By David Marburger and Brian Arnall

There are few crop inputs for winter wheat that deliver as much return on investment as nitrogen fertilizer. It takes approximately two pounds of nitrogen to produce one bushel of grain. This year that will cost approximately $0.80-$1.00 to produce a bushel of grain worth about $3.25. Of course, nitrogen is not the only yield determining factor in a wheat crop, but it does represent a significant investment in a year that will have very tight margins. Also, the law of diminishing marginal returns eventually kicks in, but nitrogen fertilizer is still one of the safest bets for wheat inputs.

Topdress nitrogen fertilizer is especially important because it is applied and utilized at a time when the plant is transitioning from vegetative to reproductive growth. Several things, including the number of potential grain sites, are determined just prior to jointing, and it is imperative that the plant has the fuel it needs to complete these tasks. Jointing occurs around the end of February in southern OK and around the second week of March in northern OK. Jointing also marks the beginning of rapid nitrogen uptake by the plant which is used to build new leaves, stem, and the developing grain head. The nitrogen stored in these plant parts will be used to fill the grain later in the season, and the plant is dependent on this stored nitrogen to complete grain fill.

Early January is our normal time for beginning to topdress nitrogen fertilizer in winter wheat in Oklahoma. However, the lack of significant rainfall in many areas during November through December has presented some interesting and frustrating challenges regarding topdress nitrogen fertilizer. The biggest of these is deciding when and how much to apply. Listed below are some considerations for topdressing nitrogen this year.

When to apply

• In order to have full benefit, nitrogen must be in the rooting zone by the time wheat is jointing, and moisture is required to move nitrogen into the rooting zone. Since precipitation is usually very limited in January and February in Oklahoma, we need the nitrogen out on the field when the precipitation hopefully arrives back.
  • If you applied nitrogen pre-plant, consider waiting until closer to jointing for a topdress application. By being patient in this situation, we are hoping for a rainfall that will not only provide soil moisture to move that topdress N down into the root zone but also provide enough N for a higher yield potential. Otherwise, if we stay dry through jointing, you will likely have enough nitrogen present to compensate for a lower yield potential.
  • If you decided not apply any nitrogen prior to planting, due to residual soil nitrogen amounts or simply did not want to invest the money into the crop at the time for example, did you happen to use a N-rich strip?

Yes I did: If you currently see a difference between the N-rich strip and the rest of the field, then now would be time to begin making applications. For those producers who are using the Sensor Based Nitrogen Recommendation (SBNRC) system, your yield predictions and nitrogen recommendations generally become more accurate as the season progresses; however, growers wishing to hedge their nitrogen bet could apply a partial topdress now and supplement with a second top dress just prior to jointing if SBNRC recommendations call for additional nitrogen. If you cannot see a difference, then wait until closer to jointing to make the call.  https://osunpk.com/2014/02/24/sensing-the-n-rich-strip-and-using-the-sbnrc/

No I did not: Now would probably be ideal to start making those applications depending on fall growth and soil moisture levels. If soil moisture is present, considering apply enough N to reach the farms break-even yield goal. At minimum, 30-40 lbs of N should be applied. The rate ultimately depends on comfortability with putting more money into this crop. Also, it is not late to apply a N-rich strip:  https://osunpk.com/2013/09/19/nitrogen-rich-strips/

• Do not apply nitrogen to frozen ground with a layer of ice or thick snow layer. Nitrogen will move with water. If melting snow or frozen rain is moving to the ditch, so will nitrogen applied to the soil surface.
• Consider splitting or delaying top dress nitrogen applications to sandy soils until closer to jointing, as leaching can occur.

How much to apply

• On average it takes about 2 lbs/ac of N for every bushel of wheat yield. In addition, dual-purpose wheat requires 30 lbs/ac of N for every 100 lbs/ac of beef or 1,000 lbs/ac of forage removed. You can subtract your soil test NO₃-N from these total requirements.
• Did you soil test? It is okay to adjust topdress N plans based on your current yield potential. When you submitted your soil test, you might have stated a 50 bu/ac yield goal which would require 100 lbs/ac of nitrogen; however, it is important to take a hard look and determine if this yield goal is still realistic based on your current crop status. This does not suggest to adjust based on what you think the weather might do, but it is okay to take inventory and adjust your topdress N up or down based on current field conditions.
• Don’t have an N-rich strip? Many more producers are trying it this year simply due the fact that producers did not want to spend much money up front on wheat inputs. An N-rich strip helps take the guess work out of adjusting your topdress N up or down based on your current crop conditions. Your county extension educator can provide more information on N-rich strips and you can find more information on the web at npk.okstate.edu

What source to use

• The plant does not care about the nitrogen source. A pound of nitrogen is a pound of nitrogen. Focus on getting the correct amount applied at the correct time, and choose your product based on price and application uniformity.
• Use a source that can be applied uniformly. Spinner trucks or buggies work but are generally the least uniform. Air trucks or streamers are the most uniform.
• Streamer nozzles almost eliminate leaf burn from UAN; however, leaf burn is generally not an issue until temperatures warm and/or you are applying fairly large amounts of UAN. Stream nozzles are also not affected much by wind and deliver a uniform pattern in a variety of conditions. There are also some studies that indicate banding of UAN through the use of stream nozzles will reduce nitrogen immobilization on crop residue. Keep in mind that you cannot tank mix herbicides when using streamer nozzles.
• One pass herbicide/topdress applications are very efficient in terms of time and input costs, but in some scenarios, it can end up costing you more money. Consider two-pass applications when dealing with no-till fields, especially when canopy coverage is below 70%. This is due to the high probability that the nitrogen will be tied up when it hits the residue and will not be available for the current wheat crop. For a more in-depth discussion on tank mixing herbicides and UAN for top-dress see
  https://osunpk.com/2016/02/07/herbicide-and-uan-tank-mixed-for-top-dress

Streamer nozzles provide uniform application of UAN in a wide variety of environmental conditions. 

Poor nitrogen application can result in a streaked field. Some of the areas in this field were over fertilized while some where under fertilized, resulting in wasted nitrogen and less than optimal crop yield.