Phyllis Bongard, Extension Educator – Dakota County
John Lamb, Extension Soil Scientist
Background In response to increasing fertilizer costs, the University of Minnesota changed its fertilizer nitrogen (N) rate recommendations for corn in 2006. Instead of using yield goals to determine how much nitrogen to apply, the new guidelines are based on economics, soil productivity and a producer’s attitude towards risk. The recommendations were developed in cooperation with universities in seven corn belt states and are based on research from over 700 sites in the upper Midwest. They are central to the voluntary Best Management Practices for nitrogen in Minnesota, agricultural practices that are economically sound and, when followed, capable of minimizing risks to the environment.
Why duplicate the research in Dakota County?
While over 700 sites were used in the original research, none of the sites represented irrigated corn on coarse-textured soils, conditions that are common in Dakota County. Since 2006, several study sites were established with the help and support of local farmers and the Vermillion River Watershed Joint Powers Organization (VRWJPO) to study the corn yield response to nitrogen rates under Dakota County conditions.
Study results from 2006 & 2007
A total of three sites, two on sandy soils and one on a loam soil, were established during the 2006 and 2007 growing seasons to evaluate corn yields in response to fertilizer N. Eight fertilizer N application rates were included and ranged from 0 to 210 pounds N per acre (0, 30, 60, 90, 120, 150, 180, 210 pounds per acre). While results from all of the sites support the new University guidelines, results from the 2007 sites are highlighted in Figures 1 and 2.
Figure 1. Corn yield response to varying amounts of N fertilizer on an irrigated sandy soil near Hastings, 2007 where the previous crop was potatoes followed by winter rye. Return over N costs based on $4.00/bu and $0.50/#N.
Yield response curves are shown by the pink lines. Economic optimum N rates (EONRs) are represented by the vertical blue line and show the point where any additional N would not be cost effective. This is also reflected by the return over N costs (yellow line) which is based on $4.00 per bushel corn and $0.50 per pound N. At Hastings, the EONR of 144 pounds per acre yielded 190 bushels per acre. An EONR of 160 pounds per acre yielded 240 bushels at the Hampton Township site. For all of the sites over these two years, the nitrogen rates that maximized economic return fell within the new University guidelines.
Figure 2. Corn yield response to varying amounts of N fertilizer on an irrigated loam soil in Hampton Township, 2007 where the previous crop was sweet corn. Return over N costs based on $4.00/bu and $0.50/#N.
In addition to yield response, basal stalk nitrate-N concentrations were evaluated. Iowa State University first studied corn basal stalk nitrate-N as a post-mortem tool to distinguish between N rates that had been adequate and those that had been excessive. In their suggested interpretation, concentrations between 700 and 2000 ppm are likely adequate, while concentrations above 2000 ppm suggest that N was applied in excess of the crop’s needs. In both of our studies, the EONR was in the optimum basal stalk nitrate-N range.
2008 Study The studies were expanded in 2008 to evaluate new technologies that help determine N needs of the corn during the growing season. In addition to the eight N rates, three new treatments were included:
“Spoonfeeding” – 180 pounds of N per acre were split into 6 applications of 30 pounds per acre. The objective was to compare a simple split application (90+90 lb/A) to a treatment that mimicked fertigation with several applications N up to the V12 stage (Table 1).
SPAD meter – The relative greenness of corn leaves in the SPAD plot were compared to leaves in a non-N limited plot (210 pound rate plot). Readings were taken at four vegetative stages and N was applied to the SPAD plots when the readings were less than 95% of the 210 pound per acre plots.
Greenseeker™ - This tool senses how much light is absorbed by the corn. The information is related to the crop’s biomass and can be used to calculate growth rates and N needs. Similar to the SPAD plots, readings were taken at four vegetative stages and N was applied when indicated by the reading.
Table 1. Time of nitrogen applications for sensor and comparable rate treatments for studies in Hampton Township and Hastings, 2008.
Yields for the checks and all of the paired treatments listed in Table 1 can be seen in Figures 3 and 4:
- Spoonfeeding – There was no advantage at either site for the spoonfeeding treatment compared to the 180 pound N rate plots.
- SPAD meter – At both sites, SPAD readings called for a total of 120 pounds of N, but application times differed. At the Hastings site, where all of the applications were completed by V8, there was no advantage to the SPAD treatment over the 120 pound N rate plot. However, there seemed to be a slight advantage for the SPAD treatment at the Hampton site, where SPAD readings called for later applications at V8 and V13.
- Greenseeker™- Sensor readings resulted in very different application rates at the two sites. In Hampton township, readings called for a total of 150 pounds N per acre, while only 90 pounds N were applied at the Hastings site. There was no yield advantage in using the Greenseeker™ in Hampton township. In Hastings, however, the Greenseeker™ plots yielded 256 bushels per acre compared to 214 bushels in the 90 pound N per acre plots. Even so, yield and return over N costs for the Greenseeker™ treatment were significantly less than for the EONR (256 vs 289 bu/A; $979 vs. $1060/A at $4/bu, $0.50/lb N).
Figure 3. Corn grain yields for sensor and comparable nitrogen rate treatments at the Hampton township site, 2008.
Figure 4. Corn grain yields for sensor and comparable nitrogen rate treatments near Hastings, 2008.
The use of sensor technology is still in its early stages in Minnesota. While results from this first year were somewhat inconsistent, they did show some potential for increasing efficiencies in irrigated corn. This study was repeated at two locations in 2009 and those results will be available in spring 2010.
Corn yields (pink lines) and basal stalk nitrate-N concentrations (black lines) for both sites can be seen in Figures 1 and 2. At the Hampton township site near Cannon Falls, the agronomic optimum N rate (AONR) of 162 pounds N per acre yielded 218 bushels per acre. Nitrogen rates above 162 pounds per acre did not result in additional yield at this site. Economic optimum N rates (EONR) take fertilizer N costs and corn price into account to calculate the most cost effective N rate. Subsequently, EONRs will be less than the nitrogen rates required to maximize yield (AONR). In these studies we used an EONR ratio of 0.15 (N cost per pound/corn price). The nitrogen rate that optimized return at this site was 141 pounds N per acre. It yielded 217 bushels per acre and fell within the new University of Minnesota guidelines.
Basal stalk nitrate-N concentration samples are taken close to harvest, so they serve as a post mortem report. Iowa State University suggests that levels between 700 and 2000 ppm indicate that nitrogen had been applied at an optimum rate. Levels above 2000 ppm would indicate that N had been applied in excess. At the Hampton township (Cannon Falls) site, the stalk nitrate concentration at the EONR (141 lb N/A) was just under 1000 ppm, well within the suggested optimum range.
Figure 5. Corn yields (pink) and basal stalk nitrate-N concentrations (black) at varying rates of nitrogen fertilizer on an irrigated silty clay loam soil in Hampton Township, 2008.
Yields were considerably higher at the Hastings site (Figure 2). The EONR of 193 pounds per acre yielded 289 bushels per acre. In addition, starter fertilizer and nitrate from the irrigation water brought the total N application to 230 pounds, a rate well above the current recommendations, but one that achieved exceptional yield. Similar results were seen at a Becker, Minnesota research site.
At Hastings, the average stalk nitrate-N concentration in the 180 pound N rate plots (slightly under EONR) was just under 2000 ppm, still within the optimum range. However, the stalk concentration spiked quickly above this rate and reached levels in the excessive range in the 210 pound N rate plots.
Figure 6. Corn yields (pink) and basal stalk nitrate-N concentrations (black) at varying rates of nitrogen fertilizer on an irrigated sandy soil near Hastings, 2008.
Since 2006, seven on-farm fertilizer nitrogen management studies in irrigated corn have been conducted in Dakota County. Four were established on coarse-textured soils while the remaining plots were located on finer loam soils. Results from the loam sites consistently support the 2006 nitrogen rate recommendations. However, while results from two of the coarse-textured sites fell within the new guidelines, following the new recommendations on the other two sites would have underestimated N needs. The resulting EONRs were closer to Wisconsin’s N recommendations for irrigated sandy soils. Since the number of coarse-textured irrigated corn research sites is still thin for Minnesota, this work will be continued.