Estimating Phosphorus Loss at the Whole-Farm Scale with User-Friendly Models

Abstract:

Phosphorus (P) loss from agricultural fields and delivery to surface waters persists as a water quality impairment issue. For dairy farms, P can be lost from cropland, pastures, barnyards, and open-air cattle lots; and all these sources must be evaluated to determine which ones are a priority for P loss remediation. We used interview surveys to document land use, cattle herd characteristics, and manure management for four grazing-based dairy farms in Wisconsin, USA. We then used the APLE and Snap-Plus models to estimate annual P loss from all areas on these farms and determine their relative contribution to whole-farm P loss. At the whole-farm level, average annual P loss (kg ha^-1) from grazing-based dairy farms was low (0.6 to 1.8 kg ha^-1), generally because a significant portion of land was in permanently vegetated pastures or hay and had low erosion. However, there were areas on the farms that represented sources of significant P loss. For cropland, the greatest P loss was from areas with exposed soil, typically for corn production, and especially on steeper sloping land. The farm areas with the greatest P loss had concentrated animal housing, including barnyards, and over-wintering and young-stock lots. These areas can represent from about 5% to almost 30% of total farm P loss, depending on lot management and P loss from other land uses. Our project builds on research to show that producer surveys can provide reliable management information to assess whole-farm P loss. It also shows that we can use models like RUSLE2, Snap-Plus, and APLE to rapidly, reliably, and quantitatively estimate P loss in runoff from all areas on a dairy farm and identify areas in greatest need of alternative management to reduce P loss.