Topdress nitrogen on wheat
Editor’s note: This article is from the archives of the MSU Crop Advisory Team
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Most nitrogen is broadcast prior to green up, usually in mid- to late March, usually in the early morning while the ground is frozen to prevent rutting of the fields. Nitrogen broadcast on the soil surface must be moved down into the soil and root zone with the spring rains. As urea dissolves and is converted to the ammonium form, some N may be lost as volatile ammonia. This may happen with granular urea and liquid UAN (50 percent urea) when it remains on the soil surface.
With cool soil temperatures in early spring, this loss is usually minimal, especially if rain occurs within five days after application. As the soil surface warms to above 50oF, the potential for volatile N loss increases. The probability of a half inch or more of rain at that time of year to move the urea into the soil is usually quite good. Once moved into the soil, volatile N loss is minimal. The risk of volatile ammonia loss can be minimized by: 1) applying urea containing fertilizer when the air temperatures will be less than 50oF, 2) timing applications within two to four days prior to predicted rain, 3) using a blend of urea and ammonium sulfate or ESN, 4) using ESN straight or in a blend, 5) using a urease inhibitor to slow the breakdown of urea, or 6) using split applications.
Timing of topdress nitrogen application is somewhat flexible in regard to effect on grain yield. Topdress N may be applied all prior to green up or split between green up and Feekes growth stage 6. The two weeks prior to and after green up is a good window for topdress N application. Getting the nitrogen applied prior to jointing prevents damage to the plants. Splitting the N application may improve yield in some years, but not always. In 2007, split applying N (urea followed by ammonium sulfate at Feekes 6) resulted in a yield 8 bu/a better than UAN applied all just prior to green up. In years where there is an extended dry period after the second application, the N may not be fully available to the crop and yield may be reduced. Spraying UAN over the foliage up to Feekes GS 6 is an option. UAN can be added to herbicides to accomplish two tasks with one trip over the field. Some injury to the wheat may occur with rates over 15 gallons (45 lbs N) per acre with or without some affect on yield. Application of liquid UAN alone into growing wheat is best done with streamers. Spraying N over wheat for foliar absorption is another option for supplying N.
In a study in Sanilac County, various N materials were sprayed over wheat at Feekes stage 6, 8/9, or 10.5 to supply 3, 6 or 12 lbs N/acre. The flag leaf N concentration was not altered and there was no improvement in yield from any of these foliar N treatments. Studies by other researchers have shown improvement in grain protein with late foliar N sprays, but if no premium is being received for grain protein there is no economic benefit.
Use of slow-release urea materials provides more flexibility in N management. Topdress application with slow-release N materials, such as ESN, is best done prior to green up. Later application may result in too little N being available to the wheat, especially if there is a prolonged dry period. Blending ESN with urea or ammonium sulfate is an option to provide immediate availability of N plus long term availability. In Michigan studies, pre-green up application of ESN has resulted in yields better than with urea or UAN. However, in Ontario and Ohio, yields have been similar or less than yields with the standard N materials. Soil moisture and rainfall appear to be very important factors