A reduction of the overall nitrogen (N) loss to the environment from intensive ruminant production systems remains a key policy and research objective. Such an outcome is essential for the development of sustainable ruminant farming systems that minimise their environmental footprints. Plant and microbial enzyme-mediated proteolysis during ensilage results in the extensively hydrolysed N fraction in grass silage (Givens and Rulquin, 2004). It is generally accepted that the highly soluble N fraction of grass silage is poorly utilised by ruminants, reflecting the low efficiency of capture of silage N in the rumen. An improvement in dietary N efficiency by ruminants is therefore a key concern. Plant tannins have in common their capacity to bind proteins and an improvement in N utilisation by ruminants has been reported for a number of tanniniferous feeds (Mueller-Harvey, 2006). Therefore, the aim of the present experiment was to study a by¬product from the wine industry, so-called ‘grape marc’, as a potential source of tannins to reduce proteolysis during ensilage.