New Manure Guidelines Help Farmers Manage Nitrogen

By Scout Nelson

Manure is a valuable source of nutrients for corn production. However, applying the right amount is important for crop growth, soil health, and water protection. The University of Minnesota Extension has updated its manure application rate recommendations for non-irrigated corn based on recent soil fertility research. These changes help farmers estimate nitrogen needs more accurately and reduce the risk of applying too much.

Melissa Wilson, Extension Specialist for Manure Nutrient Management, explained that updated recommendations use research from nitrogen rate trials conducted across Minnesota over the past two decades. More than 230 trials have helped researchers understand how much additional nitrogen corn needs under different growing conditions. The findings show that nitrogen needs depend partly on the crop grown in the previous season and the soil’s ability to supply nutrients.

What Has Changed in Nitrogen Recommendations

Earlier manure recommendations use a general estimate of 1.2 pounds of nitrogen per bushel of corn. This approach does not fully account for nitrogen already supplied by the soil. Research now shows that soil can provide a significant amount of nitrogen, depending on the previous crop.

For corn grown after soybeans, the recommended factor is 0.7 pounds of nitrogen per bushel of historical yield. For corn grown after corn, the factor is 0.9 pounds of nitrogen per bushel of historical yield.

These factors help farmers estimate the total plant-available nitrogen needed from manure and other sources. The updated approach recognizes differences between crop rotations and supports better nutrient planning.

Fertilizer recommendations may also consider the cost of nitrogen and the expected price of corn. This method is known as Maximum Return to Nitrogen, or MRTN. However, it does not directly apply to manure used during the growing season because manure costs and nutrient values vary widely among farms.

How Farmers Calculate Nitrogen Needs

Farmers first need to determine the historical corn yield for each field. This is the average yield from the previous five corn-growing seasons. Field records provide the most useful information when available.

If field yield records are missing, farmers can use the average county corn yield for the previous five years. These figures are available from the USDA National Agricultural Statistics Service.

If drought, flooding, or another major weather event causes a large yield loss in one year, farmers can leave that year out of the average and use an earlier year instead.

For grain corn, farmers multiply the historical yield by the correct nitrogen factor

• Corn after corn: Historical yield multiplied by 0.9 pounds of nitrogen per bushel.
• Corn after soybeans: Historical yield multiplied by 0.7 pounds of nitrogen per bushel.

The result estimates the total plant-available nitrogen rate needed per acre. Farmers must then account for nitrogen from manure, fertilizer, and other sources before deciding how much additional nitrogen to apply.

For corn grown for silage, historical silage yield is first converted to a grain yield equivalent. Multiply the silage yield in tons per acre, measured at 65% moisture, by 7.5. Then use the grain corn nitrogen factors to estimate the nitrogen requirement.

The University of Minnesota Extension’s manure application rate recommendations provide further details and examples.

Managing Phosphorus Through Manure Application

Nitrogen is not the only nutrient farmers need to consider. Phosphorus is also important for crop growth, but repeated manure applications can cause phosphorus levels to build up in the soil.

Farmers can help manage this risk by setting manure rates based on the phosphorus expected to leave the field in the harvested crop. This approach replaces nutrients removed during harvest instead of adding more phosphorus than the crop needs.

Soil tests and manure nutrient information help farmers make better decisions about application rates. Careful planning can support crop production while limiting excess phosphorus in the soil.

Consider Nutrients from Every Source

Not all nutrients in manure become available to plants during the first year. Some nitrogen and phosphorus are available soon after application, while other nutrients remain in organic matter and become available as it breaks down.

Farmers need to calculate first-year plant-available nitrogen (PAN) and plant-available phosphorus (PAP) when determining manure rates. The amount available depends on manure characteristics and how it is managed.

It is also important to count nitrogen from all sources. These include starter fertilizer, irrigation water, manure and other organic materials applied during the previous two years, and nitrogen credits from legume crops.

Applying more nitrogen than a crop needs can increase the risk of nitrogen leaching into groundwater and other water sources. Matching nutrient applications to crop needs helps reduce this risk and makes better use of farm resources.

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