January | 2021 | WORLD OF WHEAT

Usually, the alarm for beginning wheat nitrogen (N) topdressing gets sounded right away in early January. A significant portion of the Oklahoma wheat belt has received a good amount of moisture in the past weeks, which was great to save the crop from the severe drought in the fall. However, some parts of the state would still benefit from additional moisture (Fig. 1).

In areas of the state where it is dry and dry deeper than the majority of the rooting zone (> 6”), you should not worry about filling up the nitrogen tank as long as the water tank is empty. In this case, the best option is to wait to apply N right in front of a real chance of rain. The good news is we still have time to get N applied and not limit yield potential if we do receive rain in those areas. For regions with good soil moisture, you could start making topdressing plans.

Fig 1. 30-day rainfall accumulation in Oklahoma. Figure courtesy from Mesonet.

Few crop inputs deliver as much return on investment as nitrogen fertilizer. It takes approximately two pounds of nitrogen, costing roughly $0.80-$1.00, to produce one bushel of grain worth about $5.60. Of course, nitrogen is not the only yield determining factor in a wheat crop. Also, the law of diminishing marginal returns eventually kicks in, but nitrogen fertilizer is still one of the safest bets in the house, especially when there is adequate soil moisture.

Topdress nitrogen fertilizer is especially important because it is applied and utilized when the plant is transitioning from vegetative to reproductive growth. Several things, including the number of potential grain sites, are determined just before jointing, and the plant must have the fuel it needs to complete these tasks. 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. Our research has shown that approximately 20% of the aboveground nitrogen uptake at harvest is accumulated just prior to jointing, 50% at flag leaf emergence, and 70% at heading. The nitrogen stored in the plant vegetative parts is used to fill the grain later in the season, and the plant is dependent on this stored nitrogen to complete grain fill.

In the bullet points below, I will hit the major points regarding topdress nitrogen for wheat.

When to apply

To have full benefit, nitrogen must be in the rooting zone by the time wheat is jointing (which occurs around the end of February in southern OK and around mid-March in northern OK). 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 rain hopefully arrives back.
Suppose you decided to 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 due to the dry weather. Did you happen to use an 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 top dress now and supplement with a second top dress just before 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 farm’s break-even yield goal. An N-rich strip helps take the guesswork out of adjusting your top dress N up or down based on your current crop conditions. Also, it is not late to apply an N-rich strip. 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
Do not apply nitrogen to frozen ground. Nitrogen will move with water. If melting snow or frozen rain moves 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 to produce a bushel of wheat. 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. Keep in mind that being short in N will limit yield and protein concentration in the grain.
Did you do a soil test? It is okay to adjust top dress N plans based on your current yield potential. When you submitted your soil test, you might have stated a 50 bu/ac yield goal requiring 100 lbs/ac of nitrogen; however, it is important to take a hard look and determine if this yield goal is still realistic for your current crop status. This does not suggest to adjust based on what you think the weather might do. Still, it is okay to take inventory and adjust your top dress N up or down based on current field conditions.
If you have good soil moisture, even if you want to limit your input, you need 40 to 60 pounds/ac of nitrogen at a minimum based upon your soil test and yield goal. If you already have N in the system, make sure to apply enough N for a 30-40 bushel wheat.

What source to use

The plant does not care about nitrogen source. A pound of nitrogen is a pound of nitrogen. Focus on getting the correct amount applied at the proper time, and choose your product based on price and application uniformity.
Use a source that can be applied uniformly. In my experience, spinner trucks or buggies 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. Streamer nozzles are also not affected much by wind and deliver a uniform pattern in various conditions. Some studies indicate that banding of UAN through streamer 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