Improving dairy cattle nitrogen use efficiency (NUE) is important for both environmental and production economics perspectives. In this project we used CNCPS based modelling via AMTS software to investigate dietary approaches to improve NUE through better AA supply and reducing excess RDP to the hindgut. In Step 1a, three diets differing only in Met supply (0.86, 1.06 and 1.20 g/Mcal ME) were simulated using milk production data from Danese et al. (2023). As expected, milk yield was similar across diets while milk composition improved with increasing Met supply. Productive N increased slightly in response to the higher yields and urinary N increased in response to the higher protein supply, but NUE increased overall. In Step 1b, a diet formulated to contain 1.20 g Met/Mcal ME was balanced and used to simulate increased milk yield and milk true protein output, which led to an increased productive N and reduced urinary N, improving NUE. In Step 2, dietary urea was removed from the high Met diet to reduce RDP. This resulted in a large reduction in rumen NH₃ levels and urinary N losses while maintaining milk output and productive N, improving NUE further. Overall, the results indicate that utilizing AA balancing (Met, Lys and His) and reducing rumen NH3 and RDP can be used to greatly improve N partitioning and environmental N efficiency in dairy cow diets.