With the snow melted and a few cold mornings this past week, a few spreaders have hit the wheat fields. As we look at a potential late spring, as a time saving factor, can nitrogen be applied to wheat early to free up time for planting?
For any N application the question to ask is, “When does the crop need N?” Wheat does not require large amounts of N during green up. In fact, Ohio research has shown yield losses or at best little yield advantage from N applied prior to green-up compared to at or after green-up or Zadok 31 (first node visible) regardless of the N source. Nitrogen demand increases during stem elongation (Zadok 30), which depending on the year, is the end of April or the first of May. Soil organic matter and N applied at planting generally provide sufficient N for green up prior to stem elongation.
Nitrogen applied prior to stem elongation has the potential to be lost and unavailable when needed by the crop. Nitrogen source will also affect the potential for loss. Liquid nitrogen (28%) has the greatest potential for loss, ammonium sulfate the least, and urea would be somewhere between the two. The level of loss depends on the year and is usually moisture and temperature dependent.
Wet weather may prevent application of N at early stem elongation. In fact, research has shown a yield decrease may occur when N application is delayed until Zadok 47 (early boot). A practical compromise is to top-dress N any time fields are suitable for application after initial green-up to early stem elongation, However, there is a potential for loss even at green-up applications. To lessen this risk, the use of an N source that has a lower potential for loss such ESN (polymer-coated urea) or a nitrogen stabilizer such as Agrotain would be another option. Since ESN release is based on temperature and moisture, it needs to be blended with urea to ensure enough N will be available for the crop during early stem elongation. The source of N becomes less important as the application date approaches stem elongation. The ratio of urea and/or ammonium sulfate in the blend would need to be increased over ESN with later application times closer to green up and early stem elongation.
A split application of N may also be used to spread the risk of N loss. 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 stem elongation.
In summary, a producer may get away with applying N prior to green-up on wheat. However there is no yield advantage for these early applications, and in certain years a major N loss and yield reduction from applications prior to green-up can occur. There is really no yield reduction if N is applied later between green all the way up to just before boot stage. While the time saving may pay going early, the effects on the environment, yield and the pocket book show patience is truly a virtue.