Sustainable ruminant livestock systems must promote an efficient use of resources for food production and minimise their environmental impact. Intensive dairy cow production, based on grass silage, is a major contributor to nitrogen (N) loss to the environment, estimated as 18.2 t N/t milk for grass silage-based diets (Delaby et al., 1995). It is widely accepted that this low efficiency (milk N/dietary N) value reflects the low efficiency of capture in the rumen of the extensively degraded N fraction in grass silage (Givens and Rulquin, 2004). A common feature of plant tannins is 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 the potential of various commercial non-toxic tannin products to reduce proteolysis during ensilage.