was associated with increased intestinal villus height.