Farmers can Improve Nutrient Management with Updated Nitrogen Rate Guidelines
Manure provides valuable nutrients for corn production, but farmers need to apply the right amount to meet crop needs. Applying too much nitrogen can waste farm resources and increase the risk of water pollution.
The University of Minnesota Extension has updated its manure application recommendations for non-irrigated corn to help farmers make better nutrient management decisions.
Melissa Wilson, Extension Specialist for Manure Nutrient Management, explained that the updated recommendations are based on nitrogen rate research conducted across Minnesota during the past 20 years.
Researchers have completed more than 230 trials to understand corn nitrogen needs under different conditions. The results show that previous crops and the soil's natural nutrient supply affect how much nitrogen farmers need to apply.
Updated Nitrogen Rates for Corn
Earlier recommendations use a standard estimate of 1.2 pounds of nitrogen per bushel of corn. However, this method does not fully consider the nitrogen supplied by the soil. New research supports different nitrogen factors based on the previous crop.
For corn planted after soybeans, farmers use 0.7 pounds of nitrogen per bushel of historical yield. For corn planted after corn, the recommended factor is 0.9 pounds per bushel. These values help farmers estimate the total plant-available nitrogen needed from manure and other sources.
Maximum Return to Nitrogen (MRTN) recommendations consider nitrogen costs and expected corn prices. However, this method does not directly apply to manure used during the growing season because manure nutrient values and costs differ among farms.
How to Estimate Nitrogen Needs
Farmers should first calculate the historical corn yield for each field using the previous five corn-growing seasons. If field records are unavailable, they can use five-year county yield averages from the USDA National Agricultural Statistics Service.
When drought, flooding, or another major weather event causes a large yield loss, farmers can replace that year's yield with an earlier year's figure.
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 needed per acre.
For silage corn, farmers convert the historical yield into a grain yield equivalent. They multiply silage yield in tons per acre at 65% moisture by 7.5 before applying the grain corn nitrogen factors.
Managing Phosphorus and Other Nutrients
Farmers also need to manage phosphorus levels when applying manure. Repeated applications can increase soil phosphorus beyond crop needs. Setting manure rates based on the phosphorus removed during harvest can help maintain nutrient balance.
Soil tests and manure nutrient analysis help farmers estimate plant-available nitrogen (PAN) and plant-available phosphorus (PAP). Nutrient availability depends on manure characteristics and management practices.
Farmers should also count nitrogen from starter fertilizer, irrigation water, legume crops, and manure or other organic materials applied during the previous two years.
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