Validating N rates for corn on farm fields in southern Minnesota
Recommendations to optimize profits and protect water quality
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This publication describes the results of 29 site-years of research on farm fields in southern Minnesota to find the best rate of nitrogen (N) to apply for corn after soybeans. Results show that the N rate recommended by the University achieves optimum corn yield and may be greater than needed for maximum profitability in many fields.
Figure 1. Impact of N rate on crop yield and nitrate loss from a corn production system.
More N is added for corn than any other nutrient in Minnesota. The right amount of N optimizes crop yield while minimizing loss of N to the environment. The wrong amount eats into profits and can send excess nitrogen into ground water. Figure 1 illustrates the significant impact the N rate has on crop yield, as well as its potential loss to ground water.
Finding the right amount of fertilizer N to add for a crop means first estimating how much N is available from the soil and then adding fertilizer N to meet the crop's total N need. Because uncontrollable factors like precipitation and temperature affect the release of N from soil as well as the amount of N needed by a crop, the optimum amount of fertilizer N can change from location to location and from year to year. Continual studies measuring corn yield response to fertilizer N take these variations into account.
University of Minnesota soil scientists have conducted hundreds of field studies to find the best rate of fertilizer N for corn following soybeans since the 1960s. The N rate recommendations are updated regularly using the most recent corn hybrids with greater yield potential. The latest N rate recommendations for corn following soybeans are shown in Table 1.
Table 1. N recommendations for corn after soybeans where a soil nitrate test is not used*.
|Expected yield (bu/A)|
|Organic matter level**||100-124||125-149||150-174||175-199||200+|
|-------------------------------------------------- N to apply (lb/A) --------------------------------------------------|
|M and H||60||90||120||140||160|
|* Adapted from Fertilizing Corn in Minnesota, 03790.
**L = Low, less than 3.0%; M = Medium and H = High, more than 3.0%.
In recent years the University's N recommendations for corn have been questioned by the fertilizer industry and by some farmers and advisors. They considered the recommendations lower than appropriate, with one reason being that many of the field studies were done on University Experiment Station land. It was time to move the N rate studies to farm fields.
From 1989 through 2001, University recommendations for corn after soybeans were tested in fourteen small-plot and fifteen field-size strip experiments on farmers' fields. Data from the field-size strip trials were collected by farmers and crop advisors (see Acknowledgements section). Yield data indicated that the N rate recommended by the University produced the expected results. In fact, the recommended N rates were higher than was needed at most of the sites to reach an economic optimum. Best management practices cited later in this publication were used at all sites and are essential to efficient N use.
The fourteen small-plot studies carried out from 1989 through 1999 were equally divided between the loess soils (silt loam) of southeastern Minnesota and the glacial till soils (clay loam) of south-central Minnesota (Table 2). The small plots were 10 to 15 ft wide and 40 to 60 ft long and were replicated four to six times at each site. The farmers tilled, planted, applied pesticide, cultivated, and selected hybrid and planting rates. University scientists applied the fertilizer N, hand-harvested the yields, and collected other appropriate field data (weather, past cropping and nutrient history, etc.).
Table 2. Site characteristics for the small plot studies on farm fields.
|Site||Year||County||Soil texture||Method and time of N application, N source|
|A||1989||Dakota||silt loam||Spring preplant, urea|
|B||1989||Olmsted||silt loam||Spring preplant, urea|
|C||1989||Waseca||clay loam||Sidedress, ammonia|
|D||1990||Waseca||clay loam||Sidedress, ammonia|
|E||1991||McLeod||clay loam||Spring preplant, urea|
|F||1992||Blue Earth||clay loam||Spring preplant, urea|
|G||1992||Dodge||silt loam||Spring preplant, urea|
|H||1992||Goodhue||silt loam||Spring preplant, urea|
|I||1993||Nicollet||clay loam||Spring preplant, urea|
|J||1997||Blue Earth||clay loam||Spring preplant, urea|
|K||1998||Dodge||silt loam||Spring preplant, urea|
|L||1998||Dodge||silt loam||Spring preplant, urea|
|M||1998||Mower||loam||Spring preplant, urea|
|N||1999||Olmsted||silt loam||Spring preplant, urea|
Field-size strip studies
Because some dealers, advisors, and farmers were not sure about the relevance of yield responses from small-plot studies, 15 field-size strip studies were carried out from 1997 through 2001. All sites were on glacial till soils in south-central Minnesota (Table 3). Sites were chosen by the farmer and either local crop advisors, dealers, or state agency personnel. There is a concentration of sites in Nicollet County because of nitrate concerns in the St. Peter Wellhead area and the presence of a multi-agency project to examine N use for corn.
Spring or sidedress application was used at 10 sites and fall anhydrous ammonia plus N-Serve was used at 3 sites. Fertilizer N was applied by the dealer or farmer in strips matching the applicator width (30 to 60 ft). Strip length ranged from about 400 ft to more than 1200 ft.
Table 3. Site characteristics for the field-size strip studies on farm fields*.
|Site||Year||County||Texture||Method and time of N application, N source|
|AA||1997||Nicollet||clay loam||Spring preplant, urea|
|BB||1998||Blue Earth||clay loam||Fall ammonia|
|CC||2000||Nicollet||clay loam||Spring preplant, urea|
|DD||2000||Nicollet||clay loam||Spring preplant, urea|
|EE||2000||Nicollet||clay loam||Fall ammonia + N-serve,/|
|GG||2000||Nicollet||clay loam||Spring preplant, urea|
|HH||2001||Blue Earth||clay loam||Sidedress, ammonia|
|II||2001||Brown||clay loam||Spring preplant, ammonia|
|JJ||2001||Martin||clay loam||Sidedress, ammonia|
|KK||2001||Nicollet||loam||Spring preplant, urea|
|LL||2001||Nicollet||clay loam||Spring preplant, urea|
|MM||2001||Nicollet||clay loam||Spring preplant, urea|
|NN||2001||Nicollet||clay loam||Fall ammonia + N-serve|
|OO||2001||Redwood||clay loam||Fall ammonia|
|*All fertilizer N rates were replicated three times except for the two earliest sites (AA and BB) where only two replications were used. No zero N (0 lb N/A) strips were included at sites AA and BB and only one control strip was included at sites GG and KK. Because of these limitations, no statistical analyses were performed for these sites.|
At each field-strip site the tillage, planting, pesticide application, and hybrid and planting rate selection were done by the farmers. The strips were combine-harvested using yield monitors. All yield data were collected by the farmer and/or consultant. The strip-average yields were given to the authors of this publication for statistical and economic analyses.
Two statistical models were used to find the best rate of N for each site. The least significant difference (LSD) value gives the smallest yield difference among treatments that is statistically significant at the 90% probability level. The LSD was conducted on all yield data from the sites that had three or more control (0 lb N/A) plots/strips. With the Quadratic Response Plateau model, a response curve was fit to the yield data up to the N level where a yield plateau was reached. The economic optimum N rate (EONR) was calculated using an N price of $0.15/lb and a corn price of $2.00/bu to determine the N rate where the cost of an additional pound of fertilizer was no longer exceeded by the value of the additional yield response. EONR analysis was conducted on all sites except I (only 4 N rates) and AA and BB (no control strips).
Figure 2. The quadratic response plateau (QRP) model used to find the optimum N rate for an example data set.
Corn yields for each of the N rates and the statistical data using the LSD and QRP models in Figure 2 are shown for all 14 sites in Table 4. The optimum yield (LSD) is in bold print. Yields were optimized at the 30-lb N rate at 1 site (L), at the 60-lb rate at 4 sites (C, D, H, and K), at the 90-lb rate at 5 sites, at the 120-lb rate at 1 site (F), and at the 150-lb rate at 1 site (E). Averaged across all 14 sites, the optimum N rate (LSD) is 71 lb N/A (Figure 3), or 82 lb N/A if using only the 12 responding sites, to produce a yield of 169 bu/A. The EONR averaged across the 13 sites that fit the QRP model was 86 lb N/A. The range across sites was 0 to 140 lb N/A.
Table 4. Optiumum fertilizer N rates for corn after soybeans based on small plots in farm fields*.
|--------------------------------------------- N rate (lb/A) ---------------------------------------------||LSD||EONR**|
|------------------------------------------------------------ Corn Yield (bu/A) ------------------------------------------------------------||---- lb/N/A ----|
|* A response to fertilizer N was not obtained at two sites (A and J), although yields at these sites ranged between 150 and 180 bu/A. This can be explained by the release of N from soil organic matter. Greater availability of soil N usually follows dry years and/or long-term applications of N (fertilizer and/or manure) in excess of crop removal.
** EONR = Economic Optimum N Rate based on 15¢/lb N and $2.00/bu corn.
***Actual N rates shown in superscript.
Figure 3. Optimum fertilizer N rates using LSD (0.10) and QRP models for 13 small-plot sites.
The optimum N rate data in Figure 3 show the variability among site years. University scientists make their recommendations slightly greater than the response data suggest. This cushions the farmer from risk of yield and profit loss under unforeseen conditions. In this case, an N recommendation of 120 lb N/A was actually more than was needed for optimum yields at 13 of 14 sites when using the LSD approach and at 10 of 13 sites when using the more liberal QRP approach. Based on these yield responses to N in small-plot studies, the 120-lb N rate, recommended by the University of Minnesota for 150 to 174 bu/A corn grown on these soils, was enough to optimize yield and profit at 13 of 14 sites. In fact, the yield maximum at some sites was reached at 60 lb N/A and no more N was needed.
Field-size strip studies
Corn yields for each of the 15 field-size strip studies are shown in Table 5. Least significant differences were calculated for 11 of the sites while EONRs were calculated for the 13 sites with 0-lb control plots. All 13 sites responded to fertilizer N. The optimum yield (LSD) is in bold type. Yields were optimized at the 60-lb N rate at 3 sites, at the 90-lb rate at 6 sites, and at the 120-lb rate at 2 sites. Averaging these optimum N rates and associated yields across all 11 sites produced an optimum N rate of 87 lb/A (Figure 4) and an average yield of 150 bu/A.
Table 5. Optimum fertilizer N rates for corn after soybeans based on field-size strips.
|--------------------------------------------- N rate (lb/A) ---------------------------------------------||LSD||EONR**|
|-------------------------------------------------- Corn yield (bu/A) --------------------------------------------------||---- lb N/A ----|
|* EONR = Economic Optimum N Rate based on 15¢/ lb N and $2.00/bu corn.
** Only two replications, no statistical analysis conducted. Actual N rates shown in superscript.
***The 0-lb control strip was not replicated at these sites, so the LSD was not determined.
Figure 4. Optimum fertilizer N rates using the LSD (0.10) and QRP statistical models for 13 field-size strip sites.
The EONR using the more liberal QRP method ranged from 55 to 169 lb N/A and averaged 100 lb N/A for the 13 sites (Figure 4). Eleven of the 13 sites had EONRs less than 120 lb N/A.
As with the small-plot studies, these field-size experiments demonstrate the site-to-site variability when finding an optimum N rate for corn (Figure 4). But in total, a 120-lb N rate was enough to optimize corn yield at all sites using the LSD method, and at 11 of 13 sites using the QRP method.
Small plot vs field-size strips
Although the procedures were different for these two types of field studies, the results were remarkably similar (Figure 5). Corn yields for the 0, 60, 90, 120, and 150-lb N rates were averaged across all 13 small-plot sites to find the EONR for small plots, and across all 13 field-size strip sites to find the EONR for the strips. EONRs for the small plots and field-size strips were 105 and 99 lb N/A (QRP). Optimum yield was greater in the small plots (173 bu/A) than in the field-size strips (152 bu/A) because of high yields in the small plots in southeastern Minnesota in 1989, 1998, and 1999 (when field-size strip studies were not done). This shows that plot size does not affect fertilizer N rate recommendations.
Best management practices
The recommendations validated in these on-farm trials are based on best management practices (BMPs) that are essential for profitable and environmentally sound N management. Fertilizer form (nitrate or ammonium), timing of application, and fertilizer placement all help determine if the N will be available to the plant, or if it will be more easily lost by volatilization (ammonia), by leaching (nitrate), or by denitrification (nitrogen gas) (Figure 6).
Some practices are recommended throughout Minnesota (statewide BMPs) and others are more specific to the soils, temperatures, and moisture conditions of various regions of the state (regional BMPs). Because appropriate management practices are so critical to the effectiveness of applied N, we have listed here the essential statewide and regional BMPs that will help assure profits and reduce N loss to the environment.
- Set realistic yield goals.
- Develop and use a comprehensive record-keeping system for field-specific information.
- Adjust N rate for soil organic matter content, previous crop, and manure applications.
- Use a soil nitrate test where appropriate.
- Credit second-year N contributions from alfalfa and manure.
- Use prudent manure management to optimize N credit.
- Test manure for nutrient content.
- Calibrate manure application equipment.
- Apply manure uniformly throughout a field.
- Inject manure if possible, especially on strongly sloping soils.
- Avoid applying manure to sloping, frozen soils.
- Incorporate broadcast applications whenever possible.
- Time the application of N for high use efficiency.
- Do not apply N above recommended rates.
See Best Management Practices for Nitrogen Use Statewide in Minnesota, 06125; A Soil Nitrogen Test Option for N Recommendations with Corn, 06514; Fertilizer Urea, 00636; and Fertilizing Corn in Minnesota, 03790.
- Do not apply fertilizer N in the fall, because of the risk of yield reduction and groundwater contamination in this region of high precipitation, permeable soils, and karstic terrain.
- Apply N in the spring preplant and incorporate broadcast urea-containing sources within three days of application. This lowers the chances for nitrate leaching during late April and May, when precipitation is highest and crop demand for N and water is very low.
- Apply sidedress applications before corn is 12 inches high to make sure N is available when the plants need it most, late June through mid-August.
- Inject or incorporate sidedress applications of urea and UAN to a minimum depth of 4 inches to reduce volatilization and to reach roots during dry periods.
- Use a nitrification inhibitor like N-Serve with preplant N applications if soils are poorly drained and if moisture content is high.
See Best Management Practices for Nitrogen Use in Southeastern Minnesota, 06126; and Nitrification Inhibitors and Use in Minnesota, 03774.
- Apply N in the spring to increase yield, decrease nitrate loss to tile line drainage (Table 6), and reduce the potential for denitrification.
- If applying in the fall, use anhydrous ammonia and delay application until soil temperatures reach and remain below 50° F at the 4 to 6 inch depth. Fall application of urea is not recommended.
- Incorporate broadcast urea and preplant application of UAN within three days of application to reduce loss by volatilization and surface runoff.
- Make any sidedress applications before corn is 12 inches high so N is available when plants need it most, late June through mid-August, and during long dry periods.
- Inject or incorporate sidedress-applied urea or UAN to a minimum depth of 4 inches so N can reach roots under dry conditions or when applied later in the season.
- Use a nitrification inhibitor like N-Serve with fall applications of N, (especially in poorly drained soils) to limit denitrification and leaching.
- Carefully manage N applications on soils with high leaching potential. Apply sidedress or split applications. Do not apply fertilizer N in the fall to coarse-textured (sandy) soils.
Table 6. Long-term average corn yields and flow-weighted nitrate-N concentrations in tile drainage as affected by time of N application and N-Serve at Waseca.
|Time of application*|
|Parameter||Fall||Fall + N-serve||Spring|
|Avg. corn yield (bu/A) (1987-2001)||144.5||152.9||155.5|
|Flow-weighted nitrate-N concentration in tile drainage (mg/L) (1990-1999)||18.3||15.5||13.8|
|*Anhydrous ammonia applied after October 20 in the fall and prior to planting in spring.|
See Best Management Practices for Nitrogen Use in South-Central Minnesota, 06127; and Nitrification Inhibitors and Use in Minnesota, 03774.
Southwestern and West-central Minnesota
- Use a soil nitrate test from a 2-foot depth sample taken in early spring (preferably) or in the fall (after the daily soil temperature is below 50° F at the 4 to 6-inch depth). N fertilizer recommendations are based on residual nitrate and are adjusted for previous crop and yield goal. (This test is not recommended if the previous crop was alfalfa.)
- Spring application is recommended. Apply N before planting and incorporate broadcast urea and UAN within three days of application to reduce loss by volatilization and surface runoff.
- If applying in the fall, delay until the daily soil temperature is below 50° F at the 4 to 6-inch depth to reduce N loss. Use anhydrous ammonia or urea sources of N; UAN should not be applied in the fall. [Reminder: Nitrogen should not be applied in the fall to coarse-textured (sandy) soils.]
- Apply sidedress applications before corn is 12 inches high so N is available when plants need it most, from late June through mid-August, and during long dry periods.
- Inject or incorporate sidedress-applied urea and UAN to a minimum depth of 4 inches to reach roots under dry conditions. This is especially true when rainfall is inadequate to move surface-applied N down into the active root zone.
See Best Management Practices for Nitrogen Use in Southwestern and West-Central Minnesota, 06128; and Fertilizing Corn in Minnesota, 03790.
The authors extend appreciation to Farmland Industries; Tennessee Valley Authority, National Fertilizer and Environmental Research Center; Greater Minnesota Corporation; Agricultural Utilization Research Institute; USDA-CSRS; American Farm Bureau Foundation for Agriculture; Minnesota Farm Bureau Federation; Minnesota Corn Research and Promotion Council; and the St. Peter Wellhead Protection Project for their financial assistance. Thanks to the many farmer-cooperators who participated in the project and to the consultants/local advisors (Blue Earth Agronomics, Karyn Wassman; GMA Agronomics, Kevin Bitterman; McPherson Crop Management, Bernie Paulson; and United AgTech, Steve Sodeman) whose collection of field strip data was vital to this project. We appreciate the technical assistance of Jeff Vetsch, Andy Scobbie, Brian Anderson, and David Groh in the collection and analyses of the data. This publication was funded as part of a USEPA 319 Grant awarded by the Minnesota Pollution Control Agency and managed by the University of Minnesota Water Resources Center.
- A Soil Nitrogen Test Option for N Recommendations with Corn
- Best Management Practices for Nitrogen Use Statewide in Minnesota
- Best Management Practices for Nitrogen Use in Southeastern Minnesota
- Best Management Practices for Nitrogen Use in South-Central Minnesota
- Best Management Practices for Nitrogen Use in Southwestern and West-Central Minnesota
Corn in Minnesota
Production Manager: Catherine Dehdashti, Extension Service Communications, University of Minnesota
Editors: Kathleen Cleberg, Press 1 Production; and Les Everett, Water Resources Center, University of Minnesota
Designer: John Molstad, Studio 31 Graphics, Inc.