Many farmers are now making top-dress nitrogen (N) applications to winter wheat. Conditions of the wheat crop in Kansas are quite variable. Some fields were sown relatively early (mid- to-Late-September) and capitalized on available soil moisture, resulting in good stand establishment and early development. For a good portion of the state, however, fields planted in October may have not emerge until sometime in November or later, due to a lack of precipitation and extremely dry soil conditions.
These different crop conditions across the state result in contrasting yield potentials, which should be taken into consideration when managing N rate. Some key elements that need to be considered when deciding on the exact program you plan to use include: timing, N source, application method and N rate. Due to space limitations I won’t address N rate.
Ideally, the N in topdress applications will be moved into the root zone with precipitation well before jointing begins in order to be most efficiently utilized by wheat, if done in February and very early March. With some of the small wheat out there with limited tillers, having adequate N available to support spring tillering when it breaks dormancy will be important. Also, the potential number of kernels/meshes per head is determined right after spring green-up and prior to jointing. Thus having available N in the root zone is important to help ensure a good yield potential. Some combination of fall pre-plant or at-seeding N, and/or early topdressed N, is needed to supply adequate N to support head development.
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 waiting until spring, just prior to jointing, can be done with success, this can be too late in some years, especially when little or no N was applied in the fall.
For the well-drained, medium- to fine-textured soils that dominate our wheat acres, the odds of losing much of the N that is topdress-applied in the 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, and usually the earlier the better.
Also keep in mind that N should not be applied to the soil surface when the ground is deeply frozen and especially when snow covered. This will help prevent runoff losses with snow melt or heavy precipitation.
On both sandy soils subject to leaching and poorly-drained/higher clay content soils prone to denitrification, split applications may be a strategy to consider.
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/streamer bar on 15- to 18-inch centers. This can minimize immobilization and may provide for a more consistent crop response.
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 some of it will fall to the soil surface and be less affected by immobilization than broadcast liquid UAN, which tends to get hung up on surface residues.
Dribble band or streamer bar surface applied UAN applications would also avoid some of this tie-up on surface crop residues. However, if producers plan to tank-mix with an herbicide, they will have to use liquid UAN and broadcast it.
Controlled-release products such as polyurethane coated urea (ESN) might be considered on very sandy soils prone to leaching, or poorly-drained soils prone to denitrification. Generally, a 50:50 blend of standard urea and coated urea will provide some N immediately to support tillering and head development, and also continue to release some N in later stages of development. This would work best in settings with high loss potential.
Stacy Campbell is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact him by e-mail at scampbel@ksu.edu or calling 785-628-9430.