Grape Pomace as a Replacement for Soybean Hulls in Corn Silage-Based Diets for Dairy Cows

This study evaluated the effects of replacing soybean hulls with dried grape pomace (DGP) on feed intake, milk production and composition, and rumen N degradability in dairy cows. A 3 × 3 Latin square design was used with three rumen-fistulated Holstein cows, three corn silage-based diets containing 0%, 3%, or 6% of DGP, and three 23-day periods. Measurements were taken from days 15 to 21, with in situ incubations of two protein sources during the last 2 days. At trial end, cows continued on experimental diets to determine the degradability of soybean hulls and DGP. Compared to soybean hulls, DGP presented higher fiber content and lower in situ rumen dry matter and N degradability. Condensed tannin disappearance from DGP increased over time, reaching almost 50% after 48 h. Diet did not affect feed intake, milk production, and composition, except for milk urea N and ruminal total short-chain fatty acids concentration, which showed a quadratic response, suggesting both imbalances between fermentable energy and rumen degradable protein and potential effects of DGP bioactive compounds on rumen function. The N degradation rate of rapeseed meal was unaffected, but soybean meal degradability tended to increase with higher DGP inclusion. This was also detected when combining protein sources across diets. Although DGP influenced rumen function, further research integrating omics and detailed microbiota profiling is needed. Overall, despite its low energy content and rumen degradability, DGP comprises a sustainable feed resource for high-producing animals, supporting circular economy approaches and mitigating the environmental impacts of grape pomace disposal.