Reducing Winter Runoff 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, we tested practical management techniques that may reduce runoff on fields receiving winter applications of liquid dairy manure, and used a water-energy balance approach to identify and quantify the key drivers of runoff on frozen, agricultural soils. To test these objectives, a two-year (2015-2017) field study was conducted at the UW Arlington Agricultural Research Station in Arlington, WI, with 18 plots installed on a 6 % slope and planted in continuous corn for silage. Using a 2x3 complete factorial design in triplicate, the management treatments included tillage: a fall chisel plow with a spring finisher versus no-tillage, and three manure application timings: unmanured controls, early applications during the freezing season, and mid-winter applications. Manure was applied at a rate of 37.4 kL ha^-1 (4000 gal ac^-1) to prevent immediate runoff. The plots were monitored for runoff volume and runoff samples were analyzed for TKP, DRP, and TS. Atmospheric (albedo, wind speed, air temperature, and vapor pressure), soil (temperature, matric potential, water content, and frost depth), and hydrologic (precipitation, snow-water storage) parameters were directly measured. Over the two winter seasons, 19 runoff events occurred and the no-tillage plots, regardless of manure treatment, were twice as likely to produce significant runoff compared to the tilled plots. During the freezing season, cumulative losses of TKP ranged from 0.1 – 0.4 kg ha^-1 in the tilled plots and 0.2-3.7 kg ha^-1 in the no-tillage plots, while DRP losses ranged from 0-0.3 kg ha^-1 in the tilled plots and 0.1-1.6 kg ha^-1 in the no-tillage plots. Likewise, TS losses ranged 0.3-84.0 kg ha^-1 in the tilled plots and 98.8-836 kg ha^-1 in the no-tillage plots. Tillage created surface depressional storage, which slowed surface water movement and aided infiltration into frozen soil, while applications of manure decreased albedo, accelerating snowmelt, hence 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.