Applying nitrogen: Fine-tune for your economic optimum rate

As we approach spring and the coming growing season, corn farmers facing tight margins or operating losses with their corn crop are looking for ways to be as economical as possible. One input that always seems to need constant monitoring and fine-tuning is the nitrogen input schedule. Two Purdue experts have recently completed an updated report to a 10-year field-level study of nitrogen rates.

“Applying more than enough nitrogen is no longer the cheap insurance it once was many years ago,” said Jim Camberato, soil fertility specialist and co-author of Nitrogen Management Guidelines for Corn in Indiana. “High nitrogen fertilizer costs and environmental impacts should encourage growers to critically evaluate their nitrogen management program, including application rate, fertilizer material and timing.”

Nitrogen is the most expensive nutrient used in corn production. If applied properly, it makes individual plants stronger and increases yield.

“Beyond some level of applied nitrogen, grain yield stops increasing with more additions,” said co-author Bob Nielsen, Extension corn specialist. “Consequently, applying more nitrogen than the crop requires is dollar wasteful and environmentally distasteful.”

In the report, Camberato and Nielsen provide updated, region-specific guidelines for nitrogen use based on field trials throughout Indiana. For corn grown after soybean, the economic optimum nitrogen rate or EONR, varied considerably across the state. Note that this designation is different from the agronomic economic optimum nitrogen rate, or AONR, we have historically focused on, with probably too little attention paid to economics. “The EONR is based on the relative cost of N and value of grain and consequently is lower than the AONR,” the authors said.

Looking at northeast Indiana, including Noble and Whitley counties, the authors summarize from the 10-year study: The average AONR for trials conducted on medium- and fine-textured soils were 263 pounds N / ac for northeast (11 trials) Indiana. The average EONR for northeast Indiana was 214 pounds N / ac. “Although we report a single AONR for a region, specific AONR values often vary from field to field and from year to year for a single field,” the authors said.

The study takes into account current nitrogen price and expected average grain price to determine the economic optimum nitrogen rate for the nine regions in Indiana. Growers should access the report, plug in current numbers and consider each field’s characteristics to determine how to fine-tune nitrogen rates for the current year.

The report reveals that northeast Indiana has the highest AONR and EONR needs in the state.

Timing the application is also important, Camberato said. Applying fertilizer in the fall or early spring increases the chances of nitrogen loss. The most efficient application method is to inject the nitrogen-based fertilizer when plants are in the V6 growth stage, or about 18 inches tall with six leaves.

“The bottom line on nitrogen use in corn is that it is part of a complex biological system that interacts with everything under the sun, including the sun,” Nielsen said. “We cannot accurately predict the weather. We cannot accurately predict the soil nitrogen supply throughout the year. Yet we cannot afford financially or environmentally to simply apply ‘more than enough’ nitrogen.”

The authors further state: “We can minimize the risk of fertilizer N loss by understanding the processes and matching N source with placement and timing. We can develop average N rate recommendations that will work to optimize profit over several years. We can attempt to fine-tune those recommendations with tests, models, optical sensors or simply educated guesses.”

One precaution the authors offer regarding nitrogen sources and application timing are that practices such as fall-applied or early-spring applied N or surface-applied urea provide a larger “window of opportunity” for N loss, and therefore would require higher N rates than their results would suggest to achieve optimum yield.

Nielsen and Camberato said their long-term objective is to develop soil-specific N rate guidelines. The general protocol for their N rate trials can be downloaded at agry.purdue.edu/ext/ofr/protocols/PurdueNTrialProtocol.pdf.

If you are interested in conducting on-farm N rate trials, contact Jim Camberato (765-496-9338 or jcambera@purdue.edu) or Bob Nielsen (765-494-4802 or rnielsen@purdue.edu). You may also contact the agriculture and natural resources Extension Educator in your local Purdue Extension office.

The full report is available on Nielsen’s Web site, the Chat ‘n Chew Café, at: kingcorn.org/news/timeless/NitrogenMgmt.pdf.

John Woodmansee is Extension Educator in Whitley and Noble counties. Darrin J Pack, Purdue staff writer, contributed to this article.

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