The Timing of Winter Application of Dairy Manure and its Placement Within the Snowpack Affect Nutrient Loading to Snowmelt Runoff

Abstract

Runoff from agricultural fields can contribute to the degradation of surface waters when not adequately controlled. Application of manure in the winter is practiced in the upper Midwest of the USA, where seasonally frozen soils reduce or eliminate infiltration from precipitation events and can result in greater nutrient losses from overland runoff. While extensive research exists on the hydrological conditions that interact with manure nutrient losses from runoff during spring, summer, and fall, winter manure best management practices need further investigation. Further research is also needed due to the changing hydrological conditions from the effects of climate change, such as the increased number of freeze-thaw cycles and the shortening of the snowfall period during the winter season. Two studies evaluated whether the timing of winter manure application and its placement within the snowpack affect the nutrient content and loading of snowmelt runoff. In the first study, two dairy manure application timings were assessed under field conditions: early manure (applied in December or January and over frozen soils in the absence of snow) and late manure (applied in February or March and over a snowpack). In the second study, three manure placements were assessed within a 30-cm snow column under laboratory-based conditions: manure under snow (US), manure between snow (Mid), and manure over snow (OS). The timing of winter manure application showed a statistically significant lower nutrient loading to snowmelt runoff from the early manure treatment, regardless of the hydrological and climatic variations between years. Manure placement within the snowpack showed a significant treatment effect on nutrient loading for nitrate-N, ammonium-N, and dissolved reactive phosphorus. A significant placement treatment effect was not observed for total nitrogen (bound), total phosphorus, dissolved carbon, and dissolved organic carbon. These studies suggest that while nutrient loading to snowmelt runoff may be reduced, results may vary depending on hydrological conditions and the type of nutrient studied. This research may inform winter manure and water quality best management practices. Producers may better assess the timing of manure application to reduce nutrient losses. Goals can also be assessed depending on the nutrient of concern.