There have been some questions on topdressing N on wheat. The main sources 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 the 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. The other additive type is a nitrification inhibitor. Nitrification inhibitors can reduce the loss of N through leaching and denitrification, thus nitrification inhibitors will be most beneficial on poorly drained or excessively well drained soils where these losses are most common. Nitrification inhibitors are also more beneficial when the N is applied long before plant uptake which is usually not the case with topdressed N on wheat. Also, nitrification inhibitors will be less beneficial in cold soils with ammonium sources of N applied because of the slower microbial conversion to nitrate N due to the cold weather. Consequently, nitrification inhibitors are generally less beneficial on wheat, but can be beneficial under some conditions, however, it is very difficult to predict exactly when a nitrification inhibitor will provide a benefit on wheat. Nitrification inhibitors have no effect on volatilization losses. As with the urease inhibitors, check the label to be sure the additive contains an effective nitrification inhibitor.
One other common question is sulfur on wheat. Sulfur (S) deficiencies are still pretty rare in Pennsylvania but are becoming more common. Ammonium sulfate contains 24% S, therefore applying some of the wheat N requirement in this form will also supply S. Generally, 100 lb/A of ammonium sulfate in the wheat N program, which will supply 24 lb S/A, will supply adequate S for wheat in most situations. Ammonium thiosulfate (ATS) is often used as an S source with UAN. Three to five gallons of ATS will supply 10 to 15 lb S/A. There have also been questions about injury to wheat topdressed 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. I can find little indication that there is a problem with ATS applied with topdress nitrogen (N) on wheat. There has not 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.