Ideal nitrogen fertilizer rates in Corn Belt have been climbing for decades, Iowa State study shows

(Press-News.org) AMES, Iowa – The amount of nitrogen fertilizer needed to maximize the profitability of corn production in the Midwest has been increasing by about 1.2% per year for the past three decades, according to new Iowa State University research.

The study, published last month in Nature Communications, analyzed data from prior long- and short-term studies by Iowa State and the University of Illinois to calculate the Corn Belt’s steadily rising optimum nitrogen rates, which researchers had thought were static over time despite year-to-year fluctuations. Authors of the study primarily attributed the increase in optimum nitrogen rates from 1991 to 2021 to increased loss during wetter springs and the nutrient demands of higher yields, which also rose about 1.2% per year over the same time span.

“As much of a surprise as it was to us, it really wasn’t a surprise when you sat down and thought about it,” said study co-author Michael Castellano, agronomy professor and the William T. Frankenberger Professor in Soil Science. “It’s like a bank account. If you pull money out, you need to deposit more money to keep the account going.”

The analysis tracks with surveys of Iowa farmers, whose self-reported rates of applying nitrogen fertilizer on corn also have increased in recent decades.

“Seeing this study come out, I’m sure some of them will say, ‘I knew it!’,” said study co-author Sotirios Archontoulis, Pioneer Hi-Bred Agronomy Professor.

Efficiency improving

Data-based confirmation of corn’s climbing nitrogen needs reiterates the importance of continuously improving the efficiency of fertilizer use, which is essential to limit the impact on water quality and greenhouse gas emissions. Knowing the science is more in synch with farmers’ experiences may help build confidence in expert recommendations for effectively applying nitrogen fertilizer, said professor of agricultural and biosystems engineering and Brent and Cindy Hart Professor Matthew Helmers, director of the Iowa Nutrient Research Center, the study’s primary funder.

“Farmers hear information from lots of different sources. There’s some uncertainty there,” said Helmers, also a co-author of the study. “But the more they can fine tune their nitrogen management, the greater return they’ll see on their investment in the field and the more we will reduce environmental losses.”

While there’s still room to grow, farmers have become more efficient in applying nitrogen fertilizer. Using 0.7 pounds of nitrogen or less per bushel of corn is a common goal in recent years for Iowa farmers, but Iowa State’s recommendation was 1 pound per bushel 20 years ago and 1.2 pounds per bushel 30 years ago, Castellano said. Rising optimum nitrogen rates over the last 30 years would have been even higher if improved efficiency hadn’t slowed the increase.

Crop rotation, improved drainage and spring fertilizer application are among the highest-impact practices for increasing nitrogen use efficiency, according to an ISU Extension and Outreach tipsheet released last year by many of the same researchers who contributed to the new study.

Applying the right amount of nitrogen is also crucial, and farmers have more resources than ever for setting rates specifically tailored to individual fields. The Iowa Nitrogen Initiative conducts trials on private farms across the state, using the data and cropping system modeling to offer Iowa corn farmers more precise suggested nitrogen rates. The public-private partnership – led by Castellano and Archontoulis and supported by the Iowa Department of Agriculture and Land Stewardship and numerous commodity groups – earlier this month released the initial version of its digital decision support tool. N-FACT (Nitrogen Fertilizer Application Consultation Tool) recommends custom rates by location, weather, soil nitrogen, planting date, cropping system and market prices.

“Farmers knew optimum rates were increasing before we did, and now through our partnership with farmers, the research will closer match the actual realities of on-farm production,” Castellano said.

Trend likely to continue

Efficiency has a bigger impact on the environmental sustainability of nitrogen fertilizer than the rate at which it’s applied. That’s good news because optimum rates are likely to continue inching higher, said Mitchell Baum, a postdoctoral research associate in the agronomy department and first author of the study.

“As long as yields are going to increase, unless there are massive increases in efficiency, we don’t see any signs of this slowing down,” Baum said.

The analysis outlined in the study focused on three different types of optimum nitrogen rates: economic, agronomic and environmental. The economic optimum is based on maximizing farmer profits, the agronomic optimum is the point at which additional nitrogen has no effect on yield and the environmental optimum incorporates the estimated financial cost of nitrous oxide emissions and nitrate leaching into groundwater.  

The economic optimum is always lower than the agronomic optimum and higher than the environmental optimum, but the gaps between the rates are changing. The difference between the max-yield agronomic optimum and the max-profit economic optimum shrank by 79% over the 30 years studied, while the difference between the economic and environmental rates grew by 34%.

Reducing nitrogen fertilization rates to the environmental optimum would cause a drop in yield of about 6% while only slightly reducing nitrogen loss, an unfavorable trade-off, Archontoulis said.

“If you want to cut nitrogen fertilizer rates below the required optimum and also maintain yields, well, we can’t have everything. If you reduce the nitrogen, you reduce yield,” he said.

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