June 24, 2026

2026 June Stewardship Advocate

Iowa Nitrogen Initiative Year in Review for 2025

Several previous editions of the Stewardship Advocate have featured the Iowa Nitrogen Initiative (INI), the most extensive on-farm, in-field nitrogen rate trial ever done. Since its inception in 2022, more than 1200 variable-rate nitrogen trials have been conducted in Iowa cornfields. INI is a public-private partnership that includes Iowa State University (ISU), Iowa Department of Agriculture and Land Stewardship (IDALS), private agronomy and soil testing companies, commodity organizations, including Iowa Corn and other Iowa agricultural organizations. The INI is chiefly funded by IDALS through an appropriation from the Iowa legislature. The state funding is supplemented by the Iowa Corn Promotion Board, Iowa Pork Producers and the Iowa Soybean Association with in-kind support from ISU.

Objective of INI

The INI trials provide data that help the researchers characterize the impact of genetic and environmental factors, and management practices on the economic and agronomic optimum nitrogen rates. As more data is collected in upcoming years and combined with the datasets already in hand, the INI research team is striving to assist farmers in their nitrogen rate decisions at the local level, eventually at the sub-county level.

EONR vs. AONR

The Economic Optimum Nitrogen Rate (EONR) is the amount of nitrogen fertilizer application where the value of the final yield increase exactly covers the cost of applying that last unit of nitrogen. The Agronomic Optimum Nitrogen Rate (AONR) is the amount of nitrogen fertilizer required to maximize crop yield, regardless of cost. The INI analysis pays attention to EONR because of the implications to farmer profitability and not simply maximizing yield.

2025 by the Numbers

2025 was INI’s fourth year and with it came the highest participation level in number of trials and farmers (Table 1). About 5.2% of the trials included a “zero” rate, which means that one of the treatments in a trial did not receive an application of nitrogen. Additionally, there has been an emphasis in the last few years in recruiting fields with manure applications, and in 2025, about 18% of the trials had manure applied.

Parameter	Value
On-Farm Trials Designed	620
Farmer Participants	141
Zero Nitrogen Rate Trials	32
Manure Trials	110

Table 1. 2025 INI Trial Statistics.

Participants in the INI trials are allowed to use the source(s) of nitrogen and the application timing(s) in the trial that corresponded to the remainer of their farm. See Table 2 and Table 3 for the breakdown in nitrogen sources and timing of nitrogen applications from the 2025 INI trials.

Source of Nitrogen	Percentage
NH3	49%
UAN	23%
UAN + ATS	8%
Urea	20%

Table 2. Sources of Nitrogen in 2025 INI Trials.

Timing of Application	Percentage
Fall	44%
Spring (pre-plant)	13%
In-season	43%

Table 3. Timing of Nitrogen Applications in 2025 INI Trials.

2023-2025 Economic Optimum Nitrogen Rate Comparisons

The EONR in 2025 was 24% lower than in 2024 and 13% than in 2023 (Table 4). This observation may be due to high rates of mineralization during the middle of the growing season, which was characterized by warm, wet conditions. These conditions likely led to more mineralized nitrogen being taken up by the crop. A simple but useful metric for nitrogen use efficiency is the pounds of applied nitrogen divided by yield, which is shown in the third row of Table 4.

	2023	2024	2025
EONR	196 ± 35	226 ± 40	171 ± 40
Yield	228 ± 30	243 ± 33	228 ± 27
Pounds of N per Bushel	0.87 ± 0.15	0.94 ± 0.2	0.75 ± 0.18

Table 4. EONR, Corn Yield and Nitrogen Use Efficiency in 2023-2025.

N-FACT – Nitrogen Decision Support Tool

The N-FACT tool combines data from the INI trials with crop system models to estimate the economic and agronomic optimum nitrogen rates under different scenarios by selection location, expected crop year weather, residual soil nitrogen, crop rotation, planting date and fertilizer/crop pricing. In 2025, 8,000 unique users accessed N-FACT to estimate nitrogen rates in their local area and to compare rates across other areas of the state. Click on this link: https://n-fact.ag/start to access the N-Fact tool.

Long-Term Cover Crop Field Day

On June 4, Iowa Corn Growers Association President Mark Mueller hosted a cover crop field day in the field where he has had strips of cover crop and no-till since 2016. Mark was one of the first farmers enrolled in the Soil Health Partnership (SHP), which was initiated by the National Corn Growers Association. The SHP enrolled farmers to plant strips of cover crops in alternating strips without cover crops in large field trials to compare yields and soil health differences between the cover crop treatments. Even though the SHP wound down in 2021, Mark kept his strips in place because he continues to learn something every year to help him better manage cover crops, saying, “As a long-term practitioner of cover crops and no-till, I enrolled in the Soil Health Partnership in 2016 to compare my current practices with more traditional practices. I have been able to positively quantify my stewardship practices in the form of comparable yields and soil health benefits.”

The field day featured talks on the history of the long-term cover crop study, management tips for cover crops, a soil pit and information about edge-of-field practices such as bioreactors and saturated buffers. Mark also displayed his no-till planter at the site. The Iowa Corn trailer provided a welcome relief from the heat and information about Iowa Corn priorities.

Here are some photos from the field day: