Late nitrogen applications: A strategic advantage?

Friday, September 17, 2021

Modern farm equipment adds flexibility when applying nitrogen on corn.

By Jackie Clark

The prevalence of high-clearance machinery is expanding producers' options for corn management. Nitrogen (N) fertility is one aspect that growers and researchers alike strive to optimize.

Late-season N applications are more common because of "the introduction of aftermarket attachments and growers who started making their own, and I think the prevalence of high-clearance sprayers out there nowadays that maybe weren't around 10 or 15 years ago," Ben Rosser, corn specialist at OMAFRA, tells Better Farming. Another contributing factor is likely the adoption of corn fungicides, which made driving through tall corn more realistic.

Some debate exists around what constitutes a "late" N application compared to traditional side dressing. The dividing line is somewhere around the V6 to V9 growth stage of corn, says Dr. Joshua Nasielski, assistant professor in the University of Guelph's crop science department.

"I think the simplest way to refer to a late application is something that you can't get through with a tractor," adds Greg Stewart, agronomy lead at Maizex. Maizex is a Tilbury-based seed company.

Late N applications happen "when a tractor can no longer pull a side dresser through a corn crop because it's too tall."

Most growers who apply late N use a self-propelled, high-clearance sprayer, says Rosser.

"There's a wide range of ways growers are putting it down. On the liquid UAN (urea-ammonium nitrate) front, sometimes it's simply putting a drop hose down and dribbling it between the corn rows in tall, chest-high corn."

Some farmers use toolbar attachments on high-clearance equipment "like a traditional side-dresser," he adds. A bar mounted on the sprayer reaches down with "a coulter and an injector that's injecting UAN to the soil."

With the variety of equipment options, "there are distinctly three windows now in the N application game: the planting time, the side-dress time and the late application time," says Stewart.

This month, Better Farming checks in with corn fertility experts and a producer to find out how those three windows can be utilized most effectively.

Impact on yield?

The consensus among experts is that applying N with a late split doesn't necessarily boost corn yield automatically.

Ontario trials conducted by OMAFRA and Ontario Soil and Crop Improvement Association members show that "on average, there was a slight yield increase going from pre-plant to side dress," says Rosser. But "in a year where you wouldn't expect real losses, generally you wouldn't see a benefit for going with delayed N application, whether that was a side-dress or one of these late applications."

Late N did boost yield at a "small subset of sites where you were in an environment that had high losses and a lot of rainfall between planting and tasselling," he adds.

Nasielski agrees.

"If you're just split applying N, you will not normally get an increase in yield versus the same rate applied early, unless you're on a really sandy soil or a soil that has high N losses early in the season," he explains.

One exception "is if the farmer is pushing plant population," he adds. "Because when corn populations go up, they're going to be taking up more N later in the season and there's somewhat of a yield benefit."

Any yield benefit "will be specific to that situation," Nasielski says.

Late applications lead to "a modest increase in N recovery efficiency (NRE), about five per cent," he explains. NRE is the amount of N in the applied fertilizer taken up by the crop.

"In theory, if you get a five-per-cent higher NRE you can reduce your total N application by five per cent, because it's just being taken up more effectively," he adds. However, he's never seen that done in practice.

Typical N recovery is around 60 per cent in the above-ground plant material, but with a late application it can be 70 per cent or more, Dr. Tony Vyn tells Better Farming.

Vyn is a professor of agronomy in the college of agriculture at Purdue University in Indiana.

"We get a little more N into the plant because it stays green longer and it's photosynthesizing later," he explains.

"In many situations, especially those that already rely on early side-dress N, it doesn't help yield," Vyn says. However, he supports late application when considering environmental impact, because more N ends up in the plant instead of being lost to the water table or atmosphere.

Why doesn't more N recovery increase yield?

"Later applications seem to enhance more leaf N uptake during reproductive growth, but that doesn't necessarily mean grain-yield increase if the regular N program already had an adequate plant N status right around flowering," explains Vyn.

The ability to determine N status at essential, yield-determining development stages could help farmers achieve optimal yields through strategic N application.

"If you've done some diagnostics, whether it's a soil nitrate test or even just an accurate rainfall accumulation for the field" to determine which areas lack N, you know those areas should be targeted with a late application, says Stewart.

Rescue and risk

Late application can provide some risk management should you experience a high N-loss event, Nasielski says.

Stewart agrees.

"Late N application is a rescue treatment, and it's a rescue treatment that 20 years ago we didn't have the equipment to do," he says. "That's really a very positive story."

Farmers can apply N "as late as tassel and silk time and still recover a very significant portion of what was going to be the lost yield if additional N did not get put down," he explains.

Hauke Claussen farms with his brother near Clinton, Ont. They have invested time in a strategy that works best for their farm.

Previously "we'd put 40 pounds of N down, with P (phosphorus) and K (potassium) as well, with the corn planter in a dry fertilizer blend. We used to come back after doing (soil nitrate) samples," he tells Better Farming. "But now for seven years we've been running the GreenSeeker on our sprayer."

Claussen applies 20 lb. of N with the planter, "then the plan is coming back when the corn is still small. At one- or two-leaf stage, we come back and bump it up another 100 or 120 lb.," he explains.

"That leaves us time until waist- or shoulder-height corn, and then we come back with 28 per cent with the GreenSeeker and a hose that runs in between every other row, applying the remaining amount of N onto the corn."

The GreenSeeker sensors collect normalized difference vegetation index (NDVI) data "and according to that (data), it varies the rate while you are driving through the field," he adds.

This approach helps to split up risk.

"If we plant corn and then get inches and inches of rain, we either have leaching on lighter ground or denitrification on heavier ground," Claussen explains. "In those wet years that system really shines because you still react to those weather events and still get the correct amount of N applied. In more normal years, I think the total amount (of N put down) is pretty much the same but it's our belief that it's spread a little bit smarter."

In the late application, he's able to apply more N where it's needed, and less where it's not, Claussen adds.

The perfect split

Claussen's approach is just one of several ways to try to make the most of late N applications.

Determining the ideal way to split N applications in terms of rate and timing is an important consideration for producers, he adds.

"I think what most agronomists would suggest is applying the last 25 to 30 per cent of your total N rate late is a safe bet. It becomes riskier either the later you delay that application or the greater proportion you're applying later," Nasielski says. "If you're going to apply late, you want to do it between V9 and V10, and V13 at the latest in Ontario."

Once corn is "silking, you're going to start getting kernel abortion if there's not enough N," he adds.

Currently, "I would guess most growers are probably putting down maybe half of their N up front, with actual rate splits likely depending on when they plan to make their later application," says Rosser.

Agronomists emphasize that it's important to ensure enough N is applied to corn early.

"You want to apply a strong N rate up front, but my research has found that you need to be realistic about how much N you need early in the season to maximize yield potential," explains Nasielski. In fact, "you can apply the full rate at V13 – it's not recommended, but you can – and not get a huge yield penalty, if any."

Still, most experts would recommend leaving a relatively small percentage of N for the late-application window, Stewart says.

Strategic application

Optimizing the benefits of late N requires a strategic approach.

"There's very little compelling evidence that the spoon-feeding, that the additional late application is necessarily a return on investment, if all you're going to do is take a portion of your N and apply it late without any strategy," explains Stewart. "That is, a strategy based on rainfall or yield potential, or nitrate testing or tissue testing, or even how you use that last application strategically in the field."

Vyn and Nasielski agree.

"I think the future of late, split applications is variable rate on the late split," says Vyn. Indicators could be leaf reflectance, known soil losses or an opportunity to boost yields in high-yield zones that require more N.

"We have to accept the fact that there is more opportunity to use this pass as a variable rate application, and that justifies it economically," he explains. "Because you have to pay for this pass going across the field."

Again, technology will help farmers capture that value.

"The real benefit is the synergy that late N applications have with precision technology that allow a rate adjustment," Nasielski says.

Near the late-application window, "you have a better idea of the yield potential of that crop or any N losses," he explains. For example, in a really dry year with low yield potential, some farmers just decided not to apply the late application because the benefit wouldn't justify the cost.

Farmers can take a simple or more sophisticated approach with soil, weather and crop data to predict yield and dial in N requirements, he adds.

However, the most economic benefit will be realized in extreme years, Vyn says.

Nasielski agrees.

In most years, "if you apply a typical N rate, you're getting pretty close to an optimum N rate," he explains. "Two or three years out of 10 where N requirements are much higher or much lower than they normally are … that's the biggest bang for your buck in terms of adjusting N rates."

Caleb Niemeyer is a precision agriculture specialist at Woodrill Ltd., a crop input supply company and grain elevator in central southern Ontario. Niemeyer conducted research on using rainfall to ascertain optimal late N applications during his M.Sc. at the University of Guelph.

Previous research found that optimal N rates for corn in the same field varied from 140 to 260 pounds over the years. Niemeyer wanted to determine the driver of that variation.

"I tied that N response data to historical weather, and we found rainfall from V5 to V12 was the strongest predictor of that optimal N rate," Niemeyer explains. "If you do a late-season application, then you're able to take some of that weather data into account when you're making that rate decision."

He also considered all the data from the Elora and Ridgetown research stations "to see if that trend holds true for multiple locations and rotations," he adds. And it does, at least at those locations.

"We didn't have a very heavy clay site; we didn't have a very sandy site either. At those kinds of sites, we expect more of an importance of early-season N losses," Niemeyer explains.

Some farmers and researchers have investigated soil nitrate levels to try to dial in optimal late-N rates.

"We would go out and do the soil samples, have them analyzed for nitrate amount … but of course you can only take so many samples per field," Claussen says.

With that information, he could create field-management zones, but the process was labour-intensive and challenging. Also, because soil nitrate can be lost, it was hard to know if your single nitrate samples are representative of what the corn took up, he explains.

Instead of soil nitrate, farmers could focus on physical characteristics.

"I think the variable-rate approach in the short run has to be one that is determined most by soil property variation," explains Vyn. "By drainage variability, I would put more N on areas that are poorly drained, for instance, and also I would dare to say that I would be more likely to increase the rate on very high-yield-capable regions of the field."

Dialing in the unknowns

Though researchers have been testing many variables around N on corn for decades, there are still details to discover.

Vyn hopes to see improvements to sensor or imaging applications.

In the future, "I believe there is more opportunity associated with certain bands in hyperspectral imaging than there is with an over-the-top, single-band approach," he says. "The best readings are going to be those that are as late as possible."

Nasielski is working with other researchers and partners on a project testing one N rate with a late timing split, applied six different ways: broadcast urea, UAN streamed with a Y drop, each of those methods with urase inhibitor, injecting UAN and injecting granular urea.

Also, research results have thus far been conflicted on the impact of rainfall around the timing of late application, Nasielski adds. More work is needed to understand how rain after late N may impact uptake and losses.

Claussen's current system is working, "but we are always open to new ideas. We keep an open mind." BF