Grape By-Products in Ruminant Diets: From Nutritional Benefits to Circular and Sustainable Livestock Production

Abstract

Grape by-products are agro-industrial residues rich in polyphenols and other bioactive compounds with potential as functional ingredients in ruminant nutrition. This review evaluated their nutritional, physiological, productive, and environmental effects in sheep, cattle, and goats, with emphasis on dose-dependent responses, dietary inclusion levels, and circular livestock production. Only in vivo studies assessing grape pomace, grape marc, grape seeds, grape seed extracts, and related residues were included. Evidence indicates that grape by-products can improve antioxidant status, modulate rumen fermentation and gastrointestinal microbiota, influence nitrogen metabolism, and enhance the fatty acid profile and oxidative stability of milk and meat. Their inclusion may also contribute to reduced methane emissions and nitrogen excretion. However, responses are strongly dose-dependent. Low to moderate inclusion levels generally produce beneficial or neutral effects on animal performance, whereas excessive inclusion may reduce nutrient digestibility, feed intake, nitrogen retention, and growth because of high lignin and tannin contents. Overall, grape by-products are promising functional feed ingredients for valorising winery residues, but their practical use requires careful selection of by-product type and inclusion level.