What's the big deal with winter spreading of manure?
We frequently hear about current rules and proposals for restricting winter spreading of manure. Winter spreading has been a common practice for decades, so what is this discussion all about? Manure provides an excellent source of many nutrients essential for crop growth. There are two major considerations with winter spreading of manure: environment and economics. Phosphorus and nitrogen are two major nutrients in manure that can have negative effects in the environment when in excess.
Excess phosphorus entering our Minnesota lakes, rivers and streams contributes toward eutrophication, or rapid growth of plant life. This excess plant life eventually dies, and the decaying material causes an oxygen shortage for desirable fish and plant life. We typically observe eutrophication as algae blooms. A high percentage of the phosphorus in manure is soluble, and when quickly incorporated into the soil this soluble phosphorus rapidly attaches to the soil. Once attached, it will essentially only leave the field by plant uptake or with the soil through erosion. There is a high environmental risk when this soluble phosphorus is allowed to remain on top of the soil. A snowmelt or rain event allows this soluble phosphorus to dissolve with the water and potentially runoff into the environment. Phosphorus runoff risk increases with frozen ground, an ice layer on the soil or snow, deep hard packed snow, and manure applied closer to snowmelt.
When manure is left on the soil surface, most of the nitrogen in ammonia form is lost through volatilization. Many of the organic manure nitrogen compounds are soluble in water and are subject to spring runoff or a rain event; thus also becoming a contributing factor in eutrophication. The following table illustrates the relative availabilities and losses of dairy manure applied by various tillage methods and incorporation timing. In general, the table tells us that faster and better manure incorporation means higher nitrogen availability for crops. This table illustrates losses on unfrozen ground, so consequentially, the unincorporated column would be the best nitrogen availability we could attain when spreading manure on frozen ground. Additional losses from winter application would vary depending on an ice layer on the soil or snow, deep hard packed snow, and if manure is applied close to snowmelt.
|Dairy manure nitrogen availability and losses|
|Surface broadcast and incorporation timing||Injection|
|None||< 4 days||< 12 hours||Knife||Sweep|
|----------------------------------------------- % Total N -----------------------------------------------|
|*Losses through volatilization and denitrification.
SOURCE: Hernandez, Schmitt. 2012. Manure Management in Minnesota, University of Minnesota Extension.
As it relates to nitrogen, the question comes down to the tradeoff between applying manure in winter to possibly save time during the spring, construction expenses of a long term manure storage structure, or some other farm system modification enabling spring application and incorporation of manure after the spring snowmelt and ground thaw. A common method of facilitating this on a dairy farm is the construction of a long term storage manure pit. Pumping the pit in the spring and/or fall and incorporating it into the soil immediately or within 12 hours produces maximum economic return from the manure nutrients while also protecting the environment.
What if a dairy farmer has no pit or has manure packs that cannot be added to an existing manure pit? Another option approved for most manure management plans is temporary stockpiling of manure. According to Minnesota feedlot rules, a temporary stockpile must not be in a single location for more than one year and is not allowed on slopes greater than 6%. Stockpiling also requires following setback requirements and some common sense. A setback distance of 300 feet is required for most sensitive areas such as wetlands, road ditches, and tile-intakes; a 1000-foot setback is required from most lakes, streams, and community water supplies. Stockpiling setback requirements can vary between counties so contact your county feedlot officer or Soil and Water Conservation District staff. Manure properly stockpiled, later applied on unfrozen ground and incorporated within 12 hours will typically have nutrient content higher than manure spread on frozen ground, on an ice layer on the soil or snow, in deep hard packed snow, or when applied close to a snowmelt event.
Is a temporary stockpile the best solution to winter application of manure? No, but it is one option to consider rather than applying the manure over frozen ground. Increases in commercial fertilizer prices make the nutrients in manure very valuable; application of manure on frozen ground almost guarantees economic loss on the value of manure while also jeopardizing the environment.