By Ed Lentz
Wheat has already reached green-up across the state so spring N may be applied anytime fields are fit. Keep in mind that research has shown N can be applied up to Feekes GS 6 (one visible node) without a reduction in yield. However, wheat is growing slowly because of the cool temperatures, particularly in northern Ohio. Nitrogen applied early has the potential to be lost since wheat will use little N prior to jointing (Feekes GS 6). Urea-ammonium nitrate (UAN) or 28% has the greatest potential for loss and ammonium sulfate the least. Urea will have little potential for loss as long as it does not volatize. No stabilizer will protect the nitrate component of UAN, which is roughly 25% of the total N in UAN at application time.
Ohio State University recommends the Tri-State guide for N rates in wheat. This system relies on yield potential. 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 N 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 bushels; 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)]
No credit is given for previous soybean or cover crops, since it is not known 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. I would not credit no more than 20 lb/A even if a larger amount was applied. 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.
Wheat generally does not benefit from a nitrification inhibitor since temperatures are relatively cool at application time and the application is made to a growing crop, this is especially true as the crop approaches Feekes GS 6. However urea may benefit from a urease inhibitor (products containing NBPT) if conditions for volatilization exist for several days after application. These conditions would include an extended dry period with warm drying temperatures (risk increases with temperatures above 70°F) and evaporating winds. Urea applications need at least a half inch rain within 48 hours to minimize volatilization losses unless temperatures remain relatively cool. The urease inhibitor will prevent volatilization for 10 to 14 days with the anticipation of a significant rainfall event during this time.
Source : osu.edu