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.