Topdressing wheat with nitrogen
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.
Application method. Most topdressing is broadcast applied. In high-residue situations, this can result in some immobilization of N, especially where liquid UAN is uniformly distributed across the soil surface -- as when N is combined with a herbicide application. If no herbicides are applied with the N, producers can get some benefit from applying the N in a dribble band spaced no wider than 15- to 18-inches. This can help avoid immobilization and may provide for a 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 uniformly 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.
Some of the new 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 the coated urea -- which will provide some N immediately to support tillering and head development and also continue to release some N in later stages of development -- works best in settings with high loss potential. This approach can be an alternative to split topdress applications, especially on heavier textured, poorly drained soils where trafficability can be an issue.
Rate. Producers should have started 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. 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. Topdressing should complement or supplement the N applied in the fall, with the total application amount equaling that targeted rate. One excellent way to fine-tune N topdress rates is with the use of sensors. Extensive research in Kansas for topdressing wheat resulted in the development of N rate calculators using input from NDVI sensors. More information on the use of these spreadsheets can be found at the K-State Soil Testing Laboratory web site.
One other note: If the wheat was 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. If conditions are favorable for heavy fall and/or spring grazing, additional N maybe necessary, especially for a grain crop.