Guidelines for fertilizer use for corn production in Minnesota were never intended to be permanent. As research that focuses on fertilizer use and management continues, the information gathered frequently suggests that changes are needed. This is especially true for nitrogen. This active research is not unique to Minnesota.

In recent years, there has been extensive research in Minnesota, Iowa, Illinois and Wisconsin for the purpose of defining the optimum rate of N for corn production across a wide variety of production environments. As the results that were collected were analyzed and discussed, it became obvious that the conclusions were consistent across all states. The results for a continuous corn system shown in Figure 1 and a corn-soybean sequence summarized in Figure 2 were typical of much of the information that was collected.

In both figures, each point represents the results of a study conducted at one experimental site. Both figures show the absence of any relationship between optimum yield (X axis) and the optimum Nr Rate (Y axis) needed to produce the optimum yield. For a corn-corn sequence, for example, the optimum rate for a corn yield of 150 bu. per acre varied from 0 to about 160 lb. N per acre. The same observation was appropriate for the corn-soybean sequence.

Since research trials conducted in several states showed that there was no relationship between yield and optimum nitrogen rate, it was obvious that guidelines for nitrogen rates should not be related to yield expectation or yield goals. A new concept for nitrogen rate guidelines was needed.

In an effort to develop the new guidelines, results from nitrogen rate trials across the Corn Belt were combined into one data set. When this process was completed, data from over 600 sites split evenly between farmers' fields and experiment stations were considered. For Minnesota, information from 128 sites was used in this project. The information used was taken from crop sequences where corn followed soybean or corn. If there had been a history of manure use within the past three years, the data were not included.

The first step in the analysis of the data was to calculate the Economic Optimum Nitrogen Rate (EONR) for each site. The response to applied nitrogen shown in Figure 3 is typical of the responses measured at the large majority of the research sites. Then, using a range of values for the price of a pound of nitrogen and the value of a bushel of corn, the Maximum Return To Nitrogen (MRTN) was calculated for each site. The MRTN varied as nitrogen prices and value of the corn changed. So, the price : value ratio (price of a pound of nitrogen divided by the value of a bushel of corn) had a direct effect on MRTN.

Following the calculation of MRTN for each site, the values were averaged to produce the nitrogen rate guidelines. Those guidelines for highly productive soils are listed in Table 1. The guidelines for both the corn-corn and corn-soybean sequence are listed in this table. The Minnesota data used to develop the guidelines were collected from research sites established on soils that were considered to be highly productive. Ratios for various prices for N and values of a bushel of corn are listed in Table 2.

Not all soils and production environments are considered to be conducive to the highest yields. There are other production situations that might be classified as having a medium productivity potential. Some factors that might lead to medium productivity are: 1) poor drainage, 2) eroded soils having a low organic matter content, 3) presence of gravel in some portion of the upper root zone, or 4) any other restriction to root growth such as a compacted layer.

In the medium productivity situations, it’s not economically practical to apply additional N to achieve the very high yields. Therefore, suggested N rates are reduced for these production situations (see Table 3).

Producers do not have a universal approach. Some are conservative while others are willing to apply additional N with an expectation of achieving higher levels of production or minimizing risk. Therefore, an acceptable range of N rates is provided in Table 1. Compared to the rate listed in the MRTN column, the guidelines at both ends of the acceptable range may reduce net profit by $1 per acre. These small changes illustrate the fact that changes in the rate of N applied do not have a major effect on net profit when near the rate used for MRTN.

The nitrogen credits have not changed for production where soils are considered to be highly productive.

The various nitrogen credits that have been established are to be used in the new guidelines. The credits for a previous legume in the rotation are provided in Table 4. These are to be subtracted from the MRTN value listed for the corn-corn production system.

There are also situations where measurement of residual soil nitrate-nitrogen is appropriate. This measurement is incorporated into the guidelines by using the following equation.

N_Rec = (N rate for corn-corn from Table 1) – 0.6(soil NO₃- N_0-24 in.)

The N rate chosen from Table 1 should be the rate chosen from the corn-corn cropping sequence.

The N credits from manure are not ignored in these new guidelines. After determining the N credit for the manure applied, subtract this credit from the N rate determined from Table 1. As with other credits, the N rates used as the starting point should be the corn-corn cropping sequence.

In summary, the new N guidelines are based on an extensive data base. The concept used is consistent across the Corn Belt. They vary with both the cost of nitrogen and the value of the corn crop. Guidelines also vary with soil productivity. The producer's attitude toward risk is also included. The old concept that ties suggested N rate to expected yield or anticipated yield has been discarded.

Table 1. Guidelines for use of nitrogen fertilizer for corn grown on soils considered to be highly productive.

MRTN = Maximum Return To Nitrogen

Table 2. Calculated N Price/Crop Value ratios for various prices of a pound of nitrogen and values of a bushel of corn.

Table 3. Guidelines for use of nitrogen fertilizer for corn grown on soils considered to have medium productivity potential.

Table 4. Nitrogen credits to be used when corn follows legumes in rotation.

*Use this credit if any small grain stubble in southeastern Minnesota counties was tilled after harvest. If there was no tillage following small grain harvest, use the guidelines for corn following corn.

Figure 1. Relationship between optimum N rate and optimum yield at Minnesota sites where corn followed corn.

Figure 2. Relationship between optimum N rate and optimum yield at Minnesota sites where corn followed soybeans.

Figure 3. Corn yield in response to rate of applied nitrogen. Data shown are averages of three sites where corn followed corn in Olmsted County.

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