By Ed Lentz, CCA, Laura Lindsey, Ohio State University Extension
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 no yield benefit to early N applications as long as the application was made by Feekes GS 6 (one visible node). (If you need a reminder on how to assess if wheat is at Feekes GS 6, see this video: https://www.youtube.com/watch?v=D_f3VrqzV5c) Nitrogen applied early has the potential to be lost since wheat will use little N until after jointing. 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 Fertilizer Recommendation Bulletin for N rates in wheat. This system relies on yield potential. As a producer, you can 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 Fertilizer Bulletin recommends 120 lb N for yield goals of 100 bu/A, 110 for 90 bu/A crop, 90 lb for 80 bu/A crop, and 80 lb for a 70 bu/A crops. These recommendations are for total 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 = (1.33 x Yield potential) – 13.
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 Fertilizers Bulletin recommends that you subtract from the total (spring N) any fall applied N. I would take no more than a 20 lb/A credit even if you applied a larger amount. 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 100 lb N per acre would be recommended for a yield potential of 100 bu, 90 for 90 bu potential; and 70 for a 80 bu potential. Nitrogen rate studies at the Northwest Agricultural Research Station over the past 20 years 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 apart from one 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.
ESN or polymer coated urea will reduce the potential for N loss from leaching, denitrification, and volatilization. Since these conditions are unlikely to occur in most years, it may not be economical to use this product. Cool weather may prevent the timely release of N from ESN, so if ESN is applied, it should be mixed with urea or ammonium sulfate and be no more than 60% ESN.
A split application of N may be used to spread the risk of N loss and to improve N use efficiency. However, Ohio State University research has not shown a yield increase from this practice compared to a single application after green-up. In a split system, the first application should be applied no sooner than green-up. A smaller rate should be applied with the first application since little is needed by the crop at that time and the larger rate applied closer to Feekes GS 6.