Timing Nitrogen Applications in Corn

Introduction

The goal of timing nitrogen (N) applications to corn is to supply adequate N when the crop needs it, without supplying excess that can potentially be lost.
Applying N at multiple times spreads the risk of N loss and crop deficiency and improves profitability by reducing N rates.
To maximize nitrogen use efficiently, apply N at the time of maximum crop uptake.

N Uptake by the Corn Plant

Corn takes up (in the grain and stover) about 1 pound of N per bushel of grain produced.
Between V8 and tasseling (VT), corn will usually require over 1/2 of its total N supply to support its rapid vegetative growth. If N deficiency occurs at this time, yield losses may be severe.
- Side-dressing N by V4 to V6 is a common practice to help ensure that enough N is available to support rapid growth.
Over 1/3 of corn N needs are taken up postflowering. Recent research has demonstrated that corn yields are maximized when N is available throughout the grain-fill period.

**Figure 1.** Nitrogen uptake by corn. Adapted from Richie, et.al, 2005 *(How a Corn Plant Develops).*

Meeting Corn Needs for N

Wet weather can threaten soil N reserves and hinder resupply by ground equipment; very dry conditions can prevent applied N from moving from the point of application to the root zone.
To prevent N loss from wet weather, apply N at multiple times, and/or use N stabilizers to help protect N from losses.
N may be applied by growers at several times: in the fall, early spring (preplant), at planting, and in-season (side-dress).

Severe N deficiency symptoms — **Figure 2.** Severe nitrogen deficiency symptoms are evident in this field that remained saturated due to excessive rainfall.

Fall Application is used in areas where soil temperatures usually remain below 50 F from late fall to spring.

If soil temperatures rise above 50 F, fall-applied N is at risk of loss through leaching or denitrification.
Only ammonium sources of N should be used and a nitrification inhibitor such as N-Serve^® should also be considered to help keep N in the stable NH₄⁺ form.

Early Spring (Preplant) Application is commonly used in areas where growers are able to complete this practice without delaying planting beyond the optimum window.

This N is applied well ahead of major crop uptake, so it is also at risk of loss if wet weather occurs while soils are above 50 F.
The use of ammonium forms of N can reduce loss potential in this stage.
Depending on the time of application and expected weather conditions, a nitrification inhibitor may also be advantageous.

At Planting Application has 1 distinct advantage over the others – when the field is fit to plant, it is also fit for N application, which makes its success less dependent on the weather.

However, the amount of N that can be applied by the planter is limited, and may slow the planting process.
Application in a separate field trip immediately following planting may be preferable.

In-season (Side-dress) Application allows for adjustments to planned N supply based on weather variations.

Growers can increase side-dress rates if wet conditions and N losses are prevalent, or reduce side-dress rates if warm temperatures and less rainfall occur.
Soil fertility specialists suggest that growers target only 1/3 of total crop N supply for side-dress application (to lower the risk of N deficiency if timely side-dress is not possible).
Growers should also be prepared with a backup plan should weather interfere with the original plan.

N-Timing Research Results Vary

Studies done on the effect on yield of N application timing have shown that results vary depending on the weather conditions that occurred.
To create successful N strategies, focus on understanding the relationship between N supply, weather conditions and corn needs, rather than relying on variable research results.

**Figure 3.** Side-dress application of anhydrous ammonia at the V5 to V6 corn growth stage. *Photo courtesy of John Deere.*

Nitrogen Fertilizers and Loss Potential

The most common nitrogen fertilizers are anhydrous ammonia, urea-ammonium nitrate (UAN) solutions, and granular urea; ammonium nitrate and ammonium sulfate are less common.
N fertilizers that include more ammonium and less nitrate forms of N are less subject to loss.
- Over time, soil bacteria convert ammonium to nitrate (NO₃), a form which is readily lost when excessive rainfall leaches or saturates soils.
Urea-containing fertilizers are subject to volatilization when surface applied, unless taken into the soil.

Nitrogen Stabilizers

To help reduce N losses, nitrogen "stabilizers" or "additives" can be applied along with N fertilizers.
Several common products include Instinct^®, N-Serve^®, Agrotain^®, Agrotain Plus^® and ESN^®.

N-Serve and Instinct are nitrification inhibitors that contain the active ingredient nitrapyrin, which acts against the bacteria responsible for converting ammonium to nitrate.

N-Serve is an oil-soluble product that may be used with anhydrous ammonia, dry ammonium and urea fertilizers.
Instinct is a new encapsulated formulation of nitrapyrin for use with urea ammonium nitrate (UAN) solution that is compatible with liquid fertilizer, herbicides, insecticides and fungicides.

Agrotain helps prevent losses of N when urea is surface-applied and not incorporated into the soil.

This product inhibits urease, a naturally occurring plant and soil enzyme involved in the conversion of urea to ammonia.
It is useful when urea is broadcast (not incorporated) into the soil with irrigation or tillage.
Agrotain is most effective with urea and also UAN solutions.
Agrotain Ultra is a more concentrated formulation of Agrotain.

Agrotain Plus is an additive specifically for UAN solution.

This additive works against the nitrification and volatilization processes that lead to N losses from urea.
Does not protect the nitrate portion of the UAN solution.

ESN, Environmentally Smart Nitrogen, is another type of stabilized N.

ESN contains a urea granule within a micro-thin polymer coating, which releases the N as soil warms.
This time-release method helps reduce nitrogen losses.

Developing Your N-Supply Strategy

Where weather allows, applying N at multiple times, including the time of maximum crop uptake (V8 to VT), is a good way to spread risks and reduce costs.
New research shows that modern hybrids take up about 37% their total N needs after flowering. To optimize yield, plan to make N available throughout the grain-fill period.
Using historic weather information, growers should develop an N-timing strategy with a high probability of implementation most years. Be ready to implement a "plan B" when excessive or prolonged rainfall disrupts original plans.
Strategies should be weighted heavily for soil type and topography, which impact retention of applied N and the ability to apply additional N.