Timing is everything and the time for topdressing the winter wheat crop is now, says Kansas State University Agronomist Dave Mengel, adding that the key consideration is to make sure the plants have enough nitrogen (N) at the right time.
“The N in topdress applications should be moved into the root zone with precipitation well before jointing begins,” he says. “Ideally, the N should be available to the wheat when head differentiation occurs and head size is being determined, which can be about two weeks before jointing,”
The four main factors involved in good N management when topdressing wheat are timing, source, application method and rate, he says. It is best to apply topdress N early, preferably before the end of February, in order to have the best chance of receiving enough moisture to move the N into the root zone.
“While some producers wait until spring just prior to jointing, this can be too late in some years,” says Mengel.
“For well-drained medium-fine textured soils that dominate our wheat acres, the odds of losing much of the N that is topdress-applied in the fall or winter is low since we typically don’t get enough precipitation over the winter to cause significant denitrification or leaching. For these soils, topdressing should begin anytime now, usually, the earlier the better.”
For wheat grown on sandier soils, it is better to wait until closer to spring green-up to make topdress N applications, he adds. 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. It is also better to wait closer to spring green-up on poorly drained and/or shallow clay pan soils. He says N should not be applied to the soil surface when the ground is deeply frozen.
Mengel points out that most topdressing is broadcast-applied. In high-residue situations, this can result in some immobilization of N, especially where liquid UAN (urea ammonium nitrogen) is used. If no herbicides are applied with the N, producers can get some benefit from applying the N in a dribble band on 15-18-in. centers. This can 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.
The typical sources of N used for topdressing wheat are UAN solution and dry urea. Numerous trials by K-State 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 UAN applications would avoid much of this tie-up on surface crop residues, as well.
Mengel says growers should have started the season with a certain N recommendation in hand, ideally based on a profile N soil test done before the crop was planted and before any N was 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.
Wheat used for pasture needs a little extra N. “If the wheat was grazed in the 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,” says Mengel. “For heavy grazing, it may be necessary to make an additional N application in late winter to compensate for N removed by grazing, depending on how much N was applied earlier.”
Source: Kansas State University Go to oznet.ksu.edu.