Each year producers ask the question when is the best time to apply N to wheat? Also, is it ok to apply N on frozen ground?
For any N application the question to ask is when does the crop need N. Wheat does not require large amounts of N until stem elongation (Feekes Growth Stage 6), which is the middle or the end of April depending on the location in state. Ohio research has shown no yield benefit from applications made prior to this time period. Soil organic matter or N applied at planting generally provides sufficient N for early growth until stem elongation.
Nitrogen applied prior to rapid utilization has the potential to be lost and unavailable for the crop. Nitrogen source will also affect the potential for loss. Urea-ammonium nitrate (28%) has the greatest potential for loss, ammonium sulfate the least, and urea would be somewhere between the two other sources.
Ohio research has shown yield losses from N applied prior to green-up regardless of the N source. The level of loss depends on the year. There has never been a yield increase from applications made prior to green-up compared to green-up or Feekes Growth Stage 6 applications.
However, 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 Growth Stage 9 (early boot). Thus a practical compromise is to topdress N any time fields are suitable for application after initial greenup to early stem elongation. There is still a potential for loss even at green-up applications. To lessen this risk a producer may want to use a N source that has the least potential for loss for earlier applications. The source of N is not as critical as the application date approaches stem elongation.
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, but 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.