Farmers who are unprepared to make in-season nitrogen (N) applications to corn are putting their yield and profit at risk, says Peter Scharf, University of Missouri Extension soil scientist.

“N loss in cornfields has been widespread across much of the Midwest due to wet soil conditions for four straight springs — from 2008 to 2011,” Scharf says. “So when thinking about spring fertilizer strategies, number one is to make sure your crop will have enough N when it needs it.”

Yield boosts from rescue N applications averaged 34 bu./acre across six University of Missouri field trials during 2010, and nearly as much in previous years, Scharf says. However, in 2011, rescue N treatments may have provided a lower yield benefit, because many areas that experienced N loss from wet conditions during spring were later hit by drought, making water the limiting factor to yield instead of N, he says.

Due to unpredictable weather patterns, Midwestern farmers need to keep in mind that neither fall nor spring preplant N applications are bulletproof, Scharf notes. “When applying fall N, farmers should calculate an 8-bu./acre yield penalty under normal spring weather conditions and a much higher yield penalty if conditions turn wetter than average,” he says. “Fall N applications make great logistical sense, but you won’t come out ahead if you give up 20 or 30 bu. [due to N loss] when conditions turn wet. You need to be ready to add more N later.”

Preplant spring anhydrous applications have the lowest risk for N loss among all preplant N programs, but even they will lose significant amounts of N if conditions are wet enough, Scharf adds. As a result, he recommends farmers make arrangements now to be able to apply in-season N to cornfields where and if needed during 2012.

High-clearance kudos

For several years, farmers in southern Minnesota have been using high-clearance sprayers equipped with Trimble’s GreenSeeker sensors to apply supplemental N to corn up until tasselling, says Kevin Jeurissen, agronomist with Crystal Valley Coop, Lake Crystal, Minn.

“We’re not trying to apply all a field’s N with this equipment,” Jeurissen says. “Typically, we put a base amount of N down in the fall and spring and then come back with an in-season supplement, which varies between 2 and 15 gal. N/acre, according to what the GreenSeeker technology indicates is needed.”

Success with the technology over multiple years is generating more demand for the equipment, Jeurissen says. “We’re at the point where a certain percentage of our clientele will likely end up using these sprayers to apply in-season N on all their corn acres,” he says. “We’ve got eight high-clearance sprayers equipped with GreenSeeker sensor technology right now, and we may be getting a couple more before next season to keep up with customer demand.”

Wet springs have delayed fertilizer applications for many farmers in the area over the last three years, Jeurissen says. “However, the high-clearance sprayers have helped get the job done, and we’re seeing good results,” he says.

Early in 2011, N deficiencies were easily evident in many fields as wet weather prevailed. Despite disappointing late-summer rainfall, adding more N in-season “did have a yield advantage,” Jeurissen says.

Every year is different, but multiple, 20-acre field plots show that in-season applications consistently pay a yield dividend, he adds. “In high-management zones, we are doing repetitive block trials, with yield monitors and weigh wagons, to evaluate our practices,” he says. “We know this system works, because we’ve seen yield increases from using it over several years of varying weather climates.”

N payoffs

In-season N applications are boosting yields for Ryan Britt, a farmer and custom applicator from north-central Missouri, near Salisbury. Britt and his father, Randy, use a Hagie STS12 high-clearance sprayer and nitrogen toolbar with Ag Leader OptRx crop sensor technology.

“During the last three or four years, we’ve been consistently losing preplant N due to wet weather, but with the high-clearance capability, it’s opened up the N application window quite a bit,” says Britt, who uses smooth disc coulters on a front-mounted toolbar to dribble or inject N into the furrows. “We’ve seen some very good results from both our normal in-season and rescue-N applications.

This year, even with the dry summer weather, our yields were mostly above the historical averages, and I attribute that to the sidedress.”

Better technology and optimal timing have combined to both increase yields and protect water quality, Britt notes. “The OptRx system is configured to help grow more bushels, and that’s what it’s done for us,” he says. “Yet it’s also more environmentally friendly, because we’re not losing as much preplant N, and we’re putting N on when the plant needs it most. We bought the machine thinking it would help cut our N use, but we’ve realized now that our crops actually need more N than what we’d been applying earlier. And we’re not losing as much N to the environment this way like we once did.”

One area may be more prone to N loss than another, Jeurissen notes. “With N, we’re finding that we need more in spots, depending on soil type and soil conditions,” he says. “Some soil types typically need an extra shot of N later in the season more than other soils do.”

Britt says he’s considering making soil-based adjustments as well. “Depending on soil type, we’re looking at putting on a little more N upfront and maybe more than one postemergence N application afterward,” he says. “We’ve also been in a complete liquid N program for the last several years, but we’re looking at trying multiple N sources in the future. This year, we went to 75 lbs. liquid N per acre with preemergence herbicides and came back with the sidedress N for the rest, depending on the sensor-based rate.”

Britt adds that by working with his local agronomist, he continues to tweak his system. “I think we’re headed down the right road, but there’s still room for learning,” he says.

Jeurissen concurs that the learning curve with in-season N applications is just beginning. “We’re still in our infancy of optimizing this technology,” he says.

Other N application strategies

Crop sensors are a good option for making decisions about where to apply and where not to apply more N, but sensors aren’t always necessary, Scharf says. “The different options for applying N during the season are with urea from an airplane, with urea from a dry spinner or boom machine, UAN with pivot irrigation, or UAN with a high-clearance sprayer equipped with drop nozzles or injection arms,” he points out.

From the ground, farmers can detect N deficiency symptoms in their crop by looking for a “V-shaped brown or yellow burn up the midrib of lower leaves,” Scharf says. “From an aerial image, if your corn looks yellow or light green, you need to apply more N.”

During a wet spring, when a rescue N situation is necessary, farmers “don’t want to get too picky on how to apply the N,” he says. “I don’t recommend broadcasting liquid N, but everything else is fair game to use in order to get N on the crop in time.”

Still, the application window for applying a rescue N treatment is wide. “It’s best if you can get it done by shoulder-high corn,” Scharf says. “However, it’s still worth doing up to tasseling if you have a medium-N-stress area, and up to two weeks after tasseling with a high-N-stress area.”

Scharf says farmers can succeed or fail with any N form, depending on timing and weather, but some N applications practices should never occur. “If you are surface-applying urea and not working it into the soil, you must use Agrotain to prevent N loss,” he says. “Another practice to avoid is broadcasting liquid N solutions onto a high-residue cover. If you do, the N will get tied up in the residue and not go to the crop. Instead, either dribble it between the rows or use a coulter system to apply it.”

Problems with phosphorus (P) and potassium (K) applications are much less frequent compared to N application slip-ups, Scharf says. “Most methods to apply them are pretty reliable no matter when or how they are applied — unless you have sandy soils,” he says. “In that case, we recommend no fall K applications.”

Fertilizer application decisions typically revolve around the weather and its effects on soil conditions, Britt notes. The challenges from unpredictable weather aren’t likely to go away any time soon, but the newer application technology is a good start to overcoming them, he adds.