Fall Tillage Reduced Nutrient Loads from Liquid Manure Application during the Freezing Season.

Reducing agricultural runoff is important year round, particularly on landscapes that receive wintertime applications of manure. No-tillage systems are typically associated with reduced runoff loads during the growing season, but surface roughness from fall tillage may aid infiltration on frozen soils by providing surface depressional storage. The timing of winter manure applications may also affect runoff, depending on snow and soil frost conditions. Therefore, the objective of this study was to evaluate runoff and nutrient loads during the freezing season from combinations of tillage and manure application timings. Six management treatments were tested in south-central Wisconsin during the winters of 2015-2016 and 2016-2017 with a complete factorial design: two tillage treatments (fall chisel plow vs. no-tillage) and three manure application timings (early December, late January, and unmanured). Nutrient loads from winter manure application were lower on chisel-plowed versus untilled soils during both monitoring years. Loads were also lower from manure applied to soils with less frost development. Wintertime manure applications pose a risk of surface nutrient losses, but fall tillage and timing applications to thawed soils can help reduce loads.

Dynamics of Measured and Simulated Dissolved Phosphorus in Runoff from Winter-Applied Dairy Manure.

Agricultural P loss from fields is an issue due to water quality degradation. Better information is needed on the P loss in runoff from dairy manure applied in winter and the ability to reliably simulate P loss by computer models. We monitored P in runoff during two winters from chisel-tilled and no-till field plots that had liquid dairy manure applied in December or January. Runoff total P was dominated by nondissolved forms when soils were bare and unfrozen. Runoff from snow-covered, frozen soils had much less sediment and sediment-related P, and much more dissolved P. Transport of manure solids was greatest when manure was applied on top of snow and runoff shortly after application was caused by snowmelt. Dissolved P concentrations in runoff were greater when manure was applied on top of snow because manure liquid remained in the snowpack and allowed more P to be available for loss. Dissolved runoff P also increased as the amount of rain or snowmelt that became runoff (runoff ratio) increased. The SurPhos manure P runoff model reliably simulated these processes to provide realistic predictions of dissolved P in runoff from surface manure. Overall, for liquid dairy manure applied in winter, dissolved P concentrations in runoff can be decreased if manure is applied onto bare, unfrozen soil, or if runoff ratio can be reduced, perhaps through greater soil surface roughness from fall tillage. Both management approaches will allow more manure P to infiltrate into soil and less move in runoff. SurPhos is a tool that can reliably evaluate P loss for different management and policy scenarios for winter manure application.

Temperature and Manure Placement in a Snowpack Affect Nutrient Release from Dairy Manure during Snowmelt.

Agricultural nutrient management is an issue due to N and P losses from fields and water quality degradation. Better information is needed on the risk of nutrient loss in runoff from dairy manure applied in winter. We investigated the effect of temperature on nutrient release from liquid and semisolid manure to water, and of manure quantity and placement within a snowpack on nutrient release to melting snow. Temperature did not affect manure P and NH-N release during water extraction. Manure P release, but not NH-N release, was significantly influenced by the water/manure solids extraction ratio. During snowmelt, manure P release was not significantly affected by manure placement in the snowpack, and the rate of P release decreased as application rate increased. Water extraction data can reliably estimate P release from manure during snowmelt; however, snowmelt water interaction with manure of greater solids content and subsequent P release appears incomplete compared with liquid manures. Manure NH-N released during snowmelt was statistically the same regardless of application rate. For the semisolid manure, NH-N released during snowmelt increased with the depth of snow covering it, most likely due to reduced NH volatilization. For the liquid manure, there was no effect of manure placement within the snowpack on NH-N released during snowmelt. Water extraction data can also reliably estimate manure NH-N release during snowmelt as long as NH volatilization is accounted for with liquid manures for all placements in a snowpack and semisolid manures applied on top of snow.

Spatially explicit methodology for coordinated manure management in shared watersheds.

Increased clustering and consolidation of livestock production systems has been linked to adverse impacts on water quality. This study presents a methodology to optimize manure management within a hydrologic region to minimize agricultural phosphorus (P) loss associated with winter manure application. Spatial and non-spatial data representing livestock, crop, soil, terrain and hydrography were compiled to determine manure P production rates, crop P uptake, existing manure storage capabilities, and transportation distances. Field slope, hydrologic soil group (HSG), and proximity to waterbodies were used to classify crop fields according to their runoff risk for winter-applied manure. We use these data to construct a comprehensive optimization model that identifies optimal location, size, and transportation strategy to achieve environmental and economic goals. The environmental goal was the minimization of daily hauling of manure to environmentally sensitive crop fields, i.e., those classified as high P-loss fields, whereas the economic goal was the minimization of the transportation costs across the entire study area. A case study encompassing two contiguous 10-digit hydrologic unit subwatersheds (HUC-10) in South Central Wisconsin, USA was developed to demonstrate the proposed methodology. Additionally, scenarios representing different management decisions (storage facility maximum volume, and project capital) and production conditions (increased milk production and 20-year future projection) were analyzed to determine their impact on optimal decisions.

Quantifying the Impact of Seasonal and Short-term Manure Application Decisions on Phosphorus Loss in Surface Runoff.

Agricultural phosphorus (P) management is a research and policy issue due to P loss from fields and water quality degradation. Better information is needed on the risk of P loss from dairy manure applied in winter or when runoff is imminent. We used the SurPhos computer model and 108 site-years of weather and runoff data to assess the impact of these two practices on dissolved P loss. Model results showed that winter manure application can increase P loss by 2.5 to 3.6 times compared with non-winter applications, with the amount increasing as the average runoff from a field increases. Increased P loss is true for manure applied any time from late November through early March, with a maximum P loss from application in late January and early February. Shifting manure application to fields with less runoff can reduce P loss by 3.4 to 7.5 times. Delaying manure application when runoff is imminent can reduce P loss any time of the year, and sometimes quite significantly, but the number of times that application delays will reduce P loss is limited to only 3 to 9% of possible spreading days, and average P loss may be reduced by only 15% for winter-applied manure and 6% for non-winter-applied manure. Overall, long-term strategies of shifting manure applications to low runoff seasons and fields can potentially reduce dissolved P loss in runoff much more compared with near-term, tactical application decisions of avoiding manure application when runoff is imminent.

Testing the Wisconsin Phosphorus Index with year-round, field-scale runoff monitoring.

The Wisconsin Phosphorus Index (WPI) is one of several P indices in the United States that use equations to describe actual P loss processes. Although for nutrient management planning the WPI is reported as a dimensionless whole number, it is calculated as average annual dissolved P (DP) and particulate P (PP) mass delivered per unit area. The WPI calculations use soil P concentration, applied manure and fertilizer P, and estimates of average annual erosion and average annual runoff. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field-years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from the average annual estimates used in the WPI, the WPI and measured loads were not well correlated. However, when measured runoff and erosion were used in the WPI field loss calculations, the WPI accurately estimated annual total P loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that the WPI methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment tool for managing runoff P losses.