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Topdressing wheat
By David G. Hallauer Meadowlark District Extension agent, crops, soils, horticulture Kansas Now is the time to think about topdressing the 2007-2008 winter wheat crop. In order to get N moved in to the root zone prior to planting, producers should be making plans now to get topdress applications done by the end of February There are four main factors we consider when topdressing wheat: 1) Timing--bar none, the most important factor in a successful N topdress program is usually timing. Too late, and N can't adequately be taken up prior to jointing. Since most of our soils are well- drained medium-fine textured, the odds of losing much of the N that is topdress-applied in the fall or winter is low--we just don't get enough precipitation over the winter to cause significant denitrification or leaching. Sandy soil applications should wait until closer to spring green up. Poorly drained and/or shallow clay pan soils should consider the same to avoid denitrification. 2) Application method--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 on 15 to 18-inch centers to help avoid immobilization and maybe provide for a little more consistent crop response. The ideal application method would be to subsurface place the N into the soil. 3) Source--UAN solution and dry urea are the most common. 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 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. If producers plan to tank-mix with a herbicide, they'll have to use liquid UAN and broadcast it. 4) Rate--this topic was addressed in much more detail in this space a couple of weeks ago. Bottom line: get an accurate yield goal, then determine the N rate needed to reach it. That N formula is available at the Extension Office if you want to calculate your own. Above and beyond that, remember that graziers should add an additional 30 to 40 lbs N/acre for every 100 lbs of beef weight gain removed from the field. One common question is whether topdress N rates should be cut back, or eliminated entirely, if environmental conditions are poor and the wheat looks like it will have below- average yield potential. In general, the answer is to not be too pessimistic very early in the wheat stages of development and cut N applications too much. If conditions improve, the N that is topdressed will be needed for the wheat to reach its yield potential. If conditions do not improve and wheat yields are poor, the N will probably remain in the soil long enough for a summer row crop to utilize it. Date: 12/13/07
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