The objectives of this study were to construct an optimization model to identify opportunities to reduce land use, water use and greenhouse gas (GHG) emissions within dairy production systems and to assess how improved energy and protein use efficiency could affect opportunities to reduce environmental impact (EI) of dairy production systems. Nonlinear programming was used to adjust monthly diets fed to 10 cattle groups to minimize EI associated with an average United States (US) dairy production system. System boundaries extended from the inputs to the cropping system to the dairy farm gate. The effects of improved biological efficiencies were modeled as a 15% decrease in the allometric energy constant used to calculate maintenance energy requirements or as a 15% decrease in maintenance metabolizable protein requirements. Least-cost optimization was used as a baseline. A total of 28 scenarios were simulated which varied in objective, biological efficiency and allowable cost increase. Objectives included minimizing land, water, greenhouse gases, or all EI metrics. Allowable cost increases ranged from 1% to 20%. Baseline land use (1.22 m²/kg milk), water use (14.6 m³/kg) and GHG emissions (1.45 kg CO₂/kg) agreed well with established values for US dairy production. At cost increases between 1% and 20% above baseline, EI metrics could be simultaneously reduced by 5.9 to 16.6%. Improving energy or protein efficiency greatly improved opportunity to reduce EI. When both energy and protein efficiency were improved by 15%, EI reductions ranged from 14.6 to 21.1%. Opportunity to reduce EI varied greatly with allowable cost increase and diminishing environmental returns to cost were apparent. The cost of reducing environmental impact by 15% was decreased by 87.7% when energy and protein efficiency improved compared with the national average production efficiency scenario. Improving energy and protein use efficiency of dairy cattle represents a promising way to reduce EI without sacrificing profitability.