Illinois Fertilizer Conference Proceedings January 27-29, 1997 Main Index 1997 Index Search

On-Farm Evaluation of Variable-Rate Nitrogen Fertilizer Responses Using Farmer-Owned Equipment

P.D. Harrington, E.D. Nafziger, and R.G. Hoeft ¹

Introduction

The introduction of variable rate application technology and grain yield monitors has greatly increased farmer interest in varying input rates to match crop productivity. The use of yield monitors allows producers the opportunity to evaluate the effects of varying input rates on grain yield and ultimately profit. In order to fully utilize these technologies, recommendations from small plot research needs to be adapted to a field scale basis (Vetsch et al. 1994). The use of on-farm trials and proper statistical analysis is one way to accomplish this goal.

There have been a number of publications dealing with variable application of P and K (Wollenhaupt et al., 1994). This technology of variably applying nutrients has been adopted in several areas of the state. Nitrogen, on the other hand is most often applied at a constant rate rather than on a site specific basis, even though the controllers needed for variable-rate N application is becoming more widely available. The main reason for this slow adoption is the difficulty in quantifying the optimum nitrogen rate for a particular part of a field. Malzer et al. (1994) found that N responses varied considerably across a field. Huggins et al (1994) found that N rates required for maximum yield varied from 80 to 160 lb N/acre. The use of yield monitoring as a predictive tool for expected yields might improve our ability to apply the correct rate of nitrogen fertilizer (Kitchen et al., 1994), but a rational basis for rate adjustments must be developed.

This is the second year of a three year study designed to evaluate the potential for site specific application of nitrogen fertilizer in different parts of variable fields.

Materials and Methods

Names and locations of the eight farmer cooperators who participated in the 1996 study are listed below. Six different nitrogen rates in increments of 30 lb N/acre were applied in field length strips arranged in an RCBD with 2 or 3 replications. The total amount of N applied differed somewhat among the locations due to differing fertility programs that included varying amounts of spring-applied N.

The strips were harvested as individual loads on the yield monitor and data were collected in 1-second increments. Yield data were processed with Ag Link 3.51 and Arc View 3.0 GIS software. The pass editor features of Ag Link were used to realign GPS flyers whose pass could be identified by load number. The data were then imported and analyzed in Arc View 3.0 using Mark II Agronomy's Arc View Project, Green Plan Analyst.

For purposes of analysis, fields were segmented into grids 100 feet long by the total width of the six rate strips within a single rep. This was simply a systematic way of delineating areas of the field to determine if N response differed in different parts of the field. Among the different locations, there were thus a total of 6 to 20 grids long by 2 to 3 grids wide, depending on the length of the field and whether there were two or three reps. Regression curves were fitted to the data within each grid, with the quadratic-plateau model used where appropriate. Optimum N rates were calculated from the regression analysis using the assumptions of $2.50 bu of com and nitrogen cost of $.20/lb ($328/ton for anhydrous ammonia).

Results and Discussion

All of the trials were planted in a timely fashion in 1996. The cool and wet conditions in May caused some slow emergence problems, but stands were generally good. Yield data were collected in the fall at all eight of the sites.

The Reifsteck site in Champaign County is used as an example of how the data are being analyzed. Figure la shows the yield data with the treatments overlaid. The lightest colored areas on the map., which represent the lowest yielding areas, correspond somewhat to the lower nitrogen rates. Figure lb is corrected for N rate differences by adjusting the yield of each data point using the overall quadratic regression curve in Figure 2. This map thus shows the yield variation had the optimum nitrogen rate been applied to the entire plot area. This adjusted map will be used to delineate high and low yielding areas of the field, and to determine whether yield level corresponds to optimum N rate within the field.

Table 1 shows the yield data within each of the grids. The actual data points, the curve fitted to these points, and the N rate at which the yield reached a plateau is shown for grids 1 to 3 (Figure 3a) and for grids 4 to 6 (Figure 3b), moving across the field. The optimum N rates ranged from 65 to 180 lb N/acre among the different grids (Table 2). The second rep of this experiment shows a rather unexpected response. The plateau response seen in the other two reps occurred within the range of the applied rates in only one grid (Grid 5). The curve for grids 1, 2, 3, and 4 appears to have been affected by lower than expected yields at the 150 lb N rate in these grids. No reason is known for this response.

Tables and Figures

Table 1: Yields, bu/acre within the six grids

Table 2: Optimum N rates (lb N/acre) within the grids shown in Figure 3

Table 3: Optimum grain yields (bu/ac) within the grids shown in Figure 3

Figure 1a and 1b

Figure 2. N response as assessed by the entire trial. Reifsteck location, 1996

Figure 3a. Nitrogen response, Grids 1 to 3 (from west to east).

Figure 3b. Nitrogen response, Grids 4 to 6 (from west to east)

Footnotes and References

¹ P.D. Harrington is Graduate Research Assistant, Dept. of Crop Sciences; and E.D. Nafziger and R.G. Hoeft are Professors, Dept. of Crop Sciences, Univ. of Illinois.

Huggins D.R. and R.D. Alderfer. 1994. Yield variability within long-term management study: implications for precision farming. Proceedings of Site Specific Management For Agricultural Systems. Minneapolis, MN. March 27-30, 1994. 417-426.

Kitchen N.R., D.F. Hughes, K.A. Sudduth, and S.J. Birrell. 1994. Comparison of variable rate to single rate nitrogen fertilizer application: corn production and residual N03-N. ibid. pp. 427-442.

Malzer, G.L.,T.J. Graff, P.C. Robert, D.R. Huggins, D.J. Fuchs, W.H. Thompson, G.L. Gaalaas, and T.W. Bruulsema. 1994. Site specific crop management: N-response by soil condition at Hildreth site 1993. Field Research in Soil Science. 1994, Miscellaneous Publication 83-1994, Minnesota Agricultural Experiment Station, University of Minnesota, St. Paul.

Vetsch J.A., G.L. Malzer, P.C. Robert, and D.R. Huggins. 1994. Nitrogen specific management by soil condition: managing fertilizer nitrogen in corn. Proceedings of Site Specific Management for Agricultural Systems. Minneapolis, MN. March 27-30 1994. 465-473.

Wollenhaupt, N.C., R.P. Wolkowski, and M.K. Clayton. 1994. Mapping soil test phosphorus and potassium for variable-rate fertilizer application. J. Prod. Agric. 7:441-448.

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Department of Crop Sciences University of Illinois Extension College of Agricultural, Consumer, and Environmental Sciences University of Illinois at Urbana-Champaign Illinois Fertilizer & Chemical Association Illinois Department of Agriculture

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