Illinois Fertilizer
Conference Proceedings
January 27-29, 2003
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E.C. Varsa, S.A. Ebelhar, T.D. Wyciskalla, and C.D. Hart1
Growers who have a history of yield mapping using GPS or GIS procedures can identify areas of fields that have high-, medium-, and low-yield productivity potentials. Generally, these productivity regions are somewhat similar year after year regardless of the favorableness of the growing season or the crop species being grown. With these somewhat repeating crop growth patterns, it would seem reasonable that fertilizer requirements to achieve these yield levels would also be different and could be a basis for variable-rate fertilizer application. However, most producers apply uniform rates of fertilizers to their soils even though known productivity differences occur.
Varying the rate of nitrogen (N) within a field to the soil's productivity potential should improve the overall crop N use efficiency. Conventional wisdom would suggest that soils with a potential for high yields should receive more N, and lower productivity soils in fields should receive a lesser amount of N because of lower yields. A number of previous reports (Carr et al., 1991; Kitchen et al., 1995; Redulla et al., 1996; Sawyer, 1994) have suggested this or comparable approaches to whole-field variable-rate fertilization.
With the advent of variable-rate technology (VRT) and recent advances in fertilizer equipment, application rates can be tailored and varied as one traverses the field. It is the objective of this research to determine if agronomic, economic, and environmental benefits can be obtained by varying N application rates across the field as differing productivity areas are encountered. This would be compared with the standard practice of uniform N application based upon the average yield for the whole field.
The research contained in this report is described in two parts. First, a whole-field
comparison of two variable N application methods will be made with a fixed (uniform)
rate of N application. A second phase of this research will evaluate response
of corn to rates of N (without and with nitrapyrin) within selected historical
low-, medium-, and highproductivity regions of the field as a small plot study.
Mr. Kelly Robertson, a farmer and crop consultant located in Franklin County, Illinois with at least five years of yield monitor data on fields of his farm, agreed to collaborate with us for year three of this study. For the 2002 cropping season, he had a 27-acre field (known as the HLP3 field) available for use in these studies that was in soybean in 2001 and which has been in a corn-soybean rotation for over 10 years. The dominant soil type in the field was Cisne silt loam, with lesser amounts of Hoyleton silt loam. Topography is quite flat, with a slope averaging from 0 to 2 percent. Adequate drainage is a major limitation in this field, and surface ditching is practiced where possible to remove excess water. Tile drainage is not practiced because of the restrictive claypan layer in the Cisne soil.
Figure 1 shows a map giving the outline of the field and the normalized yields broken down into three general productivity categories. The low-yield regions were identified as those with normalized yields that were 90 percent or less, medium-yield (or average-yield) regions of the field had normalized yields that ranged from 90 to 110 percent, and high-yield regions had normalized yield that exceeded 110 percent. The areas of the field that were in the above low-, medium-, and high-yield categories were approximately 29, 46, and 25 percent, respectively. The average corn yield for this field (excluding drought years) over the past 10 years (five crops of corn) has been about 160 bushels per acre.
Sixty points for sampling were GPSidentified in the gridded map of the field (each grid being 60 feet by 60 feet). The normalized yield and the number of sampling points in each yield category were as follows: <90% = 8 points; 90 to 110% = 30 points; and >110% = 22 points (see Figure 1). Soil samples (0 to 7 inches) consisting of five cores per sample for fertility measurements were collected at each geo-referenced point. Soil test averages for the field, based upon the 60 sampling points were: pH = 5.83 (range 5.08 to 6.59); Bray P1-P = 47 (range 34 to 70 lbs/ac); exchangeable K = 223 lb/ac (range 161 to 360 lb/ac); and organic matter = 1.51 percent (range 0.75 to 2.48 percent). These geo-referenced sampling points were also the locations for other crop and soil evaluations, including ear leaf N sampling at silking and post-harvest soil nitrate analysis to a 3-foot depth.
Strip comparisons of variable-rate N and uniform N application were based upon a field average yield of 160 bushels per acre. The uniform N application rate was calculated by multiplying yield x 1.2 less a soybean credit of 40 lb N per acre [(160 x 1.2) - 40 = 152 lb N per acre]. Two computational methods of variable rate N (VRN) were used as a comparison with the uniform rate of N. The variable rate application method used in 2000, referenced as VRN-Old, is the first method. It is based upon varying the N rate with the normalized, historical yields as they occur in the field. The formula for VRN-Old is: 1.2 x normalized (proven) yield within a cell less a soybean credit of 40 lb N/ac. This method essentially reduces N rates where proven, established yields are less than 100 percent and increases N rates when proven yields exceed 100 percent.
The second variable rate N method, referenced as VRN-New, essentially reverses the process of VRN-Old. That is, it increases N rates in lower productivity areas (less than 90 percent) and decreases N rates when normalized yields exceed 110 percent. The formula for VRN-New is: 1.2 x normalized (proven) yield - 40 lb N/ac soybean credit for 90 to 110 percent yield levels; 1.0 x normalized (proven) yield -40 lb N/ac soybean credit for proven yields greater than 110 percent; and 1.4 x normalized (proven) yield - 40 lb N/ac soybean credit for proven yields less than 90 percent.
A graph of the predicted rates of N application by the three methods is given in Figure 2. Variable application of N (as anhydrous ammonia) was accomplished with a controller on the applicator programmed in synchrony with a prescription map of soil productivity indexes (normalized yield map) on a computer in the tractor. Both uniform and variable-rate strips were 60 feet wide (24 rows) for the entire length of the field, excluding head lands. The tool bar was equipped with shanks spaced between each row, and the anhydrous ammonia was applied as a sidedressing to the corn at the five-leaf stage of development.
In a selected portion of the field, where normalized yields of <90% (low), 90 to 110% (average), and >110% (high) productivity were closely contiguous to each other, a small-plot, intensive N rate study was conducted within each productivity region. N rate treatments selected were equivalent to 0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 lb N per bushel expected yield, plus a zero-N check. Additionally, nitrapyrin as Stay-N 2000 was included with the applied N for the 0.8, 1.0, 1.2, 1.4, and 1.6 lb N per bushel application rate treatments. A summary of those treatments appears below.
| N Application Rate (lb N per acre) Normalized Yield Productivity |
||||
|---|---|---|---|---|
| Nitrogen Treatment (lb N per bu) |
Nitrapyrin | Low | Average | High |
| Check | = | 0 | 0 | 0 |
| 0.8 | = | 60 | 90 | 120 |
| 1.0 | = | 90 | 120 | 150 |
| 1.2 | = | 120 | 150 | 180 |
| 1.4 | = | 150 | 180 | 210 |
| 1.6 | = | 180 | 210 | 240 |
| 1.8 | = | 210 | 240 | 270 |
| 0.8 | ± | 60 | 90 | 120 |
| 1.0 | ± | 90 | 120 | 150 |
| 1.2 | ± | 120 | 150 | 180 |
| 1.4 | ± | 150 | 180 | 210 |
| 1.6 | ± | 180 | 210 | 240 |
All nitrogen treatments were replicated three times within a randomized complete
block arrangement in each of the productivity zones. Individual plot sizes were
15 feet (6 rows) wide by 30 feet long. The nitrogen source was 28% UAN solution
knifed in with an alternate rowspaced shank applicator. Application of N was
made at the five-leaf stage of development. Measurements taken included stand
counts, ear leaf N tissue at silking, and grain yield and moisture at maturity.
Table 1 shows additional experimental details concerning
dates, cropping information, and precipitation.
Uniform N versus Variable N Results - Whose Field Study
The 2002 growing season was a tale of two extremes. April and May rainfall was much above normal (Table 1), essentially prohibiting meaningful field work from being accomplished until the later part of May. The field for the experiment was planted on May 30. Following planting, moisture conditions were nearly ideal for emergence and activation of the herbicides. Stands were nearly perfect and weed control was excellent. Sidedressing of the N treatments was made in favorably moist soil conditions and at the ideal corn growth stage. Rains at the end of June replenished soil moisture to maximum water retention capacity. However, from July 1 to about August 15, less than one-half inch of rainfall was received. This extreme dry period took its toll on the crop and its potential for high yield. In the end, the field average yield was about 126 bu/ac.
The yield obtained using the two formulas of varying N rate were both less than those obtained from uniform N application. The average amount of nitrogen applied per acre for VRN-Old was about 7 lb N per acre more than uniform N, but the amount of N applied using VRN-New was 6 lb N per acre less than uniform N. However, the VRN-New formula resulted in approximately 8 bu/ac less yield being obtained. These results should be viewed with caution because of the unusual nature of the growing season.
Strip yields over the entire field for the N application methods were observed as follows:
| Strips | Acres | N Applied (lb N/ac) |
Yield (bu/ac) |
|---|---|---|---|
| Uniform N | 9.1 | 152 | 129.2 |
| VRN-Old | 9.1 | 159 | 127.6 |
| VRN-New | 9.1 | 146 | 121.0 |
Intensive N Rate Studies on Small Plots in Soils of Varying Levels of Productivity
Corn yield response to nitrogen rates and nitrapyrin were different in the small plots on soils of low-, medium-, and high-productivity levels, respectively (Figures 3, 4, and 5). In 2002, the yields obtained in low-productivity soils remained low (average yield = 97 bu/ac). However, corn yields of plots on highproductivity soils averaged 122 bu/ac, while plots of medium-productivity soil averaged 131 bu/ac. These differences are shown graphically in Figure 6. The poorer yields in the high-productivity soils of these small plots are probably related to reduced amounts of plant-available water in these soils. The location of the high-productivity plots was on the more sloping, better drained Hoyleton silt loam soils. During seasons of normal or above normal rainfall, better yields would normally be expected on these soils because excess water is not a problem to reduce plant growth. However, these same wet seasons would greatly reduce yields in poorly drained soils (such as the Cisne series), which is the reason for the reduced productivity potential for this field. This season, more favorable moisture for corn was likely in the Cisne silt loam soils.
Nitrapyrin had no impact on yields in 2002, regardless of the soil productivity
potential (Figures 3, 4,
and 5). The dry weather conditions soon after N sidedressing
created a soil environment that did not favor N losses. Hence, only minor, non-significant
differences were observed. Despite the lack of yield response to nitrapyrin
in 2002 (Figure 7), response to nitrapyrin was more
evident in 2000 and 2001 experiments. Nitrogen use efficiency by corn was improved
by nitrapyrin addition to N in all three years of this experiment (Figure
8).
Figure 2. Predicted N to apply at Franklin County, Illinois, 2002
Table 2. Soil test measurements at the 60 sampling points, Franklin County, Illinois, 2002 1E.C. Varsa is a professor and T.D. Wyciskalla is a researcher, Department of Plant, Soil and General Agriculture, Southern Illinois University, Carbondale, IL; S.A. Ebelhar is an agronomist and C.D. Hart is a visiting research specialist, Department of Crop Sciences, University of Illinois, Dixon Springs Agricultural Center, Simpson, IL.
Carr, P.M., G.R. Carlson, J.S. Jacobsen, G.A. Nielsen, and E.O. Scogley. 1991.
Farming soils, not fields: A strategy for increasing fertilizer profitability.
J. Prod. Agric. 4: 57-61.
Kitchen, N.R., K.A. Sudduth, and S.J. Birrell. 1995. Comparison of variable
rate nitrogen fertilizer application: Corn production and residual soil NO3-N.
In Site-Specific Management for Agricultural Systems. ASA-CSSA-SSSA. Madison,
WI.
Redulla, C.A., J.L. Havlin, G.L. Kluitenberg, N. Zhang, and M.D. Shrock. 1996. Variable nitrogen management for improving groundwater quality. pp. 1101-1110. In P.C. Robert et al. (ed.), Proceedings of the Third International Conference on Precision Agriculture. Minneapolis, MN. June 23-26, 1996. ASA, CSSA, and SSSA, Madison, WI.
Sawyer, J.E. 1994. Concepts of variable rate technology with considerations
for fertilizer application. J. Prod. Agric. 7: 195-206.
