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How much nitrogen should be applied?

By Staff | Jan 31, 2020



OKOBOJI-Nitrogen (N) application on corn ground took center stage in a workshop at the Crop Advantage Series seminar, which took place Jan. 14 in Okoboji.

John Sawyer, professor of agronomy and Iowa State Extension and Outreach soil fertility specialist, told producers the Corn Nitrogen Rate Calculator (CNRC) has been around since 2004, but some still want more understanding of the approach used in the CNRC and how the rate guidelines are produced.

“We wanted to give (producers) a chance to understand some of the science behind our rate recommendations and guidelines,” he said.

Sawyer said N application recommendations vary across the state, and even across the Corn Belt states. But as producers seek out ways to receive the most return on their N investment-which Sawyer said is an expensive crop input-they can use this calculator to help them determine what amount of N could give them the most return for their money.

The CNRC uses the approach to recommended N rates called the Maximum Return to Nitrogen (MRTN). The economic return to N uses N and corn prices as determined by the producer, with the maximum return point determined as the MRTN rate. Sawyer said an under-utilized output is the Most Profitable N Rate Range -which is a range of N rates that provide nearly the same economic return to N application, but with different risks.

“When we do the calculation and analyze across the research database, we can determine a maximum return rate (MRTN), but we also know there is uncertainty around it because we also know the maximum return is pretty flat,” said Sawyer. “This provides a range around the MRTN that people can utilize. We can provide one rate suggestion, but we like to give farmers some flexibility on rate selection, to provide the Most Profitable Range.”

“The calculator uses many research trials,” he said. “We come with an output that is the point where we get the most economic return. Then we provide that Most Profitable Range around that economic return.”

Sawyer said researchers conduct N rate trials, accumulate regression equations that fit the N rate response, then analyze across the regression equations using the producer’s decision on corn and N prices. It then calculates a number (an output) across those research trials that indicate return to N for many N rates.

“When we get the complete calculation, then we can look at where the MRTN is, and we can also provide the range that is economically close to that actual point, said Sawyer, adding that they use $1/acre above and below the MRTN rate as a determinant range.

Sawyer said researchers are more interested in the amount of money producers make from a nitrogen input through the yield increase from N, rather than the crop yield level itself.

“We pick out where (producers) get the biggest net economic return. You have to pay for the input, and we want you to get the most return on your money. Farmers should understand that,” he Sawyer.

Sawyer said in the world of N, the CNRC tries to help the producer decide where they will get the most return year-in and year-out. That number is the MRTN rate.

Extension specialists across seven states in the Corn Belt-including Minnesota, Iowa, Wisconsin, Illinois, Indiana, Ohio and Michigan-have conducted and continue to conduct N rate research trials that are utilized in the CNRC. That research allows the rate guidelines to be continually updated and reflect changing weather and production practices. Suggested rates are specific to each state’s corn production conditions.

Sawyer said the CNRC and MRTN are a uniform approach to determining the most economical N rates that will provide the best return over time. He said the field trial database done in the real world provides solid information from which to draw.

“Nitrogen fertilization need and corn response is one of the most uncertain crop input decisions,” he said. “The CNRC addresses part of the uncertainty by use of a recent and large N response database, but also looks at economic response across that database to help guide decisions on the use of a rate more or less than the MRTN rate.”

More information about the CNRC and MRTN can be found on several ISU Extension and Outreach websites, including the following: Corn Nitrogen Rate Calculator (cnrc.agron.iastate.edu); Concepts and Rationale for Regional Rate Guidelines For Corn (store.extension.iastate.edu/product/12240, article number 3073); Nitrogen Use in Iowa Corn Production (store.extension.iastate.edu/Product/Nitrogen-Use-in-Iowa-Corn-Production), and Iowa State University Soil Fertility (www.agronext.iastate.edu/soilfertility).

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