Grape By-Products in Poultry Nutrition: Functional Benefits, Limits, and Circular-Economy Potential

Grape winery by-products, including grape pomace, grape seed meal, and polyphenol-rich extracts, are increasingly studied as functional poultry feed ingredients. This review summarizes 46 in vivo studies on broilers and laying hens (26 and 20, respectively) published between February 2008 and January 2026. Overall, low-to-moderate inclusion levels improved antioxidant status and reduced lipid peroxidation in tissues and poultry products. They also enhanced egg internal quality, yolk color, and meat oxidative stability during refrigerated storage, with little or no negative effect on production performance. Higher inclusion levels, particularly of unprocessed high-fiber or high-tannin fractions, may enhance some antioxidant outcomes but can impair digestibility, growth, and feed efficiency through tannin–protein interactions. These effects may be limited through fermentation, enzymatic pretreatment, or formulation aids. Responses depend on dose, by-product type, and oxidative challenge. Laying hens show strong improvements in yolk pigmentation, eggshell mineralization, and egg storage stability. Broilers mainly benefit through improved meat oxidative stability, reduced fat deposition, and gut microbiota modulation. The main evidence gaps are incomplete by-product characterization, few multi-dose, long-term, or commercial-scale trials, and no economic or life-cycle assessment. Grape by-products are therefore promising circular-economy feed inputs whose commercial viability remains to be validated.