Research Shows Winter Application
of Manure is a Bad Idea

 

Last winter in Wisconsin, improperly land-applied manure wiped out 10 years’ and $2 million worth of restoration work in one watershed. And that was just one of 50 recorded instances of improper winter applications of manure in the state. In other Wisconsin events, improper land application of manure was linked to a devastated fish population in a lake and contaminated private drinking water wells.

In the Northern U.S., where frozen soil and snow cover are common, winter manure application is generally a bad idea. In University of Vermont studies, winter application of dairy manure resulted in runoff concentrations of nitrogen and phosphorus from two to 15 times higher than those from summer application.

Considerable research has demonstrated that runoff from manure application on frozen or snow-covered ground has a high risk of serious water quality impact – and that trusted best management practices (BMPs), including vegetated filter strips, can be of little help in winter.

Run Off Increases

In warm weather, many good things can happen when you spread manure. Nutrients and organic matter enrich the soil. Most of the nitrogen and phosphorus are likely to be absorbed by the soil or taken up by growing crops. Bacteria and other pathogens die quickly in hot, dry conditions or are filtered out as water moves into and through the soil.

Manure can be an outstanding source of organic matter and crop nutrients – but timely application is a must. Photo courtesy of Don Meals

During the growing season, runoff carrying sediment, nutrients, or pathogens can be filtered by vegetated buffer strips before reaching water. Unfortunately, these things rarely happen if manure is spread in the winter, when riparian buffers or vegetated filter strips are not growing and are ineffective in removing pollutants from runoff.

In many cases, frozen soils are nearly as impervious as a parking lot. Snowmelt or rainfall following winter manure spreading are very likely to generate runoff that can carry manure constituents off the field and into waterways. While not all soils are frozen or totally impervious through the winter, the risks of runoff from thaw events or winter rains are generally quite high.

Although university studies in Minnesota and Vermont have shown that spreading solid manure in the winter can sometimes actually reduce the amount of runoff and of soil erosion by forming a mulch on the soil surface that slows down the flow of water, this effect would most likely not occur with liquid manure.

Nutrients Impair Waterways

Excessive levels of nutrients, especially phosphorus, cause algae growth and degrade fisheries, recreation, and aesthetic qualities in lakes and reservoirs across the U.S.

In a number of watersheds, runoff from winter-applied manure is a major source of nutrient loading. For example, one University of Wisconsin study estimated that 25 percent of the annual phosphorus load to a lake from animal waste sources came from winter spreading. In New York, a Department of Environmental Conservation study found that snowmelt runoff from winter-manured cropland contributed more phosphorus to a large reservoir than did runoff from poorly managed barnyards.

University researchers estimated that 40 percent of Vermont streams and lakes would suffer significant water quality impairments from the addition of just two winter-spread fields in their watersheds.

More Pathogens Run Off, Too

Bacteria and other pathogens from animal waste can cause serious illness in drinking water and in recreational waters.

Each gram of animal waste typically contains millions of indicator bacteria like fecal coliform and E. coli, and potentially contains pathogenic E. coli O157:H7 bacteria. (Although most strains of E. coli bacteria are harmless, E. coli O157:H7 is a highly infectious organism that can cause potentially deadly gastrointestinal illness and dehydration, especially in young children, the elderly, and people with impaired immune systems.) Animal waste also frequently contains the protozoa Cryptosporidium and Giardia. These protozoa cause mild to severe gastrointestinal illness and can shorten the lifespan of immune-compromised individuals.

Winter application of manure can increase pathogen losses in runoff from agricultural land compared to applications in other seasons. Cool temperatures and moist conditions in winter favor longer survival of microorganisms on the land. Even sub-freezing temperatures do not reliably kill bacteria – research shows that Cryptosporidium and Giardia can remain viable at temperatures well below freezing. In warm weather, most manure pathogens are killed or immobilized in soils by physical filtration, adsorption or predation by native soil microorganisms, especially if the manure is incorporated. In winter, when manure rests on the soil surface, interaction with soil is minimal and manure organisms are more readily carried away in runoff.

Lucky Combinations

There may, at times, be some limited local justification for winter manure application, such as reduced ammonia volatilization and odor concerns or reduction of soil compaction by spreading equipment. And it is true that several past university and USDA-Agricultural Research Service studies have sometimes reported minimal water quality impact from winter-spread manure, although such findings have usually been the result of fortunate combinations of weather, soil conditions and the exact position of manure in the snowpack.

Impervious soils and imminent runoff from snowmelt explain the extremely high nutrient loss from winter applications of manure. Photo courtesy of Don Meals

Lucky combinations, however, don’t come easily or regularly. High losses of manure constituents are nearly always reported when rain or melt closely follows manure application on frozen soil or snow. Other chance factors can make a difference, too. For example, nutrient losses in runoff tend to be greater if the manure is applied at the base or midpoint of the snowpack than if the manure is at the top of the snowpack. In general, the variability of such factors in time and space, as well as their unpredictability, makes the possibility of ideal conditions for winter manure application both unlikely and impossible to forecast. The risks of winter manure application to water quality (as well as the possibility of loss of a valuable agronomic asset) are simply too great.

Hidden Expenses

There are surely economic aspects to eliminating winter manure application. Storing manure over the winter can be a major expense. But remember that a manure storage facility has value beyond simply avoiding winter spreading. Storage allows a producer to follow a nutrient management plan by controlling the timing and amount of manure application, and that can result in savings on the purchase of commercial fertilizers.

Finally, loss of manure nutrients in runoff represent an economic loss to the farm. Studies in Minnesota, Vermont and Iowa recorded losses of 20 to 30 percent of applied nitrogen and phosphorus from winter-applied manure, so it quickly becomes obvious that keeping manure nutrients on the farm can be money in the producer’s pocket.

There’s also a public relations element to avoiding winter applications of manure. Nuisance complaints associated with agriculture are increasing in some areas, and the winter application of manure can add to a negative public impression of farming.

Just Don’t Do It

Winter manure application can have serious water quality impacts. While it may be possible to spread in winter without dire environmental consequences due to special weather and soil frost conditions, timing and placement in snowpack, and other factors, spatial and temporal variability and unpredictability of these factors makes this possibility both unlikely and impossible to predict.

This is why winter spreading is increasingly limited or banned entirely – in Vermont and Maine, in many Canadian provinces, and in some European countries. So keep manure nutrients on the farm and protect water quality at the same time by avoiding winter application of manure.

For More Information A good review of research on winter manure application appears in Flemming, P. and H. Fraser. 2000. Impacts of Winter Spreading of Manure on Water Quality - Literature Review. Ridgetown College. University of Guelph, Ridgetown, Ontario, Canada. The report is available on the web at: http://www.ridgetownc.on.ca/research/documents/fleming_manure.pdf.