SUMMARY
In this systematic review, prebiotic and probiotic dietary additives were compared for their ability to impact growth performance in turkeys. Eligible studies experimentally compared in vivo the effects of administering prebiotics (defined as dietary fiber, dietary carbohydrates, oligosaccharides, or yeast cell wall supplemented in addition to the basal diet) and/or probiotics (single or mixed cultures of living bacteria or fungi) on the average daily gain, and/or feed conversion ratio (FCR), and/or mortality. Database searches were conducted in December 2019 and updated in September 2021 and November 2022. Studies were initially screened for appropriate relevance followed by the target design elements before data were extracted. Risk of bias was performed using the revised Cochrane risk-of-bias tool for randomized trials. The search originally included broiler chickens, returning 3064 studies, and was reduced to 38 studies after limiting the results to turkeys and completing all screening phases. Mean differences were compared by treatment type for each outcome assessed for all included studies. All studies reporting experimental variability appropriately were compared at a treatment group level by outcome for FCR and mortality using mean difference or risk ratio, respectively, with 95% confidence intervals for both outcomes. A meta-analysis was performed on a subset of treatment groups for the outcomes FCR and mortality. Treatment with probiotics from the Bacillus or Lactobacillus genus was used. A meta-analysis was also conducted on prebiotic studies using oligosaccharides. The risk-of-bias assessment showed that 42.5% of studies fell into the “high risk” group, 22.5% of studies fell into “some concerns” group, and 35% fell into the “low risk” group. We found numerically that probiotic and/or prebiotic treatments had, on average, beneficial effects on mean difference for all three outcomes compared to nontreatment controls; however, results varied by individual study. We performed meta-analyses and found no statistically significant effects of Bacillus or oligosaccharide supplementation on either FCR or mortality. Supplementation with Lactobacillus resulted in a lower mortality risk, with no statistically significant effect on FCR. Strong reporting bias was identified when assessing FCR. The relatively small number of published studies, combined with clear reporting biases, indicates a need for more studies in this field with robust study designs.