When winter finally starts to come to an end, it will be time to start topdressing nitrogen on winter wheat.
Timing, nitrogen source, application method and nitrogen rate all need to be considered when deciding on the exact program to use, says Dave Mengel, Kansas State University agronomist.
Ideally, the nitrogen in topdress applications will be moved into the root zone with precipitation well before jointing begins to be most efficiently utilized by wheat, Mengel says.
"With some of the small wheat out there this spring, having adequate nitrogen available to support spring tillering when it breaks dormancy will be important," the soil fertility specialist says. "Some combination of fall pre-plant or at-seeding nitrogen, and/or early topdressed nitrogen, is also normally needed to supply adequate nitrogen to support head differentiation.
"This is the stage when head size is being determined, and can begin about 2 weeks before jointing."
Following are issues to consider when making topdressing decisions.
* Timing. The most important factor in getting a good return on topdress nitrogen is usually timing. "It's critical to get the nitrogen on early enough to have the maximum potential impact on yield," Mengel says. "While some producers often wait until spring just prior to jointing, this can be too late in some years, especially when little or no nitrogen was applied in the fall.
For the well-drained medium- to fine-textured soils that dominate Plains winter wheat areas, the odds of losing much of the nitrogen that is topdress-applied in the winter is low since the area typically doesn’t get 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, Mengel adds.
For wheat grown on sandier soils, earlier is not necessarily better for nitrogen applications.
"On these soils, there is a greater chance that nitrogen applied in the fall or early winter could leach completely out of the root zone if precipitation is unusually heavy during the winter," Mengel says. "Waiting until closer to spring green-up to make topdress nitrogen applications on sandier soils will help manage this risk."
On poorly drained and/or shallow claypan soils, nitrogen applied in the fall or early winter would have a significant risk of denitrification nitrogen loss. Waiting until closer to spring green-up to make topdress nitrogen applications on these soils will help minimize the potential for this nitrogen loss, Mengel says.
He says that nitrogen 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," he adds.
* Application method. Most topdressing is broadcast applied. In high-residue situations, this can result in some immobilization of nitrogen, especially where liquid UAN is used. "If no herbicides are applied with the nitrogen, producers can get some benefit from applying the nitrogen in a dribble band on 15- to 18-inch centers," Mengel says. "This can help avoid immobilization and may provide for a little more consistent crop response."
* Source. The typical sources of nitrogen used for topdressing wheat are UAN solution and dry urea. Numerous trials by Kansas 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," Mengel says. "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, Mengel reports. Generally a 50:50 blend of standard urea and the coated urea — which will provide some nitrogen immediately to support tillering and head development and also continue to release some nitrogen in later stages of development — works best in settings with high loss potential.
* Rate. Producers should have started the season with a certain nitrogen recommendation in hand, ideally based on a profile nitrogen soil test done before the crop is planted and before any nitrogen has been applied.
"If some nitrogen has already been applied to the wheat crop, it's too late to use the profile nitrogen soil test since it's not reliable in measuring recently applied nitrogen," Mengel says. "Topdressing should complement or supplement the nitrogen applied in the fall, with the total application amount equaling that targeted rate."
Mengel adds that if the wheat was grazed this fall and winter, producers should add an additional 30 to 40 pounds of nitrogen per acre for every 100 pounds of beef weight gain removed from the field.
If conditions are favorable for heavy fall and/or spring grazing, he says additional nitrogen may be necessary, especially for a grain crop.