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 new
guidelines 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, common conditions 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 were established during the 2006 and 2007 growing seasons to evaluate corn yields in response to fertilizer N. Eight fertilizer nitrogen application rates were included and ranged from 0 to 210 pounds N per acre (0, 30, 60, 90, 120, 150, 180, 210 pounds N 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 (below).
Yield response curves are shown by the pink lines. Economic optimum N rates (EONRs) are represented by the vertical blue lines and show the point where any additional N would not be cost effective. At Hastings, the EONR of 144 pounds N per acre yielded 190 bushels per acre. An EONR of 160 pounds N per acre yielded 240 bushels per acre at the Hampton Township site. For all sites over the two years, the nitrogen rates that maximized economic return fell within the new University guidelines, thus supporting the change in recommendations.
In addition to yield response, basal stalk nitrate-N concentrations were evaluated and are shown by the black curves. Iowa State first studied corn basal stalk nitrate-N concentrations 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 may suggest that N was applied in excess of the crop’s needs. Figures 1 and 2
both show that the EONR is in the optimum range for basal stalk nitrate-N in our studies.
Figure 1. Corn yields and basal stalk nitrate concentrations at varying rates of nitrogen fertilizer on an irrigated sand near Hastings, 2007.
Figure 2. Corn yields and basal stalk nitrate concentrations at varying rates of nitrogen fertilizer on an irrigated loam in Hampton Township, 2007.
2008 Study The nitrogen rate studies were expanded in 2008 to evaluate new technologies that help determine nitrogen needs of the corn crop during the growing season. In addition to the eight N rate plots, three new treatments were included:
- “Spoonfeeding” – 180 pounds 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 to the V12 stage.
- SPAD meter – The SPAD meter measures the relative greenness of a corn leaf when it is compared to corn in an optimally fertilized (non-N limiting) strip. In this study, corn in the SPAD plot was compared to corn in the 210 pounds N per acre plot and N was applied to the SPAD plot when the meter reading was less than 95%. Readings were taken at four vegetative stages (V6, V8, V10, V12).
- Greenseeker™ – The Greenseeker™ senses how much light is absorbed by the corn. This information is related to the crop’s biomass and can be used to calculate growth rates and nitrogen needs. Similar to the SPAD plots, readings were taken at four vegetative stages and N was applied when indicated by the reading.
All of the 2008 results will be discussed at Crops Day on Tuesday, March 3, 2009 at the Dakota County Extension and Conservation Center in Farmington. This will include an in-depth discussion of nitrogen rates, analysis of the new technologies and basal stalk nitrate results.