Much interest has been shown recently in synchronising the supply of nitrogen and energy yielding substrates to microorganisms in the rumen. It is argued that when supplies are synchronised, rumen fermentation will be more efficient and improved performance will result. Previous trials with beef cattle at Nottingham and elsewhere have investigated the effect of single protein sources (e.g. fishmeal, soya bean meal) on rumen fermentation and performance. However, the majority of silage-fed beef cattle in this country receive supplementary protein in compound feeds. It is possible that the digestion of the carbohydrate portion of the compound feed could compete with silage digestion for nitrogen supply. Therefore, degradation of silage should be investigated in animals actually fed on the compound feeds, rather than just considering the degradation of the protein components in a compound. A trial was carried out to investigate the influence of protein source in compound feeds on rumen fermentation and performance in beef steers given silage.