Extension Agronomy eUpdate Issue 672 February 2nd, 2018

Topdressing wheat with nitrogen: Timing, application methods, source, and rates



Now is a good time for producers to start planning topdress nitrogen (N) applications to winter wheat. Given the dry conditions in a large part of the state and some fairly small wheat in many fields due to delayed fall planting, there are some key elements to consider when deciding on the exact N fertilizer program for your crop. These include: timing, N source, application method and N rate.

Ideally, the N in topdress applications will be moved into the root zone with precipitation well before jointing begins in order to be most efficiently utilized by wheat. With some of the small wheat out there with limited fall tillers, having adequate N available to support spring tillering when it breaks dormancy will be important. Also, the potential number of meshes per head is determined after spring green-up and prior to jointing; thus, having available N in the root zone can help ensure good yield potential. Some combination of fall preplant or at-seeding N, and/or early topdressed N, is also normally needed to supply adequate N to support head differentiation. The following will discuss some of the issues to consider when making topdressing decisions.

Timing of N application

The most important factor in getting a good return on topdress N is usually timing. It is critical to get the N on early enough to have the maximum potential impact on yield. While waiting until spring, just prior to jointing, can be done with success, 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 get enough precipitation over the winter to cause significant denitrification or leaching. For these soils, topdressing can begin anytime, 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. Waiting until closer to spring green-up to make topdress N applications on sandier soils 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 green-up to make topdress N applications on these soils will help minimize the potential for this type of N loss.

Also keep in mind that N 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.

Split applications

On both sandy soils subject to leaching and poorly drained soils prone to denitrification, split applications may be a strategy to consider. This would involve applying enough N in the fall at or prior to planting to give good support for fall growth and tillering -- generally 20-30 pounds of N. Follow up with an additional application of about 20-30 pounds of N in late winter or early spring to support spring tillering, possibly applied with herbicides. This late-winter/early-spring application becomes especially important when stands are thin due to poor emergence, as many fields are this year. Finally, come back around jointing or a few days later with a final application to support heading and grain fill. This strategy can also provide flexibility for in a year like this with uncertainty due to dry conditions and poor fall growth allowing to hold back part of the N for later in the spring as we have a better idea of soil moisture and weather conditions for the season.

Application method

Most topdressing is broadcast applied. In high-residue situations, this can result in some immobilization of N, especially where liquid urea ammonium nitrate (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. This can minimize immobilization and may provide for a more consistent crop response.



Figure 1. Streamer bars used for topdressing wheat in a surface band. Photo by Ray Asebedo, K-State Research and Extension.

Nitrogen source

The typical sources of N used for topdressing wheat are UAN solution and dry urea. Numerous trials by K-State over the years have shown that both are equally effective. In no-till situations, there may be a slight advantage to applying dry urea since some of it will fall to the soil surface and be less affected by immobilization than broadcast liquid UAN, which tends to get hung up on surface residues.



Figure 2. Urea broadcast to tillering wheat in a topdress application. Photo by Romulo Lollato, K-State Research and Extension.

Dribble (surface band) UAN applications would also avoid some of this tie-up on surface crop residues as well. But if producers plan to tank-mix with an herbicide, they’ll have to use liquid UAN and broadcast it.

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. Generally a 50:50 blend of standard urea and the coated urea will provide some N immediately to support tillering and head development, and also continue to release some N in later stages of development. This would work best in settings with high loss potential.

Nitrogen 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 a soil sample was taken at sowing, profile nitrate-N can help determine the rate to be applied based on the yield goal. K-State recommends 2.4 lbs of N per bushel per acre of yield goal, from which credits for profile N, previous crop, tillage system, and organic matter are provided. However, it is not too late to use the profile N soil test if taken in late winter/very early spring before the wheat greens up. While it won’t be as accurate as when sampled in the fall, it can still point out fields or areas in fields with high levels of available nitrate N. Unfortunately it is not reliable in measuring recently applied N. So if a high rate of N has already been applied, a late winter profile sample probably shouldn’t be taken. Remember that topdressing should complement or supplement the N applied in the fall and the residual soil N present in the soil. The total N application, planting and topdressing, should equal the target recommended rate.

If the wheat was grazed this fall and winter, producers should add an additional 30-40 lbs N per acre for every 100 lbs of beef weight gain removed from the field. If conditions are favorable for heavy fall and/or spring grazing, additional N maybe necessary, especially for a grain crop.

Low grain prices and dry conditions this year may also play a role for N rate decisions this spring. However, it is important to keep in mind that N is the most limiting nutrient for wheat, and the optimum agronomic N application rate will likely result in economic returns. In general, producers may consider a later topdress application (around jointing) with a better idea of the overall crop condition and expectations for the rest of the season; rather than cutting back on N rates now and potentially limiting yields even further.

Some fields may also benefit from an application of sulfur and chloride. Like N, these nutrients are mobile in the soil, and a topdress application before jointing is considered an effective application time. Sulfur and chloride topdress applications should be made based on soil test and history of response. More information on topdressing with sulfur and chloride can be found in an upcoming eUpdate issue.

Dorivar Ruiz Diaz, Nutrient Management Specialist

ruizdiaz@ksu.edu

Romulo Lollato, Wheat and Forages Specialist

lollato@ksu.edu