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.
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