Adopting best practices will enhance nutrient management, protect water quality.

December 26, 2023

4 Min Read
National Pork Board

As winter approaches, the challenges associated with the application of manure become more pronounced. Adopting best practices that maximize nutrient utilization and safeguard our water resources is crucial. Daniel Andersen, associate professor of agricultural and biosystems engineering at Iowa State University, offers some critical considerations for swine manure application in the winter.

"Here are five areas to consider when making application decisions in the winter," he said. "The examples and reminders add additional information."

Compliance with regulations

In 2009, the Iowa Legislature passed a bill about the winter application of manure on snow-covered and frozen ground. The law applies to liquid manure from confinement feeding operations with more than 500 animal units in confinement.

This law does not apply to the following:

  • Manure from open feedlots.

  • Dry manure (frozen manure is not dry manure).

  • Liquid manure from small animal feeding operations (confinements less than 500 animal units).

  • Liquid manure that can be appropriately injected or incorporated on the same date of application.

Iowa prohibits liquid manure application from confinement operations with more than 500 animal units on snow-covered ground from Dec. 21 to April 1, and frozen ground from Feb. 1 to April 1.

The "winter manure application" rules have two parts: a date and condition. Both components must be true for liquid and slurry manure application to be prohibited. So, while the calendar date of Dec. 21 has approached, it is unlikely that the ground will be snow-covered (defined as 1 inch of snow or 0.5 inches of ice on the soil surface). As a result, liquid and slurry manure application would still be allowed until snow cover occurs.

DNR will allow for emergency applications when slurry manure applications are otherwise prohibited. If you must apply for manure under the emergency exemption, you must contact DNR before making a land application. You must also report certain facts and follow a certain protocol to meet the emergency exemption requirements.

Other winter manure application technicalities

If your operation is required to follow the Master Matrix, then make sure you comply with the land application for the Matrix since you may have gotten points for injecting or incorporating manure. If you need to winter apply, injection or incorporation may not be possible. If this change is made, pay extra attention to manure application setback distances required for surface application.

Understanding snow conditions

Applying manure directly onto snow-covered fields poses a significant risk of nutrient runoff when the snow melts. Especially if melt will occur quickly. Instead, wait to apply until after snowmelt or choose alternative manure management strategies. Applying manure on frozen ground is also prohibited, as the frozen surface inhibits absorption, increasing runoff potential.

Timing is key; in an emergency situation, apply manure when limited snow is present and quick melts aren't expected. Slow melting encourages water infiltration and absorption of manure nutrients into the soil. If necessary, choose fields with lesser snow accumulation that are relatively flat. Increase buffer or setback distances around field edges to reduce the risk of nutrient transport from the field.

Thoughtful land use choices

Assessing land use choices is pivotal in winter manure management. Avoid applying manure on steep slopes or areas prone to poor drainage, as these increase the likelihood of runoff. If applying in winter, for emergency, fields with a phosphorus index of less than two are required, and the field needs to be designated in your manure plan as an emergency application field. Choose fields with good water-holding capacity to minimize the risk of nutrient loss.

Additionally, consider the implementation of cover crops, which capture excess nutrients and protect the soil from erosion during winter months. Residue cover offers three main benefits – it helps hold the soil in place, slows down runoff, reduces erosion, and acts as a filter to help hold dislodged soil and manure particles. However, residue can also capture snowfall and hold it in the field. While it takes approximately 10" of snow to make an inch of water, ice-crusted soil water intake is often slow.

Protecting water quality

Prioritize water quality by maintaining setback distances from water bodies and sensitive areas. Implementing vegetative buffers helps prevent nutrient runoff into waterways. Incorporating manure into the soil soon after application is another effective strategy, reducing the risk of surface runoff and enhancing crop nutrient absorption. Where possible, maintain grass or cover crop buffers downslope of manure-applied areas to slow any runoff and ensure solids in the water settle out before trickling away. Leave a setback area around field borders to reduce the risk of manure nutrients leaving the field.

In conclusion, Andersen said that adopting these best practices for manure application in the winter will enhance nutrient management and contribute to protecting our water resources.

"While research has consistently shown greater potential risk of nutrient loss from winter manure application, research studies demonstrate loss is often related to quick snow melt or rainfall on frozen soils," he said. "Given current Iowa conditions with no snowpack and low chances of rainfall, the late risk of nutrient loss is lessened this year. However, it's important to stay informed of the forecast and be proactive to changing weather conditions to help ensure a sustainable and environmentally responsible approach to winter manure application."

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