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Feedlot nutrition will play an important role in meeting environmental challenges of beef cattle feedlots. Nutritionists are continually refining protein requirements and recently adopted a new, metabolizable protein (MP) system to more efficiently use N and allow more accurate diet formulation. Protein requirements vary by animal age and weight during the finishing period. Our hypothesis was that formulating diets with the MP system would decrease N inputs leading to decreased excretion and losses. Comparing industry average diets (13.5% CP) to phase-fed diets formulated to not exceed MP requirements decreased N inputs by 10 to 20% for calves and yearlings without affecting ADG. Decreasing inputs led to a concomitant decrease in N excretion (12-21%) and losses (15 to 33%) in open-dirt feedlot pens. Nitrogen losses are variable with time of year, with averages of 60 to 70% of excreted N lost during the summer months and 40% lost from November to May feeding periods. Protein requirements are being refined continually as more research data are collected. However, formulation to meet, but not exceed, protein requirements is an important nutritional management option for feedlots to become sustainable.