Post-Harvest Nitrogen Assessment: If You Don’t Measure It, You Can’t Manage It

Wilke Farm.

Wheat harvest is complete across eastern Washington with yields being at or mostly below average. There were also reports of elevated grain protein across the region. So, what does this imply about your nitrogen management program and what does it mean for nitrogen applications moving forward?

On the WSU Small Grains website under the Soil & Water Resources, Tools & Calculators tab (Figure 1) is a Post-Harvest Nitrogen Use Efficiency Calculator. This calculator is designed to help a farmer assess their nitrogen management program and help guide future applications. Farmers need only four pieces of information: 1. grain yield, 2. grain protein, 3. soil test nitrogen supply, and 4. total nitrogen applied. Total nitrogen applied should include all nitrogen applied with the seed, in a starter package, top-dress applications, and any nitrogen applied with herbicide applications.  

Screenshot of Wheat & Small Grains website.
Figure 1. Get to the Post-Harvest Nitrogen Use Efficiency Calculator by clicking on Tools & Calculators under the Soil & Water Resource tab.

At the WSU Wilke Research and Extension Farm located near Davenport, Washington, Piranha CL+ winter wheat was seeded into no-till fallow in 2022. Total nutrients applied per acre included 54 lb N, 15 lb P2O5, 0.2 lb K2O, 4 lb S, 10 lb Cl, 0.15 lb Zn and 0.06 lb B. Nitrogen and sulfur rates were variably applied based on yield potential across three zones. Prior to fertilization and seeding, soil samples were collected and showed a total net nitrogen (organic matter nitrogen, ammonium nitrogen, and nitrate nitrogen) supply of 185 lb N/acre available. The field yielded 72 bu/acre and had a grain protein content of 9.9%. Data were entered into the Post-Harvest Nitrogen Calculator and the results are in Figure 2.

Screen shot of post-harvest nitrogen-use efficiency calculator with inputs described in article.
Figure 2. Field 6 winter wheat post-harvest nitrogen inputs, nutrient uptake and removal, and post-harvest efficiency at the WSU Wilke Research and Extension Farm in 2023. 

So, how should these results be used to enhance our nutrient management program? The winter wheat crop had a total nitrogen uptake of 107 lb/acre, 75 lb/acre was hauled away to the elevator and 32 lb/acre remains on the field in residue.  Nitrogen uptake efficiency of 50% and greater is optimal. Field 6 had a N uptake efficiency of only 44.7%. This means we over fertilized the field and should have approximately 132 lb. N/acre remaining in the field for next year’s crop. If we do not have this, we might assume we lost nitrogen, and if that is the case, we may look at split applications or the use of nitrogen stabilizers. We had 3.32 lb/acre of nitrogen supplied per bushel of grain produced and traditionally we consider it requires 2.7 lb/acre of nitrogen per bushel of grain. Do we need more accurate yield goal expectations as the field was fertilized for 89 bushels/acre and our 10-yr APH yield is 75 bushels/acre? Other questions to be asking:

  1. How do these results compare to other wheat fields across the farm this year and possibly previous years to help determine what is seasonal versus management?
  2. Is some other nutrient yield limiting?
  3. Do I need to re-evaluate my variable rate fertilizer map? If you are not using variable rate, should you consider investing in this technology?
  4. Does my soil sampling program need adjusted?
  5. Would this field be a good option for hard red wheat next given the potential high amount of residual N?

Learn more about Postharvest Nitrogen Analysis & Nutrient Management at the 2023 WSU Wheat Academy.

Aaron Esser

For questions or comments, contact Aaron Esser via email at