By Greg LaBarge, Ohio State University Extension
Nitrogen is the highest variable cost line in the 2023 wheat production budget. In addition, it is an important variable in yield, lodging, and grain quality. Spring N fertilizer should be applied between green-up (Feekes 3-4) and the beginning of stem elongation or jointing (Feekes 5-6). Here are a few things to consider in determining your wheat topdress N rate.
Ohio wheat N rate recommendations appear in the Tri-State Fertilizer Recommendations. Bulletin 974 (2020) is yield potential based. Table 1 shows N rate recommendations. These recommendations are for mineral soils with adequate drainage and 1% to 5% organic matter. Another important note is that the N rates in Table 1 are the total N applied. Therefore, if you used fall N, subtract that nitrogen from the Table 1 rate.
Using manure as a nitrogen source is a great opportunity considering current nitrogen prices. Lodging from excess N is a genuine concern when using manure in wheat. The manure test is the key to determining how much manure to apply. Most manure tests show total nitrogen, ammonia nitrogen, and organic nitrogen. Focus on ammonia nitrogen. It is the amount readily available for plant growth.
Use the manure ammonium content from the manure test and Table 1 to determine how many gallons of manure to apply. For example, a swine nursery manure test shows 14 pounds ammonium per 1,000 gallons. For a 90-bushel wheat crop, the rate from Table 1 is 110 pounds of N. We applied 20 pounds of starter N in fall 2022, leaving 90 pounds of N to apply at topdress. Taking 90 pounds of N needed, divided by 14 pounds of ammonium-N in the manure, would equal a target rate of 6,500 gallons.
Urea and urea-ammonium nitrate (UAN) are the most common fertilizer N sources used for wheat. Other products include ammonium sulfate, ammonium thiosulfate, diammonium phosphate, monoammoniun phosphate, and ESN. Generally, UAN has the greatest potential for N loss; ammonium sulfate the least, and urea is intermediate loss potential.
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. However, urea-based products may benefit from a urease inhibitor if conditions for volatilization exist for several days after application, including an extended dry period with warm drying temperatures (above 70 degrees F) and evaporating winds. Urea-based fertilizers need at least a half-inch rain within 48 hours after application 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.
We have a great factsheet that shows how to determine wheat growth stages and highlights management decisions for each growth stage https://go.osu.edu/wheatgrowth. In addition, our CORN newsletter https://agcrops.osu.edu/ goes back to a weekly edition on March 27. It is an excellent source for wheat, corn, soybean, and forage management information throughout the year.
Nitrification inhibitors are typically ineffective on wheat since the crop is already in progress when they are applied and the weather is still cold.