Silage forms the basal diet during the winter months for the vast majority of beef cattle in the British Isles. There is currently a wide range of feedstuffs available as supplements to grass silage for beef cattle, many of these being by-products of the production of food for human consumption. The relative value of alternative feedstuffs as supplements to grass silage is dependent on the nature of the associative effects between each of the feedstuffs and the silage portion of the ration. Two experiments have been carried out to evaluate a range of by-product feedstuffs including maize gluten feed, citrus pulp, dark maize distillers grains and maize germ meal as supplements to grass silage and to compare their feeding values with those of barley and soyabean meal. As feeds such as maize gluten feed and citrus pulp have low tabulated metabolisable energy contents, the effects of adding protected fat to a low energy by-product feed to increase the energy content of the supplement was also examined.