The objective of this study was to evaluate the effect of reduced CP on milk production and environmental impact in a commercial dairy herd. Primiparous cows were completely randomized to 2 groups with 84 cows each, and each group had similar DIM (181 ± 7.14 vs. 195 ± 7.85) before initiation of the study. The control diet was the current general herd ration. The reformulated diet was supplemented with Prolak, MetaboLys and Smartamine M, reduced the dietary CP (17.7 ± 0.65 vs. 16.7 ± 0.7% DM), and increased methionine (1.8 vs. 2.2% MP) and lysine (6.6 vs. 6.9% MP) concentrations. Diets were fed in a 6-wk switch back design trial with 2 periods. DMI was recorded daily (1 pen per treatment), and milk yield and composition of individual cows were measured weekly. Urine and feces samples were mixed together and used in a closed chamber incubation to determine NH₃ emission. Feeding the reformulated diet numerically decreased DMI 0.4 kg (19.3 ± 0.49 vs. 18.9 ± 0.58kg), however milk yield (P = 0.91), protein yield (P = 0.74), % protein (P = 0.49), fat yield (P = 0.19), % fat (P = 0.09), and lactose yield (P = 0.28) did not differ. Lactose % and MUN (P < 0.001) decreased when cows were fed the reformulated diet. Cows fed the reformulated diet had higher Met (P = 0.09) and less Tyr (P = 0.06) in blood plasma than control cows. Feeding the reformulated diet numerically increased feed efficiency (1.62 vs. 1.66 ± 0.01) and milk true protein efficiency (28.4 vs. 30.8 ± 0.01%). Cows fed the reformulated diet consumed 7.2% less N, produced 0.6% more milk total N, excreted 9.9% less predicted urinary N and 18.0% less calculated fecal N than control cows. The difference in income over feed cost for the reformulated diet was $0.15 and $0.18 based on the milk price of Washington State in 2013 and 2014, respectively. The emitted NH₃ flux rates of manure were numerically similar (122.6 ± 10.26 vs. 124.1 ± 11.99 mg·h⁻¹·m⁻²) between diets. These results illustrated that feeding low CP with balanced AA diet could improve the efficiency of nitrogen utilization and reduce the environmental impact without compromising the profitability of milk production in primiparous cows.