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Report Shows Optimum Nitrogen Rates for Indiana

Published: Friday, February 19, 2021

The following is from John Woodmansee, Whitley County Extension educator.

Corn farmers always seem to be searching for that perfect nitrogen (N) application rate, probably because it can be a moving target. Apply too little, and you miss out on yield potential. Apply too much, and you waste money. Apply at the wrong time, and you may lose nitrogen. Two Purdue University experts have recently completed an updated report to a 13-year field-level study of nitrogen rates, entitled, "Nitrogen Management Guidelines for Corn in Indiana."

"Applying 'more than enough N' is no longer the cheap 'insurance' it was once considered many years ago," said Jim Camberato, soil fertility specialist and co-author of the report. "High N fertilizer costs and environmental impacts dictate that growers should critically evaluate their N 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."

The authors explained that nitrogen fertilizer recommendations were traditionally linked to expected yield level.

"For corn following either corn or wheat, the recommendation was equal to about 1.2 pounds of N per bushel," they said. "This rule of thumb implied a straight-line relationship between yield and N rate; meaning that the more N you apply the more grain you would harvest." However, instead of a straight line, the authors said that as the amount of applied fertilizer N nears the optimum rate, the magnitude of the yield response decreases to zero (flattens out).

Camberato and Nielsen provided updated, region-specific guidelines for nitrogen use based on field trials throughout Indiana. For corn grown after soybeans, the economic optimum nitrogen rate, or EONR, varied considerably across the state. Note that this designation is different from the agronomic optimum nitrogen rate, or AONR, we have historically focused on to maximize grain yield, with probably too little attention paid to economics.

"The EONR is usually less than the AONR, will usually decrease as N price increases, will usually increase as grain price increases, or may remain the same as long as the ratio between nitrogen cost and grain price (N:G) remains the same," the authors said.

Looking at Northeast Indiana, including Whitley County, the authors summarized from the 13-year study:

• The average AONR for trials conducted on medium- and fine-textured soils was 254 pounds nitrogen per acre.

• The average EONR was 212 pounds nitrogen per acre.

"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. Northeast Indiana has the highest AONR and EONR in the state.

Year-to-year variation in optimum N rate will vary due to annual variability in soil N supply, fertilizer N loss and weather.

"It is important to note that most of our trials used efficient fertilizer application methods and timings," they said. "Less efficient N management typically require higher fertilizer N rates in response to the greater risk of N loss."

"The most efficient N application method and timing for minimizing N loss is to inject N prior to the beginning of rapid crop N uptake at roughly growth stage V6 (six leaves with visible leaf collars, approximately 18 inches tall)," they said.

"Practices such as fall-applied or early-spring applied N or surface-applied urea typically result in the greatest N loss and therefore typically require higher N rates to achieve optimum yield," they said.

The full report is available at:

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