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Beware manure-runoff risk in winter
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Beware manure-runoff risk in winter

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The value of nitrogen, phosphorous, potassium and sulfur in liquid dairy manure produced by 100 milking cows annually is $9,112. That doesn’t take organic-matter credits into account.

But there are water-quality issues in Wisconsin stemming from agricultural operations and winter runoff, says Francisco Arriaga, associate professor and specialist with the University of Wisconsin-Department of Soil Science and the UW-Division of Extension. Field conditions have an effect on that runoff.

“Fields with low infiltration capacity or drainage issues have a higher runoff risk,” he said.

Because soil is a complex network of pores that conduct water, the infiltration capacity is reduced when freezing conditions plug soil pores; ice increases runoff production.

In a winter-runoff study comparing fall chisel tillage with no tillage, two winter liquid dairy-manure applications were applied in December and late January. The site was in alfalfa four years prior to being converted to corn silage for the study.

“Initially we decided to apply 7,000 gallons per acre but saw instant runoff at lower levels, so we cut the rate back to 4,000 gallons per acre,” Arriaga said. “This shows the challenges of winter manure applications.”

The plots were on a 5.8 percent slope and designed to allow a wide variety of data collection. Greater nutrient losses due to runoff occurred in the no-tillage system in both years. Data showed chisel tillage allowed more time for water infiltration compared to the no-till.

“Under the chisel there are depressions created that trap water, giving it longer contact time with the soil,” he said. “This gives it more time to infiltrate.”

Study data showed how soil frost affects runoff including brief periods when the surface of the ground thaws while frost remains below the surface. That happened in both winters during the study; data indicated a reduced runoff risk.

“This is helpful because it tells us when we’re in the middle of winter and the ground thaws even a small amount, it’s better to apply manure then versus when it’s totally frozen,” Arriaga said.

Manure management affects the rate of snow melt when liquid manure is applied on a snowpack. Because the snow darkens with the application of liquid manure it collects more solar energy through the snowpack – creating a greater chance of runoff. The opposite is true of solid-manure application on a snowpack because it acts like an insulator, slowing the melt rate.

Researchers also looked at winter runoff losses compared to the rest of the year. The two-year study showed a runoff rate 48 percent greater across the year in the no-till system compared to chisel tillage. No-tillage runoff was greatest when the soil was frozen whereas chisel-tillage runoff was dominant during the growing season. A single dramatic runoff event can make a big difference in annual totals, Arriaga said. Tillage-management decisions need to be made considering annual effects and site-specific runoff characteristics.

Producers should apply injected manure at a level to meet the phosphorous need of the crop being grown. When manure application targets nitrogen the phosphorous level is excessive and prone to runoff, he said. Field conditions are important; in a case in the study, instant runoff was measured at a rate of 4,000 gallons per acre.

Nitrogen is saved when manure application is done when soil temperatures are 50 degrees Fahrenheit or less. Avoid application to fields with greater than a 6 percent slope, especially when the ground is frozen.

“Cover-crop residue is fantastic at reducing runoff losses,” Arriaga said. “That’s something for growers to consider along with buffer strips and other conservation practices.”

It would be nice to have more data on tillage versus no tillage from other sites, he said.

“We’re currently using some other approaches including meta-analysis which should have results next year,” he said.

The Runoff Risk Advisory Forecast is a tool that assesses conditions related to runoff risk in Wisconsin. Visit manureadvisorysystem.wi.gov for more information.

Visit ipcm.wisc.edu for more information.

Francisco Arriaga was the presenter for a recent University of Wisconsin webinar titled, “Challenges of Liquid Manure Management in Wisconsin: Dealing With Winter Runoff.”

Greg Galbraith, a former dairy farmer who owns woodlot property in eastern Marathon County, Wisconsin, writes about the rapidly changing nature of the agricultural landscape. He has built a lifetime connection to the land and those who farm it.

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