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Early season N applications

It’s time for a new look at an old nitrogen question

Nitrogen demands in corn aren’t large up to V6. Instead, it’s the placement in the root zone and concentration that are important.

Significant debate has arisen in the past decade as researchers, agronomists, seed company representatives and farmers have tried to determine the best application methods and timing for nitrogen (N) fertilizers. The standard practice used to be to apply everything up front, mostly because it was easier to have it done all in one go at planting.

In the past 20 years, however, the thinking has been revised to focus on optimal timing. It’s better for the corn plant to receive split applications that recognize its physiological demand during the growing season.

Greg Stewart has spent much of the past 25 years helping growers better manage corn in an increasingly intensive regimen that blends different kinds of tillage and N applications, among other practices. He has also encouraged discussions and plot trials on the effects of higher plant populations, increasing weed pressures and in some cases, shorter rotations.

Over those years, Stewart has assessed, studied and written about everything from nitrogen calculators to high-clearance applicators.

Now, as agronomy lead with Maizex Seeds, Stewart is raising the issue of critical early-season nitrogen in corn, a concept that has become easier to study in plot trials as more growers accept and work with side-dressing. That trend has become more adaptable with the advent of high-clearance application equipment that provides growers with the option of later-applied N right up to tassel.

There are still some questions that Stewart is trying to answer, however, especially in the face of a growing trend to spoon-feed the crop in-season and to boost later-season N uptake. In some cases, growers may not be applying sufficient N either in the planting window or in the row zone to stimulate excellent growth early on. It might also be insufficient to promote crop uniformity and to ensure optimum ear size (especially in terms of “rows around” the ear).

“The actual nitrogen demands of the corn crop are not large up to the V6 stage,” says Stewart, adding it’s roughly 25 lbs. of N per acre. “However, it’s more important to consider whether you’re providing enough N in the row zone and in a high enough concentration to easily meet the crop’s demand as the nodal roots are just beginning to initiate.”

Case study

In one example that Stewart cites, a grower used UAN as a herbicide carrier, applying 15 gallons per acre with Primextra II Magnum soon after planting. Also on the planter were five gallons per acre of Alpine G24 delivered in-furrow. That brought the total amount of N applied to that field to 48 lbs. to carry the crop until the first side-dressed UAN application by mid-June.

“In the row zone, there are three lbs of N from the starter fertilizer and nine lbs. from the UAN application,” says Stewart, based on six inches worth of applied UAN across a 30-inch row width — or one-fifth of 45 lbs. “This might be enough in a silt-loam soil, following soybeans on worked ground. But if the soil is heavier or lighter, and the previous residue is wheat or corn, and if rainfall is above average or tillage is reduced, we get concerned that the row zone runs short of N.”

The advent of high-clearance application equipment is pushing more growers to split their N applications. photo: Supplied

Results from 2020

Where Stewart is trying to coax more of a response to N is in working on the notion of boosting N in the row zone and then to monitor the impact on yields. In 2019, roughly 33 per cent of the sites he tested responded with significant yield boosts with 24 lbs. of N banded over the top of the row (at eight gallons per acre of UAN).

In 2020, Stewart worked with Julie and Kyle Maw at Mooremaw Farms in Lambton County in a bid to show the advantages of ensuring adequate in-row N applications. The field used in the study was a clay soil with corn planted no till into a cover crop of winter wheat. Prior to planting, 50 lbs. of N were broadcast, with the planter delivering roughly three lbs. of N in the starter band.

Results indicate additional N was needed in the field and that the row zone was the best placement site.

“Not all field situations will react similarly,” says Stewart. “But it’s a good reminder for growers to consider whether their in-row N supply is adequate to support high yields. Also, there’s evidence to suggest that yield reductions caused by the shortfall of N can’t be recovered, regardless of how much N is applied later in the season.”

Open for more study

Stewart concedes that his dataset is limited, especially in trying to pinpoint how much nitrogen needs to be applied to the row zone in order to optimize yields in all situations. According to Dr. Emerson Nafziger of the University of Illinois, soil nitrate concentrations in the row zone should be between 40 and 50 ppm when corn reaches V2 (three leaf overs, four leaf tips visible).

How that range is reached becomes the question, notes Stewart. Pre-plant broadcast is one option, where 60 lbs of N per acre can do the job, provided there is little loss or movement below the seed zone. Another notion is a two-by-two band delivering 20 to 30 lbs. of N. In some cases, where there is no banding option, growers are delivering 30 lbs. of N by laying UAN on the soil surface near the row at the back of the planter. Spring strip tillers can also incorporate significant amounts of N to meet those early requirements.

“Traditionally, we haven’t tried to assess soil nitrate status in the row, but beginning this year, the Maizex N Tracker will be looking for this information,” says Stewart. “Soil cores will need to be pulled in a zone six inches wide by six inches deep and be done at the V2 to V3 stage. Since there’s a possibility that banded N will still be in place, several cores will need to be pulled to get a good average nitrate concentration across the row zone.”

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CG Production Editor

Ralph Pearce

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