Cows don’t stop pooping

Manure management can be as tricky as managing livestock. Responsible producers pay close attention to manure storage and application throughout the year and avoid land application during winter.

Chryseis Modderman, Extension educator, soil and manure management, University of Minnesota, explained some of the challenges that come with manure in winter. Most states discourage manure application on frozen ground.

The primary adverse conditions for manure application include wet or saturated ground and frozen and snow-covered ground. These conditions increase the risk of runoff and volatilization and are challenging because manure often can’t be incorporated. Liquid dairy manure on waterlogged soil on a 0.8% slope increases the runoff of nitrogen, phosphorus and potassium.

“Levels of pathogens like E. coli and Enterococcus in drainage water are higher from winter-applied manure compared to spring and fall,” said Modderman. “Runoff with excess nutrients in waterways leads to eutrophication, which results in hypoxic conditions in water. Eutrophication causes excess growth of plant life and death of animal life due to lack of oxygen.”

While the key to avoiding runoff risks is incorporation, tillage isn’t always ideal, especially for those with no-till practices.

“Incorporation is not possible when soil is frozen or saturated,” said Modderman. “Manure sits on the surface and can’t soak into the ground, and that’s where losses come from.”

Volatilization, the atmospheric loss of N, still occurs in winter; it’s just a slower process. Modderman referenced a study showing that runoff was reduced by more than 80% when manure was incorporated with a moldboard plow, chisel plow or knife injection.

In late winter or early spring, manure application on wet or saturated ground increases the runoff risk and creates soil compaction, which reduces crop yields. Application on wet ground can also tear up the ground and leave the operator stuck in a messy field. Modderman referenced another study showing that the most runoff occurs in February and March during snowmelt, but this can vary according to location, and even within one farm.

“Animal type also affects nitrogen losses,” said Modderman. “High-ammonium manure will lose more total nitrogen than low-ammonium manure. Broadcasting manure on the surface and incorporating it within 12 hours allows the highest percent total nitrogen available.”

Waiting more than 12 hours contributes to N loss through volatilization. Waiting more than four days results in the highest loss.

Swine and poultry manures are typically high-ammonium manures, and if left on the surface for more than four days, half of the total N is lost because there’s more ammonium to lose. Cattle manure has less immediately available N.

Modderman explained three forms of N in soil: organic, ammonium and nitrate. “Manure supplies organic nitrogen and ammonium nitrogen,” she said. “Organic nitrogen sticks around and isn’t broken down, but it isn’t plant available. However, ammonium and nitrate are taken up by the crop. For the most part, manure doesn’t directly supply nitrate.”

Nitrogen in the soil is either mineralized, in which organic N can become ammonium N; or it undergoes nitrification, which is the process of ammonium N becoming nitrate.

“This is why a lot of cattle producers apply manure in fall,” said Modderman. “It has time to mineralize in soil and becomes plant-available by spring.”

She added that what can happen, especially with beef bedded pack manure, is high carbon and organic N results in early season N deficiencies in crops because manure isn’t breaking down fast enough for the crop to use.

Mineralization and nitrification are driven by bacteria and are influenced by both the environment and weather. It’s difficult to estimate when these processes will occur and when ammonium will be plant available. Volatilization is ammonium converting to ammonia. Nitrate has the most loss pathways and is lost to the atmosphere through denitrification. It also runs off from manure that’s on top of the soil and leaches downward.

“Nitrate dissolves easily in water,” said Modderman. “Where water is running, there’s probably nitrate in it. We want to keep manure and nutrients in the soil in the organic or ammonium form so it isn’t converted to nitrate. Manure is not supplying nitrate directly but can become nitrate in the soil, and once it’s nitrate, it’s a flight risk.”

Much like volatilization, mineralization still happens in winter – it’s just slower. Modderman recommends waiting until soil temperature is around 50º to slow the process. Manure types that contain more organic N will likely be less of a nitrate flight risk.

“When applying in winter, losses happen through volatilization and runoff,” said Modderman. “Things are happening with nitrate, but in the manure world, we are worried about ammonium being lost to the environment.”

In some cases, winter manure application may be necessary when autumn was excessively wet and harvest was delayed.

“If storage is completely full, land application is more environmentally safe than storage overflow or spill, even in challenging conditions,” said Modderman. “Communicate with regulatory agencies to create a plan if storage is running out.”

If application can’t be avoided, select fields in less sensitive areas, such as away from streams, drainage ditches, steep slopes, areas that flood easily or have high water tables, and away from springs and sinkholes. Be aware of bodies of water and setback regulations. If excess manure is a problem every year, plan to expand storage, which is easier for solid manure than for liquid manure. Composting solid manure helps reduce total volume by as much as 50%.

Fields with residue will retain manure best, and fields with furrows perpendicular to the slope can help slow runoff. Fields with furrows can be as effective in retaining manure on frozen soil as high residue fields. Land application just prior to snowmelt is usually a disaster that ends with a river of manure running off fields.

“Manure production doesn’t cease,” said Modderman. “Cows don’t stop pooping – manure has to go somewhere. Manure applied in winter is going to sit. Hopefully it will lose some nitrogen through volatilization.”

by Sally Colby