Recently, Purdue experts updated their applied research findings on nitrogen management guidelines for corn in Indiana. The report summarizes corn yield response to fertilizer nitrogen (N) rate in field-scale trials conducted around the state of Indiana since 2006. The results are applicable to N management programs that use efficient methods and timings of N fertilizer application.
Co-authors Jim Camberato, Bob Nielsen and Dan Quinn recently made the report available at: https://www.agry.purdue.edu/ext/corn/news/timeless/NitrogenMgmt.pdf, and at https://thekernel.info/nitrogen-management-guidelines-for-corn-in-indiana/.
For quite a few years, N fertilizer rate recommendations were traditionally linked to expected yield level (Camberato, 2012). “For corn following soybean, the traditional rule of thumb was an N rate equal to about 1 lb. of N per bushel of expected yield,” they said. “For corn following either corn or wheat, the recommendation was equal to about 1.2 lbs. of N per bushel.” The authors said this rule of thumb implied there was a straight-line relationship between yield and N rate; meaning that the more N you apply, the more grain you would harvest. “In reality, the relationship is not a straight line,” they said. “As the amount of applied fertilizer N nears the optimum rate, the magnitude of the yield response decreases to zero.”
Two key types of nitrogen application rates are discussed in the paper: agronomic optimum nitrogen rate (AONR), and economic optimum nitrogen rate (EONR). Data are organized for various regions of Indiana, and Whitley County is within the Northeast Region.
“The Agronomic Optimum N Rate (AONR) represents the total amount of fertilizer N (including starter N) required to maximize yield, but not necessarily profit,” they said. “The AONR in these trials varied among regions of the state from about 211 to 254 lbs. N/ac, depending partly on soil organic matter and soil drainage characteristics.”
The researchers conducted paired trials at five Purdue Agricultural Centers to compare corn following soybean (corn/soy) and corn following corn (corn/corn). From 2007 to 2010, the average AONR for corn/corn was 44 lbs. greater than for corn/soy, while average corn/corn yields were 18 bu./acre less than the corn/soy yields, they said.
Economic Optimum N Rates (EONR) are defined as those that maximize dollar return from the nitrogen fertilizer investment. “Because the yield benefits from additional N decrease as N rates approach the AONR, the EONR will almost always be less than the AONR,” they said. “Region-specific EONR, calculated for various combinations of N fertilizer cost and grain price, are provided in the accompanying tables [in the report].”
Data from the report indicate that the northeast and east central regions of Indiana typically require the highest rates of nitrogen for AONR and EONR. According to the report, the AONR for the northeast region is 254 lbs. N/acre. EONR will vary depending on N fertilizer cost and grain price, as described above.
“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,” they said.
N use in corn is part of a complex biological system that interacts with everything under the sun and is difficult to model with computer programs. “We cannot accurately predict the weather. We cannot accurately predict soil N supply or availability throughout the year,” they said. “Yet, we cannot afford, financially or environmentally, to simply apply ‘more than enough’ N.”