IMPACT of Manure Application in Different Seasons on Phosphorus Loss in Runoff.

Agricultural nutrient management is an important area of research and policy due to concerns of phosphorus (P) loss in runoff and water quality degradation. Surface manure application to fields without incorporation can be a significant source of P loss. In northern states, winter manure application without incorporation is common. This fact, combined with frequent snowmelt runoff, has prompted some states to restrict winter manure spreading. Overall, good understanding of P cycling and transport associated with winter manure application is lacking. We collected precipitation and field runoff data from six monitored locations in WI. In total, there were 108 site years of runoff data representing a variety of climate and soil conditions. We divided the data into groups of high, medium, and low runoff. We then used the SurPhos model to simulate dairy manure application, allowing the model to change the day of manure application so that each day of the year was represented. Phosphorus loss in runoff is generally low for all rates of site runoff if manure was applied between March and October. For the low runoff sites, P loss in runoff increases if manure is applied in the winter, but not very dramatically. For medium and high runoff sites, applying manure during the winter can significantly increase the risk of P loss, with peak loss occurring if manure is applied around late January to early February. Results show avoiding winter manure application could help decrease potential P loss in runoff. Because the rate of runoff (high, medium, or low) is not always the same for a given field every year (i.e., the same field could have low runoff one year and high the next), it may be difficult to reliably identify low runoff fields that may be able to receive winter-applied manure.