Farm Progress

Corn Source: Updated online tool helps determine most profitable N rate for corn.

November 17, 2017

3 Min Read
USEFUL TOOL: Using the Maximum Return to Nitrogen concept, the online calculator helps farmers determine the most economical rate of N to apply based on corn and nitrogen prices and recent N rate research data.

By Virgil Schmitt

Planning for 2018 crops is in full swing — with that comes decisions about nitrogen rates for corn. An excellent tool to help farmers make future decisions is the revised Corn Nitrogen Rate Calculator. The University of Illinois, the University of Minnesota, the University of Wisconsin and Iowa State University, as well as several other universities, are partners in sharing data used in this tool.

The user needs to tell the tool where he or she is located, what the price of nitrogen fertilizer is, and what the price of sold corn is, and the tool will respond with the most profitable rate of nitrogen to apply year-in and year-out. It will also provide a range of nitrogen rates that will be within $1 per acre of the most profitable rate. This calculation is done based on hundreds of nitrogen rate response trials conducted at various locations in each state over many years.

Consider farm’s location in Iowa
A farm’s location can be narrowed to southeast Iowa or the rest of the state. Work at the ISU research and demonstration farms showed that on the poorly and very poorly drained soils in southeast Iowa, slightly higher nitrogen application rates are needed compared to the rest of Iowa. However, the farmers in southeast Iowa with better drained soils may want to use the rates recommended for the remainder of Iowa. More details are in the publication “Nitrogen Use in Corn Production.” 

Commonly, farmers will know the price of nitrogen but not the selling price of corn, so there is the ability to use multiple prices. Farmers can then see where the profitable rate ranges mentioned above will overlap. An application at a rate where the ranges overlap will provide profits within $1 per acre of maximum profit from nitrogen fertilizer, regardless of corn price at the end of the season.

‘Insurance’ applications
There is a temptation to apply extra nitrogen “as insurance” in case the summer is especially rainy and nitrogen is lost. In most years, that extra nitrogen is a cost without a benefit and serves only to add nitrates to water running down the Mississippi River.

Work done by John Sawyer at Iowa State University suggests that monitoring total rainfall in April, May and June (in all but southeast Iowa) will provide excellent insight into the need, or lack thereof, for supplemental nitrogen. His work suggests that if the total rainfall for that three-month period exceeds 15.5 inches, farmers should consider adding about 50 units of supplemental nitrogen fertilizer in very early July. And if rainfall amounts are less than 15.5 inches, sufficient nitrogen should remain in the soil. For southeast Iowa, the “trigger” is 17.8 inches of rainfall during March, April, May and June.

Following that guideline will be the correct course of action nearly 80% of the time. The remaining 20% of the time is divided almost evenly between putting on nitrogen when it is really not needed and not putting on nitrogen when it would have been beneficial.

Late-spring soil nitrate test
Farmers who are concerned about nitrogen losses due to rainfall between nitrogen (either commercial or manure) application and late spring may want to use the Late Spring Soil Nitrate Test to verify either the presence or absence of adequate nitrogen for the crop. The test is taken when corn plants are 6 to 12 inches high at the whorl. If the test shows that nitrogen is lacking, a sidedress application can be made.

Details can be found in “Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production.”  Note that this publication was updated in May with new information.

In the long run, the most profitable use of nitrogen fertilizer can be achieved by using the Corn Nitrogen Rate Calculator and then using the late spring soil nitrate test and/or monitoring spring rainfall amounts to determine if supplemental nitrogen should be applied.

Schmitt is the ISU Extension field agronomist for southeast Iowa, based at Muscatine. Contact him at [email protected].

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