Source: Ed Lentz, OSU Extension
Late-planted wheat fields had little opportunity for growth before cold and wet conditions moved into the area last November. Fall tiller production was limited because of early cold weather soon after planting. In addition, some wheat stands have been damaged this winter from lack of snow cover, standing water, saturated soils, ice sheets, and days of very cold temperatures.
In these situations, producers have asked whether they should apply nitrogen earlier to increase the number of spring tillers. Keep in mind, it is fall tillers that provide most of the yield in a wheat field. Heads developing from spring tillers generally are much smaller than heads from fall tillers.
In northern climates, the vegetative period of growth is much shorter than the other wheat regions of the country; thus, plants have a much shorter time to recover from winter damage. From my experience, producers will have limited success in improving yields of poor stands and stands with reduced-growth by applying nitrogen earlier. A producer may get a few more spring heads, but not enough to significantly change the yield situation. The earlier application will also significantly increase the risk of nitrogen loss. In fact, a producer may need to readjust their yield potential for these fields and reduce their total nitrogen rate accordingly.
Wheat does not need large amounts of nitrogen until jointing (Feekes GS 6), generally the latter part of April. Soil organic matter and/or nitrogen applied at planting generally provide sufficient nitrogen for early spring growth. Ohio research has shown no yield advantage for nitrogen applied before jointing. The longer the time between nitrogen application and jointing, the greater the risk for nitrogen loss. 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 also shown that yield losses may occur from nitrogen applied prior to green-up regardless of the nitrogen source. The level of loss depends 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 any year compared to green-up or Feekes GS 6 applications. Keep in mind that green-up is a descriptive term and not a definable growth stage. My 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 – thus, growing plants.
There is a legitimate concern that wet weather may prevent application of nitrogen at early stem elongation. Ohio research has shown a yield decrease may occur when nitrogen application is delayed until Feekes Growth Stage 9 (flag leaf fully emerged). Thus a practical compromise is to topdress nitrogen any time fields are suitable for application after initial green-up to Feekes GS 6. There is still a potential for loss even at green-up applications. To lessen this risk a producer may want to use a nitrogen source that has a lower potential for loss such as urea or ammonium sulfate. ESN (polymer-coated urea) would be another option but it needs to be blended with urea or ammonium sulfate to insure enough nitrogen will be available for the crop between Feekes GS 6 – 9. The source of nitrogen becomes less important as the application date approaches Feekes GS 6 (jointing). The percentage of urea and/or ammonium sulfate would need to be increased with ESN for application times closer to Feekes GS 6. A producer may want to consider the use of a urease inhibitor with urea if conditions are favorable for volatilization losses: warming temperatures, drying winds and no rain in the forecast for 48 hours.
A split application of nitrogen may also be used to spread the risk of nitrogen loss and to improve nitrogen efficiency; however, Ohio State University research has not shown a yield increase from this practice compared to a single application after green-up. In a split system, 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 applied closer to Feekes GS 6.
In summary, some wheat fields look rough coming out of the winter. Applying nitrogen earlier may slightly increase the number of spring heads but probably not enough for a significant yield increase. The earlier application will increase the potential for nitrogen loss. University recommendation would be to topdress nitrogen when fields are suitable for application after initial green-up to early stem elongation.