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Whole-farm phosphorus loss from grazing-based dairy farms

Peter A. Vadas, J. Mark Powell, Geoff E. Brink, Dennis L. Busch, Laura W. Good
Agricultural systems 2015 v.140 pp. 40-47
animal housing, crop production, cropland, dairy farming, decision support systems, farms, hay, information management, land use, models, overwintering, pastures, phosphorus, runoff, soil, surveys, topographic slope, water quality, Wisconsin
Phosphorus (P) loss from agricultural farms persists as a water quality impairment issue. For dairy farms, P can be lost from cropland, pastures, and open-air lots. A need remains for user-friendly, readily applied, decision-support tools that simulate all areas of dairy farms, and determine their relative contribution to whole-farm P loss. We used the Snap Plus and APLE models to estimate annual P loss from all areas on four grazing-based dairy farms in Wisconsin, USA. At the whole-farm level, average annual P loss (kgha−1) from grazing-based dairy farms was low (0.5 to 1.8kgha−1), generally because a significant portion of land was in well-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 were 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 and reliably estimate P loss in runoff from all areas on a dairy farm and identify critical P source areas in greatest need of alternative management.