Topdress nitrogen on wheat
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EmailEditor’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
