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Quest for nitrogen efficiencyQuest for nitrogen efficiency

December 20, 2013

6 Min Read

The connection between nitrogen fertilizer and corn yields is old news. But today’s new vocabulary for fertilizer management — nitrogen use efficiency, or NUE; average optimum nitrogen rate, or AONR; and economic optimum nitrogen rate, or EONR, to name a few — reflects the industry’s efforts to hone N management into a finely tuned instrument. That’s going to have a big impact on your pocketbook as well as on the environment.

Changes in the corn plant itself have made a massive difference in nitrogen use efficiency, notes Tony Vyn, cropping systems Extension agronomist at Purdue University. Purdue researchers reviewed decades of corn studies and determined that 1 pound of N produces 22% more grain today than it did in 1940. That’s one of the benefits of modern genetics.

Crop guides applications

The reality of nitrogen management is that the vast bulk of the corn crop’s N is applied in the fall or early spring based on price and workflow considerations. However, a growing set of tools is emerging to encourage more targeted, environmentally friendly in-season applications.

Ideally, says Fabian Fernandez, University of Illinois Extension soil fertility specialist, split applications would be the way to go, as 40% of corn’s N use occurs after flowering.

“Just a small amount of N is needed in the spring to get the corn plant growing early on,” Fernandez says. “Plus, people who split-apply their N don’t have to compensate as much for lost N as those who apply everything in the fall.”

Grower Ed Winkle of Martinsville, Ohio, takes this approach. Winkle says planting-time applications provide enough N for early “workhorse” hybrids, while doses of 28% UAN, along with his postemergence herbicide pass, and a later sidedress application push “racehorse” hybrids on their path toward 200-bushel-
per-acre yields.

No more blanket N rate

Determining the optimum rate for those applications is no longer a matter of an old rule of thumb such as “1 pound of N per expected bushel of corn.” Today’s rate tools are getting away from yield-based recommendations and taking into account soil characteristics, plant physiology, the price of grain and the cost of fertilizer.

That makes “the right rate” a much better reflection of what will be most environmentally sound or generate the optimal return on investment — but it makes it a lot harder to pin down “right” from year to year.

For instance, optimum nitrogen rates for corn at Purdue’s research site near West Lafayette, Ind., from 2006-2012 have been calculated at 130, 182, 179, 165, 186, 186 and 221 pounds of N per acre, respectively. That’s a massive variation, and a vivid illustration of the fluctuations in soil N levels, fertilizer N loss, crop
health, plant N efficiency and weather that can occur from year to year on a single farm.

Purdue researchers have also noted that the agronomic optimum N rate — the AONR — varies significantly in a given year based on soil characteristics. The AONR for common Indiana soils can range from 180 pounds of N per acre in better-drained, fine-textured soils in west-central, north-central and northwest areas of the state to 216 pounds of N per acre in fine-textured, poorly drained soils in eastern areas.

Factoring in the price of N and the value of grain, the researchers calculate the economic optimum N rate — the EONR — which is lower than the AONR, and fluctuates with markets.

Thinking in terms of how to put every N molecule to work in the crop is a new and savvy way to approach fertilizer management.


Efficient N tools

Some growers apply 50% to 70% of estimated N need early in the season, and then use a chlorophyll meter to compare the crop with overfertilized reference strips. The process can indicate the appropriate rate for a sidedress application to bring the crop over the finish line without wasting N.

Writing about the process in an Extension bulletin, Sylvia Brouder of Purdue University and Dave Mengel of Kansas State University noted, “It is important to remember that this particular tool is best used to manage the last 40 pounds of N, not the first.”

In fields where the primary source of N was manure or incorporated sod, a pre-sidedress nitrate test can provide insight on how much nitrate is available in soil reserves. The test is not accurate if N fertilizer had been broadcast, however. Nitrate from the application will throw off the readings.

For commercially fertilized fields, Adapt-N would be a better choice. An online computer model developed by Harold van Es and fellow Cornell University agronomists, Adapt-N analyzes local temperatures and precipitation, soil type, organic matter, slope, cropping history, and yield potential to recommend highly accurate sidedress rates in real time. The idea is to reduce preplant applications with the confidence that you can go back and provide what the crop needs later.

“You can also replay the growing season and see whether you did the right thing,” adds Jerry Hatfield, director of USDA’s National Laboratory for Agriculture and the Environment in Ames, Iowa.

Another way to review the season and learn for the future is with an end-of-season cornstalk nitrate test. The lower portion of the corn plant is like a reserve fuel tank in a truck — the plant will draw on it if the usual sources are deficient and leave it full if the roots had enough “fuel” to work with. Testing the nitrate levels of stalks when the corn is mature provides an idea of how N levels matched crop needs.

The test doesn’t allow corrective action, but it provides insight into the crop’s N needs — information that is increasingly vital as economic and environmental pressures on N use increase.


Fine-tune nutrition program with top apps

The smartphone and tablet offer ready access to essentially any information you need. However, key apps available on the market can help you target your data use in new ways.

We’re sticking to apps that look at crop nutrition, though you’ll find a growing range of ag-focused tools coming to the market. Best way to find these is to type their exact names in the search box in iTunes or Google Play to get the right app for your device.

Crop Nutrient Deficiency Photo Library — What does a nutrient deficiency look like in a specific crop? The International Plant Nutrition Institute has come up with a photo app that shows different problems by crop. Got a boron deficiency in corn? You can see just what it looks like. The app is free (iTunes only).

Fertilizer Removal by Crop — Developed by the folks at Ag PhD, this app looks at how much “X” number of crop bushels will remove for nutrients. Designed for both iPhone and iPad, the app is free (iTunes and Google Play).

Nutrient Removal — The folks at the fertilizer company Mosaic have also come up with a nutrient removal app. This is a free app available in iPhone and Android format (iTunes or Google Play).

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