This study was conducted to investigate the effects of total replacing soybean meal with dried distiller grains in commercial dairy farm. Data were collected on fifteen lactating dairy cows divided equally into three groups according to their calving date. The experimental rations were calculated and equilibrated using the ALIM32 program (Numerical data was analyzed statistically using the Statistical Package for the Social Sciences (SPSS 20). Results revealed that inclusion of dried distiller grains in the ration increase the efficiency use of nitrogen and reduce significantly urinary nitrogen excretion (p<0.05). Indeed, the efficiency use of nitrogen was higher (25.8%±2.3) and urinary nitrogen excretion lower (178g±5.2) when rations included the dried distillers grains (p<0.05). Thus, offering dried distillers grains to lactating dairy cows as a major source of protein could increase their efficiency use of nitrogen associated to a little decrease of milk production that could not affect profitability in commercial dairy farms.