Source: Dorivar Ruiz Diaz, Nutrient Management Specialist, Kansas State University 

Producers in most areas of Kansas topdress their wheat with nitrogen (N), and possibly a broadleaf residual herbicide, during the winter or early spring. As always, it is important for producers to properly plan their N fertilization programs in order to make sure they get the highest possible net return. The four main factors involved in this are timing, source, application method, and rate.


* Timing. The most important factor in getting a good return on topdress N is usually timing. It is critical to get the N on early enough to have the maximum potential impact on yield. While some producers often wait until spring just prior to jointing, this can be too late in some years. For well-drained medium-fine textured soils that dominate our wheat acres, the odds of losing much of the N that is topdress applied in the fall or winter is low since we typically don't get enough precipitation over the winter to cause significant denitrification or leaching. For these soils, topdressing can begin anytime now, and usually the earlier the better.


For wheat grown on sandier soils, earlier is not necessarily better for N applications. On these soils, there is a greater chance that N applied in the fall or early winter could leach completely out of the root zone if precipitation is unusually heavy during the winter. Waiting until closer to spring green-up to make topdress N applications on sandier soils will help manage this risk.


On poorly drained and/or shallow clay pan soils, N applied in the fall or early winter would have a significant risk of denitrification N loss. Waiting until closer to spring green-up to make topdress N applications on these soils will help minimize the potential for this N loss.


On all soils, nitrogen should be applied early enough that it has time to move down into the root zone of the wheat before jointing begins. Surface-applied nitrogen moves into the soil with rain and/or snow, and that can be unpredictable in many regions of Kansas. Therefore, particularly in drier regions the best strategy on most soils is to apply the N as early as possible to have the best chance of getting a rain that will move the N into the root zone. Another reason to apply N as early as possible on most soils is that wet and muddy fields may make timely topdress applications impossible if the applications are delayed too late in the growing season.


As temperatures warm in the early spring, there is a greater likelihood of getting significant leaf burn from topdress N applications. This leaf burn generally does not reduce yield prospects if applications are made before the last, uppermost leaves are developed. If the leaf burn occurs at a very late stage of growth — such as flag leaf emergence or boot stage — significant yield reductions are possible. Volatilization N loss is not an issue for topdress N applications made during cool weather in Kansas.


A final reminder about the timing of topdress applications: Nitrogen should not be applied to the soil surface when the ground is deeply frozen. This will help prevent runoff losses.


* Application method. Most topdressing is broadcast applied. In high-residue situations, this can result in some immobilization of N, especially where liquid UAN is used. If no herbicides are applied with the N, producers can get some benefit from applying the N in a dribble band on 15-18-inch centers. This can help avoid immobilization and maybe provide for a little more consistent crop response.


* Source. The typical sources of N used for topdressing wheat are UAN solution and dry urea. Numerous trials by K-State over the years have shown that both are equally effective. In no-till situations, there may be some slight advantage to applying dry urea since it falls to the soil surface and may be less affected by immobilization than broadcast liquid UAN, which tends to get hung up on surface residues. Dribble (surface band) UAN applications would avoid much of this tie-up on surface crop residues as well. But if producers plan to tank-mix with a herbicide, they'll have to use liquid UAN and broadcast it.


* Rate. Producers should start the season with a certain N recommendation in hand, ideally based on a profile N soil test done before the crop is planted and before any N has been applied. It is not uncommon for many producers to just use the same N rate year after year. This may result in too much N being applied some years, and too little in others. Where conditions have been dry and recent crop yields have been low, there could well be some very high levels of residual N remaining in the soil, and easily within the root zone of wheat. In those cases, a topdress N application may not increase yields at all. The only way to know for sure is to have a profile N test done. However, if some N has already been applied to the wheat crop, it is too late to use the profile N soil test since it is not reliable in measuring recently applied N.


The K-State wheat N guidelines suggest that all potential soil N contributions (soil organic matter (SOM), residual soil profile N (ProfN), manure-N (ManN), etc.) be subtracted from the total N requirement. For no-till systems, 20 lbs N per acre is added to the recommendation. And another 30 lbs N per acre is added if sunflowers or grain sorghum immediately preceded the wheat crop. Below is the suggested N guideline equation:


N Rec = (2.4 x Expected Yield) - (10 x %SOM) - (ProfN) - (Man-N) + Adjustments


Additionally, if the wheat will be grazed this fall and winter, producers should add an additional 30-40 lbs N/acre for every 100 lbs of beef weight gain removed from the field. For heavy grazing, it may be necessary to apply some N preplant or as an early topdress, then make an additional N application to compensate for N removed by grazing.


One common question is whether topdress N rates should be cut back, or eliminated entirely, if the wheat looks like it will have below-average yield potential. In general, the answer is to not be too pessimistic at this point and cut N applications too much. If environmental conditions are more favorable in winter and spring, the N that is topdressed will be needed for this wheat to reach its yield potential.