Wheat does not require large amounts of nitrogen (N) until stem elongation/jointing (Feekes Growth Stage (GS) 6), which is typically at the middle or the end of April depending on location and spring temperatures. Ohio research has shown no yield benefit from applications made prior to this time period. Soil organic matter or N applied at planting generally provide sufficient N for early growth until stem elongation.
N applied prior to rapid utilization has the potential to be lost and unavailable for the crop. The type of N source also will affect the potential for loss. Ureaammonium nitrate (28%) has the greatest potential for loss, and ammonium sulfate the least, with urea somewhere between the two.
Ohio research has shown that yield losses may occur from N applied prior to green-up regardless of the N source. The level of loss can depend on the year. Losses would be smaller if the ground is not frozen or snow/ice-covered. This same research did not observe a yield increase from applications made prior to green-up compared to green-up or Feekes GS 6 applications. Keep in mind that green-up is a descriptive, relative term and not a definable growth stage. Our definition of green-up is when the new growth of spring has covered the dead tissue from winter, giving the field a solid green color from growing plants.
There is a legitimate concern that wet weather may prevent application of N at early stem elongation. Ohio research has shown a yield decrease may occur when N application is delayed until Feekes GS 9 (early boot). A practical compromise is to topdress N any time fields are suitable for application after initial green-up to early stem elongation. There is still a potential for loss even at green-up applications. To lessen the risk a producer may want to use an N source with a lower potential for loss such as urea or ammonium sulfate. Environmentally Smart Nitrogen (ESN) (polymer-coated urea) would be another option but it needs to be blended with urea or ammonium sulfate to insure enough N will be available for the crop between Feekes GS 6 to 9. The source of N becomes less important as the application date approaches stem elongation. The percentage of urea or ammonium sulfate would need to be increased with ESN for application times closer to Feekes GS 6.
A split application of N may also be used to spread the risk of N loss and to improve N efficiency; however, Ohio State University research has not shown a yield increase from this practice. The first application should be applied no sooner than green-up. A smaller rate should be applied with the first application, since little is needed by the crop at that time, and the larger rate should be applied closer to Feekes GS 6.
In summary, a producer may get away with applying N prior to green-up on wheat. However, university data has not shown a yield advantage for these early applications, and results have shown in certain years a major N loss and yield reduction from applications prior to green-up. Why take the risk? Just wait until green-up; the wheat does not need most of the N until April and May anyway.
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