Effects of Fermented Feed Supplementation on Production Performance and Egg Quality Parameters in Laying Hens: A Meta-Analysis

Fermentation-based feed processing has been proposed as a nutritional approach to improve nutrient availability and metabolic efficiency in laying hens. However, information on its impact on production performance and egg quality remains limited. This meta-analysis statistically assessed the impact of fermented feed supplementation on the production performance and egg quality parameters of laying hens while investigating potential sources of heterogeneity across studies. A comprehensive literature search was performed in PubMed, Scopus, and Web of Science using the PRISMA 2020 guidelines. The inclusion criteria were randomized controlled trials and controlled intervention studies with non-fermented control diets. Two primary outcomes were examined: (1) production performance and (2) egg quality. Multilevel random-effects meta-analyses employing restricted maximum likelihood were conducted to address multiple effect sizes. Heterogeneity was evaluated using Cochran's Q statistic and I² estimates. Meta-regression analyses were performed considering dietary inclusion level, trial duration, and total hens, with subgroup analyses based on fermented feed type, assessment of publication bias using Egger's test and trim-and-fill methods, and leave-one-study-out sensitivity analysis. Twenty-four studies were included in the quantitative synthesis. Supplementation with fermented feed markedly enhanced the egg-laying rate (MD = 2.11 percentage points; 95% CI: 0.92-3.30; <i>p</i> = 0.0005), Haugh unit (MD = 1.99; 95% CI: 0.61-3.38; <i>p</i> = 0.0048), and eggshell thickness (MD = 0.0081 mm; 95% CI: 0.0037-0.0124; <i>p</i> = 0.0003), whereas no significant overall effect was noted on the feed-to-egg ratio (MD = -0.0384 g feed/g egg; 95% CI: -0.0871-0.0103; <i>p</i> = 0.1218). Significant heterogeneity was observed across outcomes (I² ≈ 73-93%). Subgroup analyses revealed notable feed-type-specific effects, whereas meta-regression indicated that dietary inclusion level is a significant moderator of the feed-to-egg ratio. Sensitivity analysis validated the robustness of the pooled estimates, and publication bias did not significantly influence the results. Supplementation with fermented feed was associated with improvements in egg-laying rate, albumen quality (Haugh unit), and eggshell thickness in laying hens. However, substantial heterogeneity across studies and variability in effect sizes indicate that responses may depend on feed type, inclusion level, and study conditions. These findings should therefore be interpreted with caution, and further well-designed and standardized trials are needed to confirm the consistency and magnitude of these effects.