Value of More Uniform Nitrogen Application Across the Toolbar
Bruce Erickson, Fulgence Mishili, and Jess Lowenberg-DeBoer
While the application of some crop inputs is becoming more precise, many corn growers continue to apply one of their most important crop inputs in a very inexact way. With conventional technology, anhydrous ammonia application is difficult to control and application may vary widely from knife-to-knife and one part of a field to another. The goals of this research were to test the economic consequences of nitrogen application variability across the toolbar, and the profitability of investing in equipment to reduce application variability.
Anhydrous ammonia is often the nitrogen (N) choice for corn producers since it is relatively less expensive per unit of actual nutrient compared to other nitrogen fertilizers. And although it has some unique human safety issues, ammonia is held tightly to soil particles upon application to the soil, putting it in a favorable view from an environmental loss standpoint. With a boiling temperature of -28°F, anhydrous ammonia is necessarily stored and transported as a pressurized liquid. As it is applied, ammonia moves by its own pressure from the tank out through metering devices and a distribution manifold that allocates the ammonia among hoses leading to row outlets. Ammonia begins to change to a gas as its pressure decreases, occupying a far greater volume. Since the metering and distribution is by volume but nutrient amounts are by weight, the actual amount of nitrogen applied can vary dramatically.
The difficulties of ammonia regulation can result in significant application variability. Nebraska on-farm measurements show an average of a 16 percent variation among different parts of fields Other research has shown a high degree of knife-to-knife distribution variability. Testing at Iowa State University has shown row outlet differences from 4 to 16 percent on average from the mean, depending on the rate of nitrogen and the type of manifold used. At a 150 lb/A rate using the conventional manifold, the highest output outlet was putting out about twice the amount of N of the lowest output outlet. Other studies have shown up to four-fold differences among outlets. To compensate for variability some growers use higher N rates, so that even the lower rate areas receive enough N for optimal yields.
Equipment manufacturers are claiming that new types of equipment can reduce variability. With nitrogen prices increasing and intense scrutiny of the link between N fertilizer and nitrates in ground and surface water, there is strong interest in ensuring that nitrogen is being applied more precisely. When N was less expensive, higher N rates to compensate for variability made some economic sense, even if it was environmentally risky. With higher N prices more accurate application can help achieve both economic and environmental objectives.
Effects of Non-Uniform Ammonia Applications on Yield
Anhydrous ammonia applicators that do not apply nitrogen uniformly leave an uneven pattern in a field, which should cause a corresponding variation in corn response. Nitrogen responses can be unpredictable and influenced by many factors, but rate response curves remain a foundation of nitrogen recommendations. University of Illinois response functions served as the basis of this analysis (Figure 1).
To test the ramifications of nitrogen variability, a series of analyses were conducted that simulated corn yields in response to the levels of variability across the toolbar. Three levels of variability of nitrogen rate were simulated in a spreadsheet—a high variability set designed to mimic the application pattern when using a conventional ammonia manifold, a medium variability set that reduces the variability by about half, and a low variability set that mimics a nearly uniform application. Numbers representing the rate of nitrogen applied for knives across a 7-knife toolbar were randomly generated based on the variation and nitrogen rate.
Corn roots extend outward about three feet in all directions, so it was assumed that corn in 30-inch rows would draw as an average from two fertilizer bands. No factor was added for variability among various areas of fields, only for variability across the toolbar.
Average yields resulting from various combinations of nitrogen rates and crop histories (Tables 1 and 2) are based on the Illinois response curves. The long term average yield gain from greater N uniformity across the toolbar is modest. For a corn-soybean rotation the average yield gain from moving from a conventional high variability system to medium variability equipment is only around 0.3 bu/A at the economic optimum N rate for the Illinois response curves of 140 lb/A, and going to the low variability application equipment adds 0.1 bu/A, for a total gain of 0.4 bu/A with the low variability technology. These responses are low since the yield response curves are relatively flat near the economic optimums—changing the N rate has a small affect on yield. The gains are slightly more at lower N rates and for the continuous corn because the N response is greater.
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. Utilizing the yield information generated in Tables 2 and 3, economic analyses were conducted to determine the relative returns of equipment/variability options, rates of nitrogen, and ammonia knife spacings. The base analysis was calculated using prices from the Purdue Crop Cost and Return Guide (http://www.agecon.purdue.edu/extension/pubs/crop_guide_04.pdf):
- Cost of anhydrous ammonia $0.24/lb
- Price of corn $2.00/bu
- Corn acres 1000 acres
- Cost of manifold to achieve medium variability $1000.00
- Cost of manifold to achieve low variability $12,000.00
- Equipment life expectancy 10 years
- Depreciation and Interest 10%
- Variable cost per bushel for corn $0.63/bu
Variable costs assume nutrients removed by the crop are replaced at a cost of $0.28/lb P2O5 , $0.14/lb K2O, $16/T lime. In addition, hauling is charged at $0.20/bu and drying at $0.25/bu. The results of this analysis are presented in Tables 3 and 4. In each table, all combinations are compared to a conventional, high variability manifold. For corn following soybeans (Table 3), the 140 lb/A rate of N was closest to the economic optimum with conventional equipment, so this was chosen as the basis for comparison. For corn following corn (Table 4), 170 lb/A was the nitrogen rate closest to the economic optimum, and was chosen as the basis for those comparisons.
Note in Table 3 for corn following soybeans that it is economically advantageous to utilize equipment that applies ammonia somewhat more uniformly, but the cost of going to a low variability situation is prohibitive if the equipment costs $12,000. Even more advantageous is the combination of investing in medium-variability equipment to minimize low application areas and reducing nitrogen rates, with the economic benefits coming both from uniformity and lower N cost. With more uniform application higher N rates to compensate for variability is no longer the most profitable option. In corn after corn there is an advantage for moving to a medium level of application uniformity, but it is not profitable to use medium uniformity equipment and cut N rates, due to the steeper slope of the corn after corn response curve. No accounting was made for some of the other advantages claimed by manufacturers of precision application equipment, such as an ability to apply ammonia at low temperatures, or more even application from one portion of a field to another from better regulation ahead of the distribution manifold.
A set of sensitivity analyses were conducted to test the consequences of changing nitrogen costs and grain prices, and the effects of equipment cost and farm size. As expected, higher grain prices, larger farming operations, and less expensive equipment favor the investment in equipment to lower the variability of N rates across the toolbar.
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 in the long run over many crop seasons, the yield gains to greater N uniformity may be quite modest. For example, the analyses suggest a long-run gain of less than a half bushel per acre for a corn-soybean rotation. At current prices 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. The right decision for a specific farming operation depends on grain prices, fertilizer cost, site-specific N responses and other factors.
For More Information
Boyd, P.M., Hanna, H.M., Baker, J.L., and Colvin, T.S. 2004. Field evaluation of anhydrous ammonia manifold performance. Appl. Eng. Agric. 20(6):745-756. (http://asae.frymulti.com/request.asp?JID=3&AID=17720&CID=aeaj2004&v=20&i=6&T=2)
Kranz, W., Shapiro, C., and Grisso, R. 1994. Calibrating anhydrous ammonia applicators. EC94-737D. University of Nebraska Extension, Lincoln, NE. (http://ianrpubs.unl.edu/farmpower/ec737.htm)
Miller, W.A., and Dobbins, C.L. 2004. Purdue crop cost and return guide: Estimated per acre crop budgets. ID-166W (Rev.). Department of Agricultural Economics, Purdue University, West Lafayette, IN. (http://www.agecon.purdue.edu/extension/pubs/crop_guide_04.pdf)
Nafziger, E.D., Sawyer, J.E., and Hoeft, R.G. Formulating N Recommendations for Corn in the Corn Belt Using Recent Data. Presented at the North Central Extension-Industry Soil Fertility Conference, Des Moines, IA, Nov. 17-18, 2004. (http://extension.agron.iastate.edu/soilfertility/info/nrecNCEISFC-04.pdf)
Bruce Erickson is a Visiting Assistant Professor; Fulgence Mishili is a Graduate Student and Jess Lowenberg-DeBoer is a Professor in the Department of Agricultural Economics at Purdue University.
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