In many parts of Ohio, wet conditions during the last part of April have prevented the second application of N in split programs and in a few cases, first application. As a result of the cloudy and wet weather, much of the wheat in the northern two-thirds of the state is still at a growth stage where timely applications of N can still be made.
However, Ohio research has shown that yields drop as much as 10-15% when single N applications have been delayed to Feekes Growth Stage 9 (boot). In a split program there may not be any yield reduction with the delayed application. If application is delayed to Feekes GS 9, a single application program may want to reduce the N rate to reflect the reduction in yield potential.
A split program probably should stay with its original rate. Nitrogen source will be dependent on price, availability, and application options. Ohio research did not observe any yield differences among urea, urea-ammonium nitrate (28% solution) or ammonium sulfate when applied at later growth stages. To minimize leaf damage, nozzle selection and the use of streamer bars would be recommended with UAN — damage to the flag leaf may significantly reduce yield.
Also consider, if the weather forecast is for an extended drying period with windy and warm temperatures (>75°) and no rain for several days, you may want to consider a urease inhibitor with urea applications. Slow release and controlled-release N products should probably be avoided since the wheat crop needs the N now and until flowering and not so much later. Since the growing crop will use N now, nitrification inhibitor-type products would be of little benefit and most likely not worth the extra cost.
In summary, for those who have wheat beyond stem elongation (Feekes 7) and N has not yet been applied, you should still get out and apply N. You might have lost some yield potential if the crop has been N stressed, but the crop will still respond.