Evaluating agricultural management practices for reducing phosphorus losses from grasslands receiving manure
Butler, David Michael
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Poultry and dairy production are important aspects of the agricultural economy in the Southern Piedmont of Georgia. Manures associated with this production are typically surface-applied to pastures as an economic fertilizer for forages. However, nutrients such as phosphorus (P) applied in manures may contribute to agricultural nonpoint pollution through contamination of surface runoff. This has potential to accelerate eutrophication of surface waters, which can harm aquatic life and complicate the water treatment process. Given that surface-applied manures can contribute to P in runoff, a study was conducted to examine agricultural management strategies to reduce export of P from grasslands with applied manures. Following application of either broiler litter or dairy slurry at the plot-scale in the first objective of the study, core aeration was shown to have the greatest potential for reducing P losses in runoff compared to discontinuous slit aeration with tines, continuous-furrow disk aeration, and a control of no aeration. The second objective, conducted at the field-scale, determined that continuous-furrow knife aeration was effective in reducing runoff volume (22%) and export of total P (18%) and dissolved reactive P (41%) compared to a control on well-drained soils and less effective on fields with more poorly-drained soils or a relatively high seasonal water table. In the third objective, the use of on-farm, field-scale runoff data determined that the Georgia P Index was well-suited for estimating the risk of edge-of-field P losses from fields under pasture or hay management in the Southern Piedmont. Furthermore, nutrient source (broiler litter, dairy slurry, inorganic N, and no amendments), forage system (hay or pasture), and year were significant factors affecting edge-of-field P losses. Results from this study suggest that core and continuous-furrow knife aeration procedures may have a widespread impact in reducing the levels of P exported from pastures receiving broiler litter in the Southern Piedmont. Additionally, the impact of agricultural management practices on P export from pasture and hay systems can be effectively modeled by farmers using the Georgia P Index, especially for farms with a low or medium risk of P export.