Soon wheat will be greening up across the state. Ohio State University recommends applying nitrogen between green-up and Feekes Growth Stage 6 (early stem elongation), which is generally the latter part of April.
The potential for nitrogen loss will decrease by waiting to apply closer to Feekes 6. Green-up will be later this year and a common sense approach would recommend applying as soon as field conditions allow application equipment, particularly since days available for field activities may be limited between now and Feekes 6.
Ohio State recommends the Tri-State guide for N rates in wheat. This system relies on yield potential of a field. As a producer, you can greatly increase or reduce your N rate by changing the value for yield potential. Thus, a realistic yield potential is needed to determine the optimum nitrogen rate.
To select a realistic yield potential, look at wheat yield from the past five years. Throw out the highest and lowest wheat yield, and average the remaining three wheat yields. This three-year average should reflect the realistic yield potential.
Table 10 in the Tri State guide recommends 110 lb N for yield goals of 90+; 70 lb for 75 bu; and 40 lb N for 50 bushel yield goal (these recommendations are for total N and include any fall N). If you prefer to be more specific the following equation may be used for mineral soils, which have both 1 to 5% organic matter and adequate drainage:
N rate = 40 + [1.75 x (yield potential - 50)]
We do not give any credit for the previous soybean or cover crop, since we do not know if that organic N source will be released soon enough for the wheat crop. The Tri-state recommends that you subtract from the total (spring N) any fall applied N up to 20 lb/A. Whether you deduct fall N depends how much risk you are willing to take and your anticipated return of investment from additional N. Based on the equation above and deducting 20 lb from a fall application, a spring application of 110 lb N per acre would be recommended for a yield potential of 100 bu, 90 for 90 bu potential; 70 for a 80 bu potential and 40 lb N per acre for a 60 bu potential.
Nitrogen rate studies at the Northwest Agricultural Research Station have shown the optimum rate varies depending on the year.
However, averaged over years, yield data from these studies correspond well with the recommendation equation given above. These studies have also shown that regardless of the year, yields did not increase above a spring rate of 120 lb N per acre. Nitrogen rate work in 2013 at the OARDC Northwest Agriculture Research Station once again showed no response above a spring rate of 120 lb N per acre.