With the increased cost of commercial fertilizer and the ever-increasing pressure on producers to protect and preserve water quality, University of Wisconsin (UW) – Discovery Farms staff get a lot of questions on how farmers can reduce their losses of manure.

Manure management will always have some risk; it’s inherent in every system. However, there are ways for producers to reduce these risks and to apply manure at times and using methods which reduce the potential for nutrients to enter into waters of the state (surface or groundwater).

The first step is to identify high-risk areas and time periods (Table 1*). This is relatively easy for sites that impact surface water because most of the factors are visible, such as slope and distance to water. Nutrient management rules place limits on when you can apply manure on fields based on slope and distance to surface water. Timing is more of an issue, and if we review three years’ worth of data collected on seven sites in Wisconsin (tilled, no-till and grazing operations), you can see that most of the runoff occurs on frozen soils in early spring (57 percent February and March). The majority of the rest of runoff comes off before June 15th.

One of the critical time periods for producers in northern climates such as Wisconsin is late in the winter when the ground is frozen and the potential for snow melt or rain on snow is high. Another critical time period is early in the spring when the soils are saturated and rain is likely. Determining when soils are saturated is usually not easy for producers, and the staff at UW – Discovery Farms are working on another method of identifying these conditions.

Work over the past three years indicates that soil moisture can be used to predict runoff potential. The data collected on several of our farms indicates that the majority of runoff on non-frozen soils occurs when soil moisture levels exceed 32 to 35 percent. While there are intense storms that can produce runoff when soils are drier, even small storms can produce runoff when the soils are at the upper range of this moisture level. Our goal is to be able to link soil moisture levels with acceptable application rates of liquid manure and then tie these measurements into both short- and long-range weather forecasts. This would provide producers with site conditions and a prediction of rainfall potential so that manure systems could be managed to apply manure when the risk of runoff is low.

Now that we have identified the critical conditions and time periods for surface water runoff events, let’s tackle another of the challenges for manure management – groundwater protection. It’s one thing to know what is leaving your farm and entering surface water, but how do you know what is filtering through your soil and entering groundwater? In Wisconsin and several neighboring states this is a very important question because we have karst topography. Karst topography is a landscape shaped by the dissolution of a layer or layers of soluble bedrock (usually carbonate rock such as limestone or dolomite). Many karst regions display distinctive surface features, with sinkholes being the most common. The challenge in karst regions is that many times the soils are shallow and the underlying bedrock has cracks which connect the soil surface to groundwater.

Studies in Wisconsin have shown that nutrients can quickly move through cracks and other macropores where the soils are thin or the soils have large openings due to unusual weather conditions. One key to managing manure in these areas is identifying how much soil you have (depth of soil to bedrock or groundwater), the types of soil and the potential for your soil to hold nutrients and water. Matching your soil’s capacity to hold and utilize nutrients is the key to reducing losses to groundwater. It is important for producers to understand that we’re not talking strictly about nutrient application rates, but also the water-holding capacity of the soil. Applying high rates of dilute products (low levels of nitrogen or phosphorus) can increase the leaching potential. ANM

Table omitted but is available upon request to editor@progressivedairy.com.