Effects of Dietary Crude Glycerol Supplementation on Performance, Egg Quality, and Yolk Fatty Acids in Laying Hens: A Systematic Review and Meta-Analysis

This study evaluated the effects of dietary crude glycerol supplementation on laying performance, egg quality traits, and egg yolk fatty acid composition in laying hens. A systematic search of peer-reviewed literature from major scientific databases identified thirteen eligible studies. Effect sizes were calculated as mean differences using random-effects models, and subgroup and meta-regression analyses were conducted to assess the influence of inclusion level, hen age, strain, and dose–response relationships. Moderate inclusion of crude glycerol (≤4%) significantly increased egg production (MD = +0.71 percentage points; p = 0.019) and egg mass (MD = +1.08 g; p = 0.007). External egg quality traits, including shell thickness, shell breaking strength, and shell percentage, were not significantly affected, while albumen height decreased slightly (MD = −0.14 mm; p = 0.004) and Haugh unit remained unchanged. Crude glycerol supplementation also increased linoleic acid, total unsaturated fatty acids, and total polyunsaturated fatty acids (p < 0.001). Responses varied with inclusion level, hen age, and strain, and a positive dose–response relationship between crude glycerol level and yolk linoleic acid was observed in white-egg strains. Overall, moderate crude glycerol inclusion can partially replace conventional dietary energy sources with minimal adverse effects on egg quality.