The aim was to analyze the effects of a supplementation of concentrate mixtures (CM) differing in their concentrations of rumen-degradable crude protein and readily fermentable carbohydrates on feed intake, diet digestibility, milk performance, and nitrogen (N) partitioning in lactating dairy cows grazing a mixed grass-legume sward. Three dietary treatments were tested with two groups of dairy cows during three experimental periods (14 d adaptation and 5 d sampling) following a 2 × 3 Youden square design. Each cow daily received 4.12 kg CM (as-fed basis) distributed over two meals in addition to maize silage and the grazing of an alfalfa-clover-rye grass sward. The three CM contained (as-fed basis): 4 kg/d of dried distillers’ grains with solubles (DDGS) and no maize kernel meal (MM) (HPLS; high protein-low starch), 2.5 kg/d DDGS and 1.5 kg/d MM (MPMS; medium protein-medium starch), and 1 kg/d DDGS and 3 kg/d MM (LPHS; low protein-high starch). Additionally, 0.12 kg/d of a mineral-vitamin mixture was added to each CM. Fecal excretion was determined using the external marker titanium dioxide and apparent total tract digestibility of ingested organic matter (OM) estimated from crude protein concentration in fecal spot samples in order to calculate daily OM and N intakes of cows. Total OM intake and OM intake during grazing did not differ between CM. However, digestibility of ingested OM and daily milk yield were lowest for LPHS. Total N intake and urinary N excretion decreased from HPLS to MPMS and LPHS. Accordingly, N use efficiency (in g milk N per g of N intake) was higher for LPHS (0.249) than for HPLS (0.189) and MPMS (0.209). Feeding CM low in rumen-degradable crude protein and rich in readily fermentable carbohydrates may slightly reduce diet digestibility and milk yield in dairy cows grazing mixed grass-legume swards, but can greatly improve N use efficiency and lower the risk of N emissions into the environment.