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Beef cattle feedlots contain unconsolidated material that accumulate on the feedlot surface during a feeding cycle. This study was conducted to: (1) determine the effects of varying diameter of unconsolidated surface material on nutrient concentrations of runoff and (2) identify the effects of selected runoff rates on nutrient concentrations of runoff from unconsolidated surface material containing variable particle diameters. Unconsolidated surface material with a mean diameter of 4.76, 9.53, 19.1 or 47.5 mm and a composite sample with a 10.8 mm mean diameter were placed within a 0.75 m wide by 4.0 m long plot area. Flow was then introduced at the top of the plots in successive increments and runoff samples for water quality analyses were obtained. Particle diameter significantly influenced runoff concentrations of dissolved phosphorus (DP), particulate phosphorus (PP), total phosphorus (TP), and NH4-N. Concentrations of DP, PP, TP and NH4-N for the composite material were 1.90, 1.28, 3.18, and 3.81 mg L-1, respectively. Runoff rate also significantly affected concentrations of DP, PP, TP, and NH4-N for each of the particle size classes except the 4.76 mm diameter material. For the composite material, concentrations of DP, PP, TP, and NH4-N decreased from 4.30 to 0.34 mg L-1, 5.52 to 0.41 mg L-1, 9.82 to 0.75 mg L-1, and 25.8 to 0.49 mg L-1, respectively, as runoff rate increased from 0.02 to 1.10 L sec-1. Thus, both the diameter of unconsolidated material from feedlot surfaces and runoff rate may significantly influence nutrient concentrations of runoff.