Feeding corn dried distillers grains with solubles (DDGS) in low CP diets improves nitrogen (N) utilization. However, it reduces metabolizable AA supply, especially Lys, and compromises milk production. Our objective was to determine the effects of feeding by-pass protein/AA supplements in low or high CP diets containing DDGS on milk production and N utilization. Six multiparous Holstein cows (619.3 ± 49.8 kg BW; 26.8 ± 6.2 DIM) were subjected to a split-plot, 3 × 3 Latin square design with 21 d periods. The whole-plot factor was dietary CP content; low (14.6%; LP) or high (16.6%; HP), and the subplot was by-pass protein/AA supplement (top-dressed); Control (CON), Lactivate (LAC; 0.11 kg/cow/d) or ProLak (PRO; 0.45 kg/cow/d). All diets contained 10% corn DDGS. Feed intake and milk yield were recorded daily. Milk samples were collected from d 19 to 21. Spot urine (d 19 to 21) and blood (d 21, 3 h post-feeding) samples were collected to measure urine urea-N (UUN) and BUN, respectively. Data were analyzed using PROC MIXED in SAS. There was no CP level x supplement interaction (P > 0.05) for all data. Dry matter intake (26.2 vs. 26.9 ± 1.68 kg/d), milk (41.3 vs. 44.6 ± 4.69 kg/d) and milk component yield did not differ (P ≥ 0.38) in cows fed the LP than HP diet. Although N intake did not differ (0.615 vs. 0.718 ± 0.0422 kg/d; P = 0.16), BUN (8.3 vs. 13.0 ± 0.78 mg/dL), MUN (11.9 vs. 14.5 ± 0.60 mg/dL) and UUN excretion (40.0 vs. 54.9 ± 4.03 g/d) were lower for cows fed the LP than HP diet. However, there was no supplement effect (P ≤ 0.82) on milk (42.0, 43.5 and 43.2 ± 3.55 kg/d) and milk protein yield (1.19, 1.22 and 1.23 ± 0.092 kg/d for CON, LAC and PRO, respectively). Overall, feeding corn DDGS in LP than HP diets improved N utilization. Although not statistically significant, the numerical changes in milk and milk protein yield in cows fed the LP than HP diet; supplemented with LAC and PRO vs. CON, are important from an economic standpoint when evaluating income over feed costs.