Spring nitrogen management for winter wheat
Spring nitrogen application is very important for production of top wheat yields. It is the one practice that has the most affect on wheat yield. The current Michigan State University nitrogen (N) recommendation for winter wheat (soft white or red) is [(1.33 x projected yield) – 13].
For example, if the projected yield is 100 bushel per acre then the N recommendation is 120 lbs per acre. Normally 20 to 25 lbs N per acre is applied at seeding time in the fall. Therefore, the recommended spring topdress N rate will be 100 or 95 lbs N per acre. However, the actual amount of supplemental N needed in the spring varies from year to year depending on weather conditions.
In field studies over the past five years 100 + bu/a yields have been maximized with anywhere from 40 to 120 lbs N/acre topdressed at green up. Most commonly the top yields have occurred with 80 to 90 lbs N/a, which is slightly less than the recommended amount. In years where soil moisture and temperature conditions have been favorable throughout the months of April and May there has been good mineralization (release) of N from the soil reducing the amount of supplemental N required. However, in years like 2009 when the spring was cool and wet, N release from the soil was limited. The excess rain in many areas of Michigan in late April and throughout May resulted in significant loss of N that was topdressed at the end of March or in early April. As a result, where those conditions existed, it took 120 lbs of topdress N to maximize yield.
A variety of N carriers can be used successfully for enhancing wheat production. The more common fertilizers used to topdress wheat are urea (dry granular), urea-ammonium nitrate (UAN - liquid), ammonium sulfate and ESN (a polymer coated urea). Various combinations of the dry materials can be used. ESN provides gradual release of N over several weeks whereas the other materials dissolve fairly quickly and move into the soil with adequate soil moisture and rain. Volatile N loss is more likely to occur from urea on the soil surface than with other materials. This is more of a concern as soil temperature increases above 50°F. Once urea is moved into the soil by a good rain, the potential for volatile N loss is minimized. In four years of studies at MSU yields tended to be best where ESN was the topdress material. When ESN was blended 50:50 with urea or ammonium sulfate, yields were slightly lower and similar to yields with UAN or urea. In one year, which was dry in the spring, the wheat yield was lower with urea than with UAN. This was likely due to more volatile N loss from urea. Treating urea with Agrotain, a urease inhibitor, will delay the potential for volatile N loss for about two weeks and provide more opportunity for rain to occur.
Timing of application
It is most commonly recommended to apply spring N on wheat just as the wheat is greening up. At this time there is good potential for spring rains to occur and move the N into the soil and root zone, and thereby minimize the potential for volatile N loss. The N will be available to the root system as it begins to actively grow and utilize N. Studies over the years have shown this timing to be most favorable. Splitting the N between green up and tillering (Feekes Growth Stage 6) or later has most often given no yield benefit. However, some yield benefit may occur about one in every four years. Benefit from split application is more likely to occur when there is an extended wet period after topdressing N which results in soils being saturated and N being lost by leaching and denitrification.
In a 2009 study, yield was increased by applying 30 lbs N/a at Feekes Growth Stage 8 (tip of flag leaf just emerging). Delaying the N application to late boot stage resulted in no yield benefit. Therefore, when making split N applications it is best to make the second application between Feekes Growth Stage 6 and 8. In MSU field studies, foliar N applications with various materials have not shown any yield benefit. In some cases there was actually a slight decrease in yield even though there was no sign of injury. In these studies the N materials were applied to supply 3 or 6 lbs N/a at GS 6 (tillering) or 8/9 (flag leaf emerging).
In studies over the past two years, yield response to N rates has been similar across the six varieties used (3 red and 3 white). Grain yield did vary among the varieties. Nitrogen rate greatly increased the grain protein content even when it had no additional benefit on yield. This effect was also similar among the six varieties.