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

## TL;DR

This study explores using dried grape pomace as a sustainable alternative to soybean hulls in dairy cow diets, finding it supports circular economy goals without harming milk production.

## Contribution

The study introduces grape pomace as a viable, environmentally friendly feed replacement in dairy diets, offering insights into its impact on rumen function and sustainability.

## Key findings

- Dried grape pomace has higher fiber and lower ruminal degradability compared to soybean hulls.
- Milk urea N and ruminal short-chain fatty acids showed quadratic responses with DGP inclusion.
- Soybean meal degradability increased with higher DGP inclusion, suggesting bioactive effects.

## Abstract

The valorization of dried grape pomace (DGP), a by-product of the wine industry, in ruminant feeding supports circular economy principles, reduces reliance on imported feedstuffs, and mitigates environmental issues related to grape pomace disposal. A 3 × 3 Latin square design was conducted using three rumen-fistulated Holstein cows fed corn silage-based diets containing 0%, 3%, or 6% DGP, replacing soybean hulls, over three 23-day periods. In situ degradability of DGP, soybean hulls, and two protein sources was assessed. Compared with soybean hulls, DGP exhibited higher fiber content and lower ruminal dry matter and N degradability, with condensed tannin disappearance approaching 50% after 48 h. Dietary treatment did not affect feed intake, milk yield, or composition, except for milk urea N and ruminal short-chain fatty acids, which showed quadratic responses, suggesting effects on rumen fermentation. Rapeseed meal degradability was unaffected, whereas soybean meal degradability tended to increase with increasing DGP inclusion. Overall, despite its low energy value and ruminal degradability, DGP represents a sustainable feed resource for dairy production.

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.