Phosphorus Losses From Simulated Dairy Land Uses of Management Intensive Grazing and Concentrated Animal Feeding Operations
Johnston, Michael Rhodes
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Dairy grazing systems have been promoted as an environmentally friendly alternative to concentrated animal feeding operations (CAFOs). However, questions remain regarding the amount of phosphorus (P) loss from pasture-based dairies. Therefore, the goal of this study was to quantify P losses via runoff from grazing dairy systems and CAFO hay production. Four land use treatments were simulated on runoff release plots planted in two forage treatments. Land use treatments were management intensive grazing (MIG) and CAFO hay production to which manure was applied; grazing and CAFO hay production without manure application served as controls. The forage treatments were orchardgrass (Dactylis glomerata) and broad-leafed forage (buckhorn plantain [Plantago lanceolata], red clover [Trifolium pretense], and alfalfa [Medicago sativa]). The four land use treatments and two forage treatments had four replications for a total of 32 (4 land use treatments e n2 forage treatments e n ¤ nreplications) runoff release plots. Thirty minutes of runoff was collected from each runoff release plot during six rainfall simulation series. Grab samples of runoff were collected and analyzed for dissolved reactive phosphorus, total phosphorus, and total suspended solids. Particulate P (PP) was determined indirectly by subtracting dissolved reactive P (DRP) from total P (TP). No distinction was observed between DRP concentrations from simulated MIG plots and simulated CAFO plots for the duration of the study. However, a clear divergence of TP concentrations from CAFO hay plots with manure from all other land uses was observed during simulation series 1. DRP concentrations for all land uses were fairly consistent for the duration of the study with the exception of simulation series 2. All land uses had DRP concentrations that would be considered of concern (> 1.0 ppm DRP) by the US-EPA during simulation series 4 and 5. The elevated TP concentrations and mass losses from CAFO hay plots with manure were primarily due to PP losses. Thus, it was concluded that MIG has less potential for P loss than CAFO hay production. Broad-leafed forage had significantly higher DRP, PP, and TP losses than orchardgrass on CAFO hay with manure. However, no significant difference was found between forage types on MIG plots. Thus, either forage could be planted in grazing land without a significant difference in P losses.
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