It is well established that increasing the protein concentration of supplements increases milk yield in cows given grass silage-based diets. The responses are attributed to improved diet digestibility, increased silage dry matter (DM) intake and supply of amino acids from the small intestine. However, physical and chemical treatments of protein supplements used to decrease ruminal protein degradability have produced variable responses in animal production. Another alternative to increase amino acid supply is the processing of the grain supplements, which could affect both the energy availability and protein degradability in the rumen. Kassem et al. (1987) showed some improvements in milk production when barley was treated with a formaldehyde reagent. The aim of this study was to examine the effects of physical treatments to reduce ruminal protein degradability of barley and rapeseed meal (RSM) on milk production in cows fed grass silage ad libitum.