In-Season Timing For N

New corn-fertilization research shows ways to fine-tune application timing so more nitrogen goes and stays where it needs to be—in the plant.

By Marilyn Cummins

One of the big decisions every year is when to apply nitrogen fertilizer to a corn crop so the nutrient is present and available when the plant needs it most. Researchers learn more each season about exactly what’s happening to nitrogen in the soil and in the corn plant itself. What follows are recent findings that can inform the choices you make regarding in-season nitrogen application.

Modern Hybrids Draw More N Later

In ongoing studies looking at the nitrogen recovery efficiency of modern hybrids, Purdue Professor of Agronomy Tony Vyn and doctoral student Sarah Mueller have found that new hybrids not only take up more total nitrogen, but they are also much more likely to take up 30 to 40% more nitrogen after flowering than older hybrids did.

Aerial photography used to develop yield loss prediction maps and prescriptions for where more nitrogen fertilizer is needed.

Peter Scharf, specialist and founder of NVsion Ag, uses aerial photography to develop yield-loss prediction maps and prescriptions for where more N is needed.

“So there appears to be more room for a positive response to applying 20 to 30% of total nitrogen after the V10 stage in modern hybrids,” Vyn says, although he cautions that a positive yield response doesn’t happen with every hybrid or in every soil and environment situation. He also adds, “If you’re already putting on too much nitrogen, don’t expect any yield response to late-split nitrogen. Don’t waste this precious resource, and don’t compound the environmental problem.”

Mueller, the lead author on the latest study, says they saw the biggest yield response when there was stress, such as drought, early in the season, or if the nitrogen was applied at that later timing in continuous corn. She also noted that while late-split nitrogen timing may not always mean a bump in yield, “we’re shifting the availability of nitrogen in the soil to when the most rapid accumulation is happening—from V6 until silking. And more nitrogen tied up in the plant means it’s not being leached into the groundwater.”

Vyn concurs. “We’re really excited about this from an environmental point of view,” he says. “We know we can improve the nitrogen recovery efficiency. The farmer may be less excited about that if it doesn’t pay, so we hope to continue our research to figure out the best niche in terms of hybrid, field conditions and nitrogen-rate range.”

Synchronizing N with Need

Vyn also says one of the benefits of the late-split program may be to cut back on the total nitrogen application in areas of the field that don’t need any more, which can help save nitrogen costs. Research at the University of Missouri is focusing on ways to identify variable needs for nitrogen in real time as well as by analyzing soil conditions in advance, such as low-lying areas in claypan soils.

Peter Scharf, Missouri Extension nutrition management specialist, studies how to manage the great variation in nitrogen needs within a field, and from field to field, based on soil properties, and from year to year in terms of weather conditions. He uses color sensors to see what’s happening in real time during the growing season and then prescribe treatment to match application with the need at that spot in the field.

Peter Scharf

Peter Scharf

Scharf’s recent three-year study of sensor-based variable-rate nitrogen management showed that using canopy sensors on the fertilizer applicator to read the color of the corn plants on the go—which adjusted the nitrogen rate every second—made a big difference. Nitrous oxide emissions were cut by 50%, nitrates in the drainage water were cut by 24% and the grain yield went up 13%, all without changing total nitrogen use. (Results were compared with applying 140 pounds of nitrogen per acre, preplant.)

Scharf also has taken the color sensing to the sky to develop yield-loss maps and nitrogen application files based on georeferenced aerial images of corn fields, allowing growers to look at all their fields in a hurry. “It’s taking the temperature of the corn real-time,” he says. “If it’s hot—dark green—it doesn’t need very much nitrogen. But if it’s light green, you’d better pour it on.”

Using the aerial image system especially helps when all the nitrogen is applied preplant and wet weather may have caused losses that need to be replaced with rescue treatments. Scharf is also studying its application to planned split applications.

Scharf notes that today’s farms are so large that many growers can’t cover all their acres quickly enough when sidedressing with anhydrous in-season. The big influx of fast, high-clearance spinners, however, like AGCO’s RoGator, he says, “has greatly increased our capacity to get corn taken care of and supplied with fertilizer, whether as a planned two-thirds in-season pass or as a rescue.”

Photo: Courtesy of Jason Young, AgVision

Tony Vyn and Sarah Mueller wish to acknowledge the support of their research by the Indiana Corn Marketing Council and DuPont Pioneer.

Peter Scharf wishes to acknowledge support of his research by the USDA National Institute for Food and Agriculture. More information about NVision Ag is available at