Monitoring runoff from cattle-grazed pastures for a phosphorus loss quantification tool

Abstract Nitrogen (N) and phosphorus (P) loss from agriculture persists as a water quality impairment issue. For dairy farms, nutrients can be lost from cropland, pastures, barnyards, and outdoor cattle lots. We monitored N and P loss in runoff from dairy and beef grazed pastures for two years in southwest Wisconsin, USA and tested the accuracy of the Annual P Loss Estimator (APLE) model to predict runoff P from pastures using study and literature data. About 3–10% of annual precipitation became runoff from the pastures, and sediment loss was very low due to well-established vegetation. Measured annual nutrient loss in runoff was also low, averaging 1.0 kg ha −1 for total P and 2.9 kg ha −1 for total N. Runoff sediment and particulate N and P concentrations were well related to each other and tended to be greater in rainfall-induced runoff than snowmelt runoff. Conversely, dissolved N and P runoff concentrations were greater in snowmelt runoff. APLE was able to reliably predict annual P loss in runoff, estimating that the average relative contribution to total pasture P loss was about 10% from fertilizer, 15% from soil dissolved P, 30% from dung, and 45% from soil erosion. Our study has increased the ability to develop reliable models for estimating the impact of cattle grazing pastures on nutrient runoff, which will be valuable in estimating whole-farm P loss from dairy production systems and identifying areas on dairy farms where P loss remediation should be targeted.