Optimal levels of crude (CP) and undegraded (UDP) dietary protein were studied in feeding systems based on whole maize fed free-choice and on complete mixed diets composed from by-products of the milling industry. On the whole maize free-choice system, animal performance was improved when more CP and UDP were fed. With by-product feeds, the higher CP treatment did not improve animal performance, which suggests that the lower CP treatment was adequate in meeting requirements. Better results were obtained with treatments that contained 35% and 40% UDP in the CP than 45% UDP. Cost benefits were realized when CP level was decreased steadily in agreement with requirements. We recommend that the CP of feedlot diets should contain 35 to 40% UDP and that the CP level during the fattening period should be reduced from 14% to 10% in the dry matter by adopting the principle of three diets (‘starter’, ‘grower’ and ‘finisher’).