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Aerial view of nitrogen deficient corn from central Iowa August 2008
Aerial view of nitrogen deficient corn from central Iowa, August 2008

Wrong year for late season nitrogen application

If you apply enough nitrogen early and don’t lose it during spring, you won’t likely need more.

Having equipment that allows you to apply nitrogen (N) late in the season is well worth having — even if you don’t use it every year, says Clay Mitchell, Buckingham, Ia.

“Adding late-season N can be like putting on a booster rocket for corn production in a wet year, when it’s needed,” says Mitchell, who farms approximately 2,800 corn and soybean acres with his great-uncle Philip near Waterloo, Ia. “In dry years, you can save a lot of money by holding off on early-season N applications that don’t pay off.”

Mitchell’s strategy is to apply N in a way that ensures little, if any, will be wasted. “Typically, we split-apply anhydrous ammonia N applications,” he says. “We start with some preplant and then apply more as an early sidedress. If it’s been warm and wet, like last year, we use our high-clearance sprayer to apply a third N application.”

In 2011, Clay’s dad, Wade, helped equip the farm’s John Deere 4700 self-propelled sprayer with drop tubes that ensures the Mitchells can safely apply urea ammonium nitrate (UAN) to corn up until tasseling stage. They welded oval, stainless-steel screw-thread links to the spray boom where they could hang chains connected to 5-ft. stainless-steel tubes that attach to the nozzle couplers.

“Having the tubes hang on the chains makes it easier to take the tubes on and off,” Mitchell says. “The drop-nozzle tubes get the UAN product closer to the roots, so they don’t burn the leaves.”

For farmers who use sprayers with drop tubes to apply liquid N, the tubes need to be long enough to go well below the ear, says Peter Scharf, University of Missouri Extension soil specialist. “Avoid spraying the ears or the leaves near the ears, or you’ll hurt yields, not help them,” he advises. “Farmers should also avoid broadcasting UAN solution on corn larger than the V7 growth stage.”

Just add water

Cornfields typically need late-season N applications only if soils lost their N earlier in the season, due to excessively wet conditions or if the field didn’t receive enough N early on, Scharf says. “There is nothing wrong with an early N application that can supply the crop’s needs all the way through the season,” he says. “The only problem with that approach is if you lose N after it’s applied and you don’t get back in the field later on to apply more.”

When relying solely on spring preplant or sidedress anhydrous applications, farmers may be unable to complete operations on all their acres, if conditions are wet. “Sometimes it can be mid-June to early July before the ground dries up enough to get in the fields, and by then the corn might be too tall to do so,” Mitchell says. “That’s where a high-clearance sprayer is a really good risk-management tool to be able to apply N when it’s a challenge to put it on by any other means.”

Having the high-clearance sprayer gives his family’s operation “a third backstop for putting on N during the season,” Mitchell says. “We always have a window, sometime between the V6 to tasseling growth stage, when ground conditions are dry enough to allow us to make these high-clearance N applications. We also try to time our applications to when we know rain is coming so that the N is made available to the roots.”

The biggest concern in a rain-fed system, which includes most of the Midwest, is not having enough moisture after a supplemental application to move the N into the root zone, says Fabian Fernandez, University of Illinois Extension soil fertility specialist. “A half-inch of rain or more is typically needed to do this,” he says.

For late-season applications, the supplemental N also needs to be in a readily available form, such as urea or UAN, Fernandez adds. “Either liquid or granular N is fine, as long as it’s not in a slow-release form,” he says. “Irrigation can also make a huge difference in the success of late-season supplemental N applications, because with irrigation, farmers can make sure to move the N into the root zone, where plants can use it.”

Still, sometimes the soil will contain enough moisture that only minimal rainfall is needed to move N into the root zone, Scharf says. “If you throw a handful of dry N on the ground and it disappears overnight, that shows there is enough moisture in the soil to dissolve the product and wick it into the soil,” he says.

Late N needs are predictable

“A successful, late-season N application is very, very weather dependent, and that’s exactly what makes it attractive,” Mitchell says. “You can predict when it will be an effective practice and when it won’t be. If it’s been warm and wet during spring, it will more than pay for itself. In a year like last year, we would easily see a 1 bu./acre yield increase per unit of N applied.”

Late-season N applications can be particularly valuable in places with inconsistent rainfall, such as on the fringes of the Corn Belt, Mitchell points out. “In these areas, if you apply all the N upfront, either prior to or soon after planting, then you risk throwing it all away or losing a lot of money if it turns out to be a very dry year and the corn can’t take it up,” he says. “In a very wet year, unless you’re able to apply N later in the season to make up for lost N early on, you risk not having enough N available to the crop to optimize yields. So, especially on marginal corn-growing areas, this is a great way to deal with the uncertainty and risk over weather.”

Farmers who can raise a fall cover crop after corn would still benefit from a late-season N application, even if dry conditions prevail, Mitchell adds. “The cover crop will take up the N and release it in the future,” he says. “So the late-season N application is a really good option. It’s much better than putting a whole bunch of N on upfront and taking a risk that it might be wasted.”

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