Nutrient Concentrations of Runoff as Affected by the Diameter of Unconsolidated Material from Feedlot Surfaces

Beef cattle feedlots contain unconsolidated material that accumulates on the feedlot surface during a feeding cycle. This study was conducted to measure the effects of varying diameters of unconsolidated surface material and varying flow rates on nutrient concentrations in runoff. Unconsolidated surface material with an average diameter of 4.76, 9.53, 19.1, or 47.5 mm and a composite sample with a 15.2 mm mean diameter were placed within 0.75 m wide × 4.0 m long plot areas. 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), NH₄-N, and solids transport. 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, and solids transport was 19.8 g min^-1. Runoff rate significantly affected concentrations of DP, PP, TP, NH₄-N, and solids transport 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, from 5.52 to 0.41 mg L^-1, from 9.82 to 0.75 mg L^-1, and from 25.8 to 0.49 mg L^-1, respectively, as runoff rate increased from 0.02 to 1.10 L s^-1. Nutrient concentrations of runoff from feedlot surfaces are affected by both varying diameters of unconsolidated surface material and varying flow rates.