• 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.

• 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.

• 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).

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.

• 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.

• 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.

• 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.

• 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.
• Pioneer released Encirca^® Yield to help growers more effectively manage critical crop production inputs, including nitrogen.