Lucerne (Medicago sativa) is a major, high protein forage fed to dairy cattle. However, during ensiling, much of the CP in lucerne silage (LS) is broken down to nonprotein N (NPN); high levels of NPN in LS depress protein utilisation in lactating dairy cows. Red clover (Trifolium pratense) is a high quality legume forage that can be grown in Britain and Northern Europe. Polyphenol oxidase, an enzyme system in red clover, converts plant phenols into quinones that react rapidly with forage proteins in the silo and result in red clover silage (RCS) having less NPN than LS. Earlier (Broderick et al., 2000), we reported that replacing LS with RCS lowered milk yield but improved feed efficiency and apparent digestibility. Our objective was to compare the production of dairy cows fed equal amounts DM as LS or RCS, with or without maize silage (MS) and supplemental protein added to the diet.