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Topdress Wheat by Feekes Growth Stage 5
Winter wheat in the Central Plains has been slower than usual to break dormancy, thanks to colder than normal temperatures this winter. Now that the crop is beginning to green up and grow, farmers are scurrying to apply a post-winter application of nitrogen fertilizer, a practice commonly called topdressing.
Roger Barrett, crop consultant with Farmway Co-op in Courtland, Kansas says the slow start of the wheat crop breaking dormancy could mean it is prone to tissue injury from topdress fertilizer applications. The harsh winter conditions burned back or killed tillers on many wheat plants. Smaller tillers are coming on to replace those lost due to freeze, and he is wary of burning those secondary tillers back due to a nitrogen application. Thus, waiting a bit before topdress application occurs may be a way to save those young plants.
Feekes Growth Stage 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, according to Dave Mengel and Dorivar Ruiz-Diaz, soil fertility specialists at Kansas State University.
Winter wheat producers should getting wheat topdressed as soon as possible when it breaks dormancy, and definitely by the time the plants begin jointing, or Feekes Growth Stage 6. (Image courtesy New Mexico State University).
Timing. Enough nitrogen fertilizer should have been applied last fall or winter in order for wheat yield to have maximum yield potential by producing adequate numbers of tillers and large heads. Waiting until just prior to jointing to topdress can be too late in some years, especially when little or no N was applied in the fall. For well-drained medium- to fine-textured soils, the odds of losing much of the N that is topdress-applied in the winter is usually low since there is seldom enough winter precipitation to cause significant denitrification or leaching. For these soils, topdressing can begin anytime.
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 in the case of unusually heavy winter or early spring precipitation.
On poorly drained and/or shallow claypan soils, waiting until closer to spring greenup to make topdress N applications on these soils will help minimize the potential for this N loss.
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. Dry urea and UAN solution are equally effective, based on several years of research by K-State. Dry urea 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 the solution.
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.
If no soil test was conducted, a good rule of thumb is two pounds of N per bushel of expected yield, Barrett says.
Wheat planted into corn, sorghum and soybean stubble last fall may benefit from an additional 15 to 20 pounds per acre of additional nitrogen, to compensate for fertilizer that could be tied up in crop residue.