G.V. Johnson and W.R. Raun


Department of Plant & Soil Sciences

Division of Agricultural Sciences & Natural Resources

PT 2001-18                                                         July 2001                                                  Vol. 13, No. 18


What is a Response Index?

A response index is a number which will identify the amount of yield response to expect from an application of fertilizer-nitrogen and that may range from 1 to as high as 3 or 4.  For example, for a field with a response index of 1.5 one could expect to obtain a 50% increase in yield from the application of fertilizer-nitrogen (a yield of 60 bushels with added fertilizer compared to 40 bushels without fertilizer).


Why is it important to know the Response Index for a field?

Review of grain yield results from OSU long-term (30-yr) nitrogen trials showed that the yield of wheat when no fertilizer was applied varied greatly, and was unpredictable (Fig 1a and Fig 1b).


C.V. = 31



Some years there was less than a 20% increase in yield from fertilization, other years there was as much as a 300% increase, and there was no pattern in the changes over time (Fig. 2).  The yields of fertilized plots were also variable over time, but less so (CV= 21%) than the yields observed for unfertilized plots (CV = 31%) for the 31 year period.


So, how much topdress fertilizer is really needed?

The answer to this question will vary greatly from year to year depending on what the yield maximum is for the year, and how much of the nitrogen required for the yield maximum will be provided from non-fertilizer nitrogen (mineralized soil N).  First, we calculate the difference in yield between the maximum yielding fertilized plots and yield of control (unfertilized) plots.  Then, by multiplying that yield increase from fertilizer times the 2 lb N required to produce a bushel of wheat, we obtain the amount of nitrogen that should have been applied for maximum yield each year.  As illustrated in Fig 3., that amount varies from year to year even more (CV= 62) than the control yield or the maximum fertilized yield.


This variation in fertilizer need from year to year is directly related to the variation in response index from year to year (Fig. 4), and is the reason why estimating a response index is so critical to identifying the correct N fertilizer rate.

How is a Response Index for a field calculated?

The first response indexes were calculated by dividing the yield from unfertilized research plots into the best yielding fertilized plot for a given year.  Recent research has shown that a reliable response index can be calculated by comparing the forage growth of fertilized and unfertilized areas of wheat just before it is time to consider topdressing nitrogen.


The in-season calculation is made by measuring some crop characteristic that responds to available nitrogen, such as plant height, and dividing the number obtained from the fertilized area by the number obtained from the unfertilized area.


But how will farmers have both fertilized and unfertilized areas in a field?

Through cooperation with their fertilizer dealer, farmers should arrange to fertilize (preplant) a strip the length of the field with a rate that is twice the normal preplant nitrogen rate (2X-STRIP).  If they don’t plan to put down any preplant nitrogen then they should arrange for a strip applied at a rate that is 1½ times more than the total applied to the field last year (or is normally applied to the field).  If they have never applied nitrogen to the field, they should apply a strip at a rate of 80 lb N/acre.  As a general guide, the preplant nitrogen rate should be only half the total amount of nitrogen the crop (grazing, grazing plus grain, or grain alone) is expected to need in a high yielding year.


How can a farmer use the Response Index once it is known?

Once the response index is calculated, it can then be multiplied times the preplant rate used for the entire field.  This value is an estimate of the total nitrogen the crop can use.  By subtracting the rate applied preplant from the total the crop can use, the difference, after adjusting for an uptake efficiency of 70%, results in the final topdress rate.  In some years, it is likely that the uptake efficiency will be less than 70% for topdress applications of N.  Here is an example:


Preplant rate for entire field = 40 lb N/acre

2X-STRIP = 80 lb/acre.


Crop height at X-rate = 12”

Crop height at 2X-rate = 20”


Response Index = 20/12 = 1.6


Crop nitrogen need = 1.6 x 40 = 64 lb N/acre

Topdress need = (crop need – preplant application)/0.70

Topdress need = (64 – 40) = 24/0.70 = 34 lb N/acre


So, what’s in it for the farmer?

We looked at the yield results from applying 80 lb N/acre each year (typical for farmers that average 40 bu/acre yields) and compared those yields to the highest yields (at any rate of N), and the rate of N required for maximum yields.  In some cases there was a yield increase from a higher rate of N (yield loss at 80 lb rate), and in some cases there was a higher yield from a lower rate of N (wasted N and yield loss at 80 lb rate).  These losses were calculated for each year, using a wheat value of $3/bushel and N cost of $0.25/lb (Fig. 5).


$3/bu wht; $0.25/lb N



How do farmers ‘cash in’ on this new technology?

Most farmers won’t be able to benefit from this technology without the services and help of their local fertilizer dealer.  By servicing a large number of fields, fertilizer dealers and crop consultants will become the local experts.  Dealers will be responsible for applying the 2x strips that will ultimately serve as the foundation for increasing the payback from nitrogen inputs through topdressing, also provided by the dealer.

















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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.