Reducing Winter Phosphorus Losses from Dairy Agroecosystems through Tillage and Manure Application Timing.

Wintertime land-applications of manure are a longstanding practice for many dairy producers, but the presence of frozen soil and snowpack increases the potential for nutrient transport through surface runoff processes. Therefore, the objective of this study is to test practical management techniques that may reduce runoff on fields receiving winter applications of liquid dairy manure (DM = 2-3%). A replicated field study will be conducted for three years (2015-2018) under conventional and no tillage with three manure application timing treatments: unmanured controls, early applications to frozen ground (prior to snowfall), and mid-winter applications to snow-covered ground. A total of 18 plots, each 5 x 15 m, were installed in triplicate using a 2 x 3 complete factorial arrangement on a 5% slope planted in corn for silage. The plots are hydrologically isolated from the landscape and monitored for runoff volume and flow rate using a multislot divider runoff collector system and load cells. Runoff samples were tested for DRP, TS, VS, and EC. Manure was applied at a rate of 37.4 kL ha^-1 (4000 gal ac^-1) to prevent immediate runoff. During the first season (Winter 2015-2016), nine runoff events occurred, during which 84% of no till plots and 23% of conventionally tilled plots produced runoff in each event. DRP, TS, VS, and EC were significantly greater in treatments with no tillage and mid-winter applications of manure compared to treatments with conventional tillage, unmanured controls, and early applications of manure. Tillage created surface depressional storage, which slowed surface water movement and aided infiltration into frozen soil, while mid-winter applications of manure accelerated snowmelt processes, increasing runoff and nutrient losses.  This field study provides a mechanistic understanding of winter processes and a replicated dataset to help inform prediction tools that evaluate nutrient losses from agroecosystems to balance environmental and economic viability.