G.V. Johnson, W.R. Raun, John Solie and Marvin Stone


Departments of Plant & Soil Sciences and Biosystems and Ag Engineering

Division of Agricultural Sciences & Natural Resources

PT 2002-21                                                         July 2002                                                  Vol.14,   No.21   


            The new nitrogen management strategy proposed from OSU research calls attention to the fact that wheat grown in the same field year after year, needs vastly different amounts of fertilizer nitrogen (N) each year to reach maximum yield.  The new strategy relies on being able to assess, in-season, the kind of year the crop is experiencing and fertilize accordingly (PT 2002-19).  Some frequently asked questions are addressed below.


What is the projected difference in fertilizer-N used with the new management strategy?

            The estimated N used in the new “N-Rich Strip” management strategy depends some on how and when the N is applied. However, over the long-term, because the new strategy better estimates actual yield and N need, N used is expected to be less than for the conventional approach based on a yield goal.  An interesting approach to examining this is to look at how much N was applied for highest yields, and of the N applied, how much was actually harvested for the highest yields each year from OSU long-term research (Figure 1).

Figure 1.  Nitrogen rate associated with highest yield and the estimated amount of fertilizer N harvested in grain from highest-yield treatments (Lahoma Research Station, Experiment 502).


            The graph shows the N rate for highest yield varied greatly over time, but averaged 75 lb N/acre, which would be close to the conventional strategy of applying 80 lb N/acre for a 40 bu/acre yield goal (highest yield average = 43 bu/acre).  Estimated fertilizer N removed in the grain (assumes 13 % crude protein) averaged only a third of that applied and is consistent with other estimates of 33 % nitrogen use efficiency (NUE) in cereal grain production.  While the new strategy is expected to improve NUE, it will not likely be 100 %.  OSU research has shown NUE of 90 % when N was topdressed to correct deficiencies.  If the fertilizer-N in the grain (Figure 1) had been supplied by topdressing with 90 % NUE, the average fertilizer rate would have been only about 28 lb N/acre. Topdressing needs would be determined from sensor-based estimated yield potential and a response index (determined from the N-Rich Strip) for each field.


What are the estimated N rates using the new strategy?

            Using the new strategy, N would not have been applied even as a topdress application for nine years where the “field practice” had almost the same growth and development as the N-Rich Strip (RI less than 1.1).  We used the yield of the 20 lb N/acre rate each year to simulate a “farmer” field practice, rather than the zero N, since most farmer fields would have a history of annual N fertilization.  The average yield loss for the nine years when no N was applied was 1.4 bu/acre (0.4 bu/acre averaged over the 30-yr period).

            Assuming a minimum topdress rate of 15 lb N/acre, estimated amounts of topdress-N required for the highest fertilized yields would be slightly less than the amounts taken up in the grain (Figure 2).  The lower amounts are a result of using the 20 lb N treatment yield as the “unfertilized” farmer field.

Figure 2.  Estimated annual amounts of topdress N required for the yield potential, assuming the lowest rate that could be applied is 15 lb N/acre, grain will have 13 % protein, the yield increase is equal to the difference between maximum yield and the 20 lb N/acre treatment, and no fertilizer is applied when the RI is less than 1.1.


            The estimates in Figure 2 emphasize the extreme variability in year-to-year N required to support the yield potential for a particular field, and that if only the needed N is applied as a topdress the total amount of N required over time may be less than one-half that used by conventional preplant application (80 lb N/acre/year).


What is the estimated profitability of the new N management strategy?

            Depending on the assumptions used, the return from using N fertilizer varies, but is always quite good (Table 1).  Compared to no N being used, simply applying 80 lb N/acre as anhydrous ammonia (AA) each year provides a good return ($30.61/acre).  If 40 lb N had been applied as AA preplant and additional N applied as topdressing, when needed based on sensor readings to determine yield potential and the chance for a response using an N-Rich Strip, the projected return ($38.11) would be about $8/acre better.  If we use the 20 lb N/acre/yr yield as a “control” to better represent fields that have a history of N fertilization, response to N and return from either strategy is less, with the new strategy being about $11/acre better.  The reason for the improvement is that the new strategy minimizes the chance of over fertilization by only applying topdress N as needed.  Over fertilization is worsened when a flat rate is used (80 lb N/acre/yr) in fields that have a history of residual (20 lb N/acre) from last year.


Table 1.  Estimated annual net return to fertilizer-nitrogen


Compared to zero-N control (no N used)



Sensor-based topdress

Preplant-N (lb/acre)

80 (as AA)

40 (as AA)

Topdress-N (lb/acre)


19 (as UAN)

Return ($/acre)




Compared to 20 lb N control (fields with fertilizer history)

Preplant-N (lb/acre)

80 (as AA)*


Topdress-N (lb/acre)



Return ($/acre)



Net return calculated as the difference between value of wheat yield response at $3/bu, and the cost of fertilizing.  Preplant anhydrous ammonia (AA) charged at $0.15/lb N plus a $5/acre application cost plus 2.5 % interest on the investment (compared to delaying the investment until topdressing).  Topdress N charged at $0.25/lb plus a $2/acre application cost.


What about variable rate application?

Based on the average response from 10 field trials in 2002, sensing and treating every 4-square feet is expected to add about a $5/acre return from fertilizer use.


Oklahoma State University, in compliance with Title VI and Vii of the Civil Rights Act of 1964, Executive Order 11246 as amended, Title IX of the Education Amendments of 1972, Americans with Disabilities Act of 1990, and other federal laws and regulations, does not discriminate on the basis of race, color, national origin, sex, age, religion, disability or status as a veteran in any of its policies, practices or procedures.  This includes but is not limited to admissions, employment, financial aid, and educational services.


Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1913, in cooperation with the US Department of Agriculture, Sam E. Curl, Director of Oklahoma Cooperative Extension Service, Oklahoma State University, Stillwater, Oklahoma.  This publication is printed and issued by Oklahoma State University as authorized by the Dean of the Division of Agricultural Sciences and Natural Resources.