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Plan, Execute, and Evaluate: In-Season Nitrogen Application

Being Minnesota weather is as variable as the soils across the state, utilizing the nutrients in the soil is something farmers are tasked with each year. With nitrogen being arguably the most important nutrient when it pertains to corn growth, there is much debate as to when to apply nitrogen and how to optimize the availability of this nutrient during the important stages of growth.

The factors that go into choosing which nitrogen fertilizer to apply, and when, are extensive and revolve around the 4Rs – right source, right rate, right time and right place. These four metrics play a role in nitrogen application but are especially important as it pertains to in-season application. This method of fertilizer application has been on the minds of growers and retailers for many years, and it’s on AFREC’s radar too. When tackling the topic, there are three areas in which farmers and ag retailers should focus: plan, execute, evaluate.


In-season application can become overwhelming and time consuming as the months of May and June are often filled with sporadic rain showers and primarily taken over by herbicide application. If a farmer is planning to apply nitrogen in-season, it is imperative that a plan is in place as to when the ideal timing for application would be, backed by plan B.

The ideal timing for in-season nitrogen application is between stages V2 and V6. After the V6 stage, nitrogen uptake accelerates as the plant starts to increase its vegetation and start preparing for the reproductive stages. Timing is a key component to the plan but unfortunately can be impacted by many uncontrollable variables. Farmers must have a plan in place so when weather or time is a limiting variable, there is still an opportunity to fulfill the nitrogen needs of the crop to prevent reduction in yield due to nitrogen deficiency.

The chart below, sourced from Iowa State University Extension demonstrates the importance of applying nitrogen at the correct timing. The y-axis shows the percent of nitrogen uptake as well as the amount of nitrogen in pounds of N per acre. The x-axis shows both the growth stage of the corn as well as the growing degree days. There is a significant increase following the stage V6 as described above. This reiterates the importance of timing during in-season nitrogen application.

Chart that demonstrates the importance of applying nitrogen at the correct timing.
Source: Abendroth et al., Iowa State University Extension, 2011.

In some parts of the state, irrigation systems are more prevalent, in which in-season nitrogen application timing may vary. Being that there is an ability to apply nitrogen later in-season, when the crops are taller and more mature, there is opportunity to apply nitrogen right before or after tasseling. The argument for this application is to help increase the quality of grain production being the majority of nitrogen use in the plant goes to grain production after tasseling (refer to chart above). If a farmer has the correct equipment, either high clearance Y-Drop machines or ability to drop appliate near base of plant, there may be an increased yield potential in applying nitrogen around the tasseling stage.


The variables of execution are many, but one variable that impacts management decisions is the type of fertilizer. There are four main types of nitrogen fertilizers: anhydrous ammonia, urea-ammonia-nitrate (UAN), urea, ammonium sulfate. Each of these types comes with benefits, drawbacks and challenges of their own in terms of the amount of plant available nitrogen and application.

In terms of application, there are a variety of ways the products below can be applied to fields. Injection is primarily used in anhydrous ammonia and can also be an option for UAN solutions as it can slow volatile losses from products containing urea. Broadcasting is also a popular application method for urea. When utilizing broadcasting with UAN fertilizer, be wary of distributing this on high-residue fields as the UAN may latch onto the crop residue, making the fertilizer less effective. Similar to the variability of fertilizer options, application options may change on a field-by-field basis.


Nitrogen Fertilizer Forms and Application Types

Nitrogen Fertilizer Form Application Type % of Element
Anhydrous Ammonia Gas
82 N
UAN Liquid
Injected, Dribbled, Broadcasted
28-32 N
Urea Dry
46 N
Ammonium Sulfate Dry
21 N
24 S

Source: Minnesota Crop News – The Three Biggies: Urea, Anhydrous Ammonia, and UAN by Fabian G. Fernández.


In terms of management for these products, it is important to evaluate the fertilizer and read through the labels carefully when applying to fields. The first management practice to evaluate is volatilization. This is when the fertilizer loses its nutrients because of exposure to elements, either too much heat or rain, that render it unavailable for the plant. When using surface applied UAN or Urea with very warm temperatures, without rain in the forecast, it is important to look into additives like Agrotain to help reduce volatilization to the atmosphere.

Another management practice that is important when applying nitrogen in-season is to reduce the amount of plant foliage burn. This is especially important when applying UAN liquid. When applied as a liquid, UAN has the opportunity to land on the plant, rather than the soil, which can leave burn marks on the foliage of the plant. When there is a large amount of canopy on a field, one way to help reduce the side effects of foliage burn is to dilute the product using water to reduce the concentration and protect the leaves. Other methods of reducing burn is to apply in early morning or late evening when there are cooler temperatures or direct the UAN to the soil or base of the plant.

Lastly, being the V2-V6 stages of plant growth is a common time to apply herbicides and pesticides, there is opportunity to mix UAN liquid fertilizer with those products. Always reference the labels to be sure the two products are compatible and when mixed, they will not have a negative burn effect or other negative effects on the plants. Although this can save time, having to complete one pass on the fields to apply both products, there is also risk this may have an adverse effect on the field if the products are not compatible.


A main question behind the implementation of in-season nitrogen application is: what is the return on investment, and will it pay for the extra time and labor to apply the products? A study conducted in partnership by AFREC funding and the University of Minnesota evaluates fixed and variable rates to the University of Minnesota recommended rate of application at four locations around the state. Although nitrogen is not the only metric to study, others including soil sampling and yield, economic variability is the number one consideration of whether or not to utilize a product.

The chart below reports yield response to in-season nitrogen application varies on a field-to-field basis. This data shows split application performed well, in terms of yield, but better in some fields over the others. These trials located in Waseca (WA16, WA14) and Blooming Prairie (BP14), show that it is important for a farmer to evaluate the opportunity on their individual farm. There is opportunity for great return, but not on every field, in every location.


AFREC Research on Nitrogen Rates and Split Applications of N

Site Treatment Timing Pre-plant N (lbs./acre) Sidedress nitrogen (lbs./acre) Yield


WA16 Split 150 lbs. N 50 lbs. N 205 bushels
WA16 One Rate 165 lbs. N 0 188 bushels
BP14 Split 105 lbs. N 46 lbs. N 150 bushels
BP14 One Rate 120 lbs. N 0 136 bushels
WA15 Split 100 lbs. N 40 lbs. N 210 bushels
WA15 One Rate 120 lbs. N 0 206 bushels

Source: AFREC and U of M “Evaluation of Variable Nitrogen Rate Technologies for Corn in Minnesota” research report.

Continuous Research

AFREC understands the importance of nitrogen application research and, in response, has funded an on-farm trial program. Research has found that repetition and replication are the best method of evaluating a new practice on farms. AFREC encourages farmers to conduct their own studies in their fields to evaluate practices in their normal conditions. Data from these on-farm trials will be available in the upcoming months as the field results are analyzed and broken down into valuable insights for both growers and ag retailers.

Below is a graph that contains data collected from 2018 on-farm research trials. This chart only contains three out of the 48 trials. The chart below shows the corn yield response to varying rates of nitrogen application. The “Farmer Rate” was the base rate which was determined by the University of Minnesota standards as well as the farmer’s personal insight on their farm.


Corn Yield Response to Nitrogen Application Trial

– 30 lbs. of N


Farmer Rate


+ 30 lbs. of N


Site A 221.5 228.9 233.4
Site B 176.4 175.6 197.0
Site C 214.9 214.7 216.1

Source: AFREC funded on-farm research in 2018. Formal report will be available in summer 2019.


It is apparent in this graph that increased nitrogen application does not always garner a greater yield, and in turn greater economic ROI, but in some cases it does. In site A, we see the “classic” yield response to nitrogen application which is a linear increase in yield as the application of nitrogen increases. Although this site only had a 4.5 bushel per acre increase in yield, economically, the nitrogen will potentially still pay for itself which deems it a smart decision in terms of ROI. Site B shows the largest response to the increased nitrogen application, showing a 21.4 bushel per acre response to the 30-pound nitrogen increase per acre. In terms of economics, this trial saw the greatest ROI. Lastly, site C saw a fairly flatline yield for the three different nitrogen rates which lends nitrogen may not be the limiting factor in this field. With over 2,100 data points collected during the harvest of these three trials, this data will give insights related to soil type, yield response and critically evaluate management practices.

Final Word

The bottom line is no one knows fields better than the farmer does. Farmers and ag retailers working together to find a solution consistently renders the best results. AFREC advises growers to continue to plan, execute, and evaluate fertilizer for their operations and strive towards the best management practices to produce the highest ROI for their farms and assist in improving water quality.

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