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© 2005 Plant Management Network.
Accepted for publication 25 May 2005. Published 14 July 2005.

Value of More Uniform Nitrogen Application Across the Toolbar

Bruce J. Erickson, Visiting Assistant Professor, Fulgence J. Mishili, Graduate Research Assistant, and Jess M. Lowenberg-DeBoer, Professor, Department of Agricultural Economics, Purdue University, West Lafayette, IN 47907

Corresponding author: Bruce Erickson. berickso@purdue.edu

Erickson, B. J., Mishili, F. J., and Lowenberg-DeBoer, J. M. 2005. Value of more uniform nitrogen application across the toolbar. Online. Crop Management doi:10.1094/CM-2005-0714-01-RS.

Abstract

Variability of nitrogen (N) application rates with conventional anhydrous ammonia equipment is quite high. Technology is on the market to reduce variability of anhydrous ammonia application across the toolbar. A series of analyses were conducted to test the consequences of anhydrous ammonia application variability on both corn yields and the economics of investing in equipment to reduce that variability. In individual years and for specific locations, greater N application uniformity can have a yield advantage and/or promote more efficient use of N. Over several crop seasons, the yield gains to greater N uniformity may be less than 1 bu/acre. At current prices economic analysis indicates that this modest yield gain may justify an investment in equipment which would reduce the application rate variability by about half. However, more expensive equipment which provides an almost uniform application rate is not justifiable on benefits of N uniformity alone.

Variability in Nitrogen Applications

As the application of inputs for crop production becomes more precise, many corn (Zea mays L.) growers continue to apply one of their most important crop inputs, anhydrous ammonia, in a very inexact way. Anhydrous ammonia is in a complex state of continuous physical change as it is being regulated and applied (6), which leads to variability in its application.

Soil differences within a field (12), difficulties in regulating anhydrous ammonia (2,3,5,7,9,11,12,16,18), and knife-to-knife differences (2) each have contributed to variability in anhydrous ammonia applications. Knife-to-knife variability in anhydrous ammonia application is often the result of the manifold performance in delivering anhydrous ammonia to each knife. Boyd et al. (2) measured outlet differences from 4 to 16% on average from the mean, depending on the rate of nitrogen and the type of manifold used. At a 150 lb/acre rate, the conventional manifold had a standard deviation of about 25 lb/acre, and the N rate at the highest output outlet was about twice the rate at the lowest output outlet. Other studies have shown up to three-fold or four-fold differences among outlets (5,9).

Equipment manufacturers are claiming that new types of manifolds improve application uniformity. With nitrogen prices elevating in recent years (15) and the relationships among nitrogen rates and nitrate concentrations in ground water (10), there is intense interest in ensuring that nitrogen is being applied more precisely. The goals of this research were to test the yield and economic consequences of nitrogen application variability across the toolbar, and the practicality of investing in equipment to reduce application variability.

In addition to claims of more N rate uniformity across the toolbar, manufacturers have claimed that equipment providing reduced variability in anhydrous ammonia application will allow greater precision in overall application rates, applications at lower temperatures, or will result in more uniform applications from one portion of a field to another. These factors were not considered in the analysis, nor were any considerations made for equipment that might be worn, damaged, or incorrectly adjusted.

Effects of Non-Uniform Ammonia Applications on Yield

Variations in anhydrous ammonia rate applications likely will cause a corresponding variation in corn response, based on yield response curves generated from nitrogen rate studies in Illinois (Fig. 1). The Illinois yield responses follow a diminishing returns quadratic plus plateau function (4,14) and the data represented in Figure 1 (8,9,14; E. Nafziger, personal communication) were the basis of this analysis.

Fig. 1. Nitrogen response curves for two crop rotations based on field studies in Illinois over 34 site-years, 1999-2003.

To test the yield and economic ramifications of anhydrous ammonia variability across the toolbar, a series of analyses were conducted in an Excel spreadsheet (Microsoft, Redmond, WA) that simulated corn yield response to the levels of nitrogen rate application variability as reported by Boyd et al. (2) and knife spacings across the toolbar. Three levels of variability of nitrogen rate were simulated: a high variability (standard deviation 25 lb/acre) set designed to mimic the application pattern when using a conventional anhydrous ammonia manifold; a medium variability (standard deviation 9 lb/acre) set designed to mimic the application pattern when using some of the improved manifolds used by Boyd et al.; and a low variability set that mimics a truly uniform application. Numbers representing the rate of nitrogen applied for knives across a 7-knife toolbar were randomly generated in a normal distribution based on the variation and nitrogen rate, then replicated 499 times. A sub-sample of the simulation results, including the mean and standard deviation of all 500 iterations for one target N rate/standard deviation level, are shown in Table 1.

Table 1. Example normal distributions of ammonia applications, including the mean and standard deviation of all 500 iterations for an applicator applying 150 lb/acre with a standard deviation of 25 lb/acre.

Corn roots extend outward about three feet in all directions (17), so it was assumed that corn in 30-inch row spacings would on average draw from two fertilizer bands when applied with knives 30 inches apart. Some growers use 60-inch knife spacings to minimize soil disturbance and decrease horsepower requirements. For the 60-inch comparisons, each corn row was assumed to draw from only one fertilizer band, thus the effect of variability would be magnified. In a side-dress situation these row/knife spacing relationships would be nearly exact, and as an average would also be relative in a late fall or spring pre-plant situation where ammonia is often applied at an angle to crop rows. With narrower spacings (e.g., 15-inch) knife-to-knife variability would be less important because each corn plant would have access to N from three or four knives, so this scenario was not considered. With nearly uniform applications in the low variability situations, the amount of fertilizer nitrogen accessible to plants shouldn’t differ between 60-inch and 30-inch knife spacings. No consideration was made for variability among various areas of fields (12), only for variability across the toolbar. In addition, no other factors were considered, such as the reported claims of applying anhydrous ammonia during cooler soil temperatures with the newer manifolds.

Average yields resulting from various combinations of nitrogen rates and crop rotations (Tables 2 and 3) are based on the Illinois response curves (Fig. 1). The long-term (5 or more years) average yield gain from greater N uniformity across the toolbar is modest. For a corn-soybean rotation with 30-inch knife spacing the average yield gain from moving from a high variability system to medium variability equipment is only 0.5 bu/acre at the recommended N rate. Low variability application equipment adds 0.6 bu/acre to corn in a corn-soybean rotation. The yield gain from N uniformity across the toolbar under environmental conditions favoring N response and in a row which was under-applied would be greater than 1 bu/acre. For example, with a target application of 150 lb/acre, an anhydrous ammonia applicator with high variability (standard deviation of 25 lb/acre) could occasionally result in individual row N rates below 110 lb/acre (Table 1), with an expected yield reduction of 1 to 2 bu/acre compared to the long-term average yield with the target N rate (Fig. 1).

Table 2. Estimated long-term average yields for corn following soybeans by nitrogen rate, anhydrous ammonia knife spacing, and level of variability across the toolbar.

 ^a High variability = standard deviation 25 lb/acre; medium variability = standard deviation 9 lb/acre; low variability = standard deviation 0 lb/acre.

Table 3. Estimated long-term average yields for corn following corn by nitrogen rate, anhydrous ammonia knife spacing, and level of variability across the toolbar.

 ^a High variability = standard deviation 25 lb/acre; medium variability = standard deviation 9 lb/acre; low variability = standard deviation 0 lb/acre.

Economic Consequences of Non-Uniform Ammonia Applications

Nitrogen recommendations based on response curves place the economic optimum rate of N at that point where the last pound of N is just paid for by the yield increase from that N (3,7). Growers often exceed economic optimums, due to uncertainty about estimating yield goals, nitrogen soil concentrations, or the weather (1), but economic optimums served as the basis for the analysis.

Utilizing the yield information generated in Tables 2 and 3, a set of economic analyses were conducted to determine the relative returns of application equipment options, rates of nitrogen, and ammonia knife spacings. The base analysis was calculated using the following as standards (13):

Variable costs assume nutrients removed by the crop are replaced at a cost of $0.28/lb of P₂O₅, $0.14/lb of K₂O, $16/ton of lime. In addition, hauling is charged at $0.20/bu and drying at $0.25/bu (12).

The results of this analysis are presented in Tables 4 and 5. In each table, all combinations are compared to a conventional, high variability manifold in 30-inch spacings. For corn following soybeans (Table 4), the 140 lb/acre rate of N was closest to the recommended rate (9), so this was chosen as the basis for comparison. For corn following corn (Table 5), 170 lb/acre was closest to the recommended rate (9), and was chosen as the basis for those comparisons.

Table 4. Estimated long-term differences in average income for corn following soybeans by nitrogen rate, anhydrous ammonia knife spacing, and level of variability across the toolbar.

 ^a High variability = standard deviation 25 lb/acre; medium variability = standard deviation 9 lb/acre; low variability = standard deviation 0 lb/acre.

Table 5. Estimated long-term differences in average income for corn following corn by nitrogen rate, anhydrous ammonia knife spacing, and level of variability across the toolbar.

 ^a High variability = standard deviation 25 lb/acre; medium variability = standard deviation 9 lb/acre; low variability = standard deviation 0 lb/acre.

Application equipment resulting in medium variability will provide an economic advantage for corn following soybeans (Table 4). However, low variability equipment with an assumed cost of $12,000 is not justified (Table 4). The combination of investing in medium-variability equipment and cutting nitrogen rates, with the economic benefits coming both from uniformity and lower N cost is warranted in the corn-soybean rotation. In corn after corn there is an advantage for moving to a medium level of application uniformity, but the advantages of using medium uniformity equipment in combination with reduced N rates are smaller, due to reduction in corn yields associated with the steeper slope of the corn after corn response curve.

In both corn following soybeans and corn following corn there were economic penalties for utilizing 60-inch knife spacings, but the penalty was far less at the medium level of application variability, and it should make no difference at low application variability.

A set of sensitivity analyses (Tables 6 and 7) were conducted using the yield information from Tables 2 and 3, to test the consequences of changing nitrogen costs and grain prices, and the effects of equipment cost and farm size. For corn following soybeans in Table 6, all values compare against 140 lb/acre N rate applied with a conventional manifold in 30-inch spacings, $2.00/bu grain, 1000 acres, $12,000 low variability applicator and $1000 medium variability applicator. For corn following corn in Table 7, all compare against 170 lb/acre N rate applied with a conventional manifold in 30-inch spacings, $2.00/bu grain, 1000 acres, $12,000 low variability applicator and $1000 medium variability applicator. As expected, higher grain prices, larger farming operations, and less expensive equipment favor the equipment to lower the variability of N rates across the toolbar.

Table 6. Estimated long-term differences in average income for corn following soybeans for differences among grain price, number of acres, and costs of applicators.

Table 7. Estimated long-term differences in average income for corn following corn for differences among grain price, number of acres, and costs of applicators.

Conclusions

This analysis used long-term nitrogen response curves from Illinois to test the economic benefit of greater uniformity in nitrogen application. The analysis shows that over many cropping seasons, the yield gains to greater N uniformity may be quite modest (i.e., a gain of less than one bushel per acre for a corn-soybean rotation). At prices used in this study, economic analysis indicates that this modest yield gain may justify an investment in an improved manifold which would reduce the application rate variability by about half, but more expensive equipment which provides an almost uniform application may be difficult to justify on benefits of N uniformity alone. Factors such as fertilizer costs, grain prices and uniformity of application across soil types would affect a final decision on purchasing and using lower variability anhydrous ammonia application equipment.

Literature Cited

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