Topdressing wheat with nitrogen
Some wheat has begun to green up, so this is a good time to start planning for topdressing nitrogen (N) on the winter wheat crop. With fairly small wheat in some fields due to late planting plus concerns over stands, winter survival, and dry soils in some areas, there are several key elements that need to be considered when deciding on the exact program you plan to use. These include: timing, N source, application method and N rate.
Ideally, enough N was applied in the fall to meet the needs of the crop through greenup. With poorly developed wheat in particular, having adequate N available to support spring tillering when it breaks dormancy will be important. Some combination of fall preplant or at-seeding N and/or early topdressed N is also needed to supply adequate N to support head differentiation a little later in the spring. Feekes 5 is the stage when head size is being determined, and this begins about two weeks before jointing. It’s important the N in topdress applications be moved into the root zone with precipitation well before jointing. The following will discuss some of the issues to considering when making topdressing decisions.
Timing. The most important factor in getting a good return on topdress N is usually timing. It is critical to get some N on early enough to have the maximum potential impact on yield, by producing adequate numbers of tillers and large heads. While some producers like to wait until spring just prior to jointing to topdress, 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 receive 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 or early spring. Waiting until closer to spring greenup to make initial topdress N applications on sandier soils, and following that with a second topdressing later will help manage this risk.
On poorly drained and/or shallow claypan soils, especially in south central or southeast Kansas, N applied in the fall or early winter would have a significant risk of denitrification N loss. Waiting until closer to spring greenup to make topdress N applications on these soils will help minimize the potential for this N loss. This is another situation where a split N application could reduce loss. However, getting this accomplished on poorly drained soils can be difficult.
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.
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