Source: Penn State Extension
Timing nitrogen (N) applications as close to the time of crop uptake as practical is one of the best practices for improving N use efficiency by crops. Right now we need to be thinking about topdressing wheat.
In many parts of Pennsylvania we've had constant snow cover and now that there has been some melting, the soils are very wet, making topdressing challenging at best. This is especially a concern if there was poor tillering in the fall, because in these situations, topdressing early, at green-up, is recommended.
If you had good growth and tillering going into the winter, delaying N until GS 5, just before stem elongation, is most efficient. Topdressing very early before green-up puts N out on the field before the crop can use it.
With cold temperatures there probably won’t be much loss from volatilization of urea before the next rain. Ammonium forms of N are stable and won’t be lost from conversion to leachable nitrate under cold soil conditions. The main risk of loss is under conditions where the fertilizer, in any form, is washed off the surface before it gets into the soil. Generally, at our level of winter grain production we do not usually see a significant practical advantage to splitting spring N applications further.
The main sources for top dressing winter grains are UAN (30-0-0), urea (46-0-0), and ammonium sulfate (21-0-0-24S). All are good sources of N for wheat. UAN and urea are both volatile forms, which should be a consideration with these sources because of the potential for significant loss of N. The potential loss from UAN is only half that of straight urea because UAN is only ½ urea. Losses are minimized if the urea or UAN can be applied just before around ½ inch of soaking rain.
Also, the losses tend to be less at cold temperatures but even a brief warm up at the time or application can result in greater losses. A urease inhibitor can effectively reduce volatilization losses from surface-applied urea or UAN when there are the conditions for volatilization loss.
Not all N additives are urease inhibitors. Check the label carefully to make sure the additive contains an effective urease inhibitor. Ammonium sulfate is non-volatile and therefore there is no benefit to adding a urease inhibitor with ammonium sulfate.
One other common question is sulfur on wheat. Sulfur deficiencies are still relatively rare in Pennsylvania, but are becoming more and more common. Ammonium sulfate contains 24% sulfur, therefore applying some of the wheat N requirement in this form will also supply sulfur. Generally, 100 pounds per acre of ammonium sulfate in the wheat N program, which will supply 24 pounds sulfur per acre, will supply adequate sulfur for wheat in most situations. Ammonium thiosulfate (ATS) is often used as a sulfur source with UAN. Three to 5 gallons of ATS will supply 10-15 pounds sulfur per acre.
There have also been questions about injury to wheat top-dressed with UAN containing ATS. While ATS can cause crop injury, this is usually only when the ATS is applied with starter fertilizer close to the seed. We can find little indication that there is a problem with ATS applied with topdress nitrogen (N) on wheat. There hasn't been any research done in Pennsylvania on this.
However, in looking at work done in other states and recommendations from many wheat-producing areas, there does not appear to be a problem directly related to ATS injury with topdressing. For example, in one study in Montana, ATS was applied straight with no dilution or other fertilizer and there was no injury. Remember that UAN itself can cause leaf burning, which can be significant at higher rates, such as in overlaps. Usually, this burning is not serious and does not result in a yield reduction. However, if this injury occurs along with other stresses, it could impact yield.