The objective of this study was to compare the effects of a grain-based conventional concentrate (GC) with a concentrate based on agro-industrial by-products (BC), each fed with 2 different grass silages harvested at early (ES) or late (LS) maturity stage, on dairy performance and efficiency of N and energy utilization when fed to lactating dairy cows. Twenty lactating Nordic Red cows were assigned to a replicated 4 × 4 Latin square design with 4 periods of 21 d. Dietary treatments were in a 2 × 2 factorial arrangement. The silages were harvested with 2-week interval from the same primary growth grass ley. The ES was higher in CP (173 vs. 108 g/kg DM) and lower in NDF (443 vs. 607 g/kg DM) than LS. The GC was made from grain and soybean meal, while the BC contained sugar beet pulp, wheat bran, canola meal, distillers dried grain, palm kernel expeller and molasses. The GC contained more starch (444 vs. 51 g/kg DM) and less NDF (162 vs. 351 g/kg DM) than BC. The diets were fed ad libitum as total mixed rations and were formulated from 661 g/kg of silage, 326 g/kg of concentrate and 13 g/kg minerals on DM basis. Experimental data were subjected to ANOVA using the GLM of SAS by applying a model correcting for the effect of block, period, cow within block and dietary treatments. The effects of silage maturity, concentrate type and their interaction were evaluated. The milk yield and composition were unaffected by concentrate type, except that milk protein decreased 0.7 g/kg from cows fed BC. These results were paralleled by decreased CP digestibility and increased NDF digestibility for cows fed BC. Cows fed ES on average consumed more dry matter (23 vs. 21 kg/d) and yielded more milk (28.7 vs. 25.2 kg/d) than cows fed diets with LS. The edible feed conversion ratios showed greater improvements with ES than LS when replacing GC with BC. Feeding diets with LS reduced digestibility and energy utilization efficiency, but improved N efficiency. In conclusion, a conventional concentrate can be replaced by agro-industrial by-products without compromising production in lactating dairy cows, and the silage maturity had a stronger effect on dairy performance than the concentrate type.