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Wheat Topdress Options for a Soggy Spring

In dealing with wet conditions, consider timing, plus nitrogen source and rate.

Ken Wood isn’t sure he’s ever seen a winter like this one. 

The Chapman, Kansas, farmer says some of his wheat fields have had water standing in them since last October. “I don’t think I’ve ever seen it this wet,” says Wood, who raises winter wheat and corn in north-central Kansas.

The winter wheat crop is just beginning to break dormancy, a time when most farmers give the young crop another shot of nitrogen fertilizer to help carry it through the spring. But not this year. For winter wheat producers, winter 2018-2019 is a perfect storm of awfulness, due to late planting last fall, followed first by extreme cold, and then excessive precipitation in the winter.  

Normally, Wood – and other wheat growers throughout the Winter Wheat Belt – would be in the middle of topdressing N fertilizer right now. But wheat fields have been frozen and muddy, exacerbated by even more rain falling throughout Kansas and Nebraska this week. “We’re not close to getting anything put on now,” Wood says.

In Kansas, the nation’s leading winter wheat producing state, about half of the Kansas wheat crop is extremely small (less than one tiller), well behind its normal growth rate. 

“The only good thing is, wheat is behind, too,” Wood says. “If it’s broken dormancy at all, it’s just broken dormancy.” Therefore, there is still time to get the topdress N application on.

However, wheat growers have some decisions to make about spring fertility, including timing, N source, application method, and N rate, says Dorivar Ruiz Diaz, nutrient management specialist at Kansas State University. 

Ideally, the N in topdress applications will move into the root zone with precipitation well before jointing begins in order to be most efficiently used 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. Therefore, having available N in the root zone can help ensure a 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. 

Timing

It is critical to get the N on early enough to have the maximum potential impact on yield, especially in a year with limited fall tillering. While waiting until spring just prior to jointing can be done with success, in some years it is too late – especially when little or no N was applied in the fall. 

Soil type is a big factor in timing.

  • Well-drained, medium- to fine-textured soils. The odds of losing much of the N topdress applied in the winter is low. For these soils, topdressing can begin now; usually the earlier the better. 
  • Sandier soil. 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. Waiting until closer to spring green-up to make topdress N applications on sandier soils will help manage this risk.
  • Poorly drained and/or shallow claypan soils. 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 N loss.

Nitrogen should not be applied to the soil surface when the ground is deeply frozen – and especially when covered with snow, Ruiz Diaz says. This will help prevent runoff losses with snowmelt or heavy precipitation. This will be a special challenge this year, as most Kansas soils are deeply frozen. Additionally, once the soils start to melt, they will likely be too wet for any fieldwork. Therefore, every field should be considered for characteristics such as slope, N source, tillage system, and the short-term forecast for temperature and precipitation.  

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 to 30 pounds of N. Then follow up with an additional application of about 20 to 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 in a year like this with 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 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, which can minimize immobilization and may provide a more consistent crop response.

Nitrogen source

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 some slight advantage to applying dry urea since some of it will fall to the soil surface (Figure 1) and be less affected by immobilization than broadcast liquid UAN, which tends to get hung up on surface residues.
Dribble (surface band) UAN applications would also avoid some of this tie-up on surface crop residues. However, if producers plan to tank-mix with an herbicide, they will 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 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

Ideally, wheat growers started the season with a certain N recommendation in hand, preferably based on a profile N soil test done before the crop was planted and before any N had 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. However, it is not too late to use the profile N soil test if taken in late winter/very early spring before green-up. While it will not be as accurate as when sampled in the fall, it can still identify fields or areas in fields with high levels of available nitrate N. 

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.

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