In late fall of 2005, the University of Minnesota Extension Service changed their corn nitrogen fertilizer guidelines for those situations where deep nitrate soil sampling (0-24 inch depth) is not practiced. This new method does not include the yield goal equation we are all familiar with because of the poor correlation of corn yield response to added nitrogen in their many of research studies. Nebraska, North and South Dakota, and other Great Plains states, which use the deep nitrate soil sample to make N fertilizer guidelines, have not changed their guidelines process. Several of these states are conducting additional N fertility trials to evaluate this approach.

The new University of Minnesota corn guidelines are for nitrogen only, not phosphorus, potassium, zinc, sulfur or lime. These new N guidelines have been derived from many years of replicated small plot and field trials and are now focused on the relationship of nitrogen price and corn price, called the NP/CP ratio. Table 1 shows some common scenarios of this ratio concept.

Table 2 shows the new MN nitrogen rate guidelines based upon these ratios, previous crop and assumes a high productive potential field. The new N rate guidelines is called the MRTN (Maximum Return To Nitrogen). As seen in the table 2, there is about a 30-pound difference in N rate depending upon the previous crop. Also, as the NP/CP ratio gets higher the MRTN rate gets lower.

Also incorporated into these new N guidelines is a Range of Economically Acceptable N. These ranges are shown in Table 3 along with the NP/CP ratio and are called the MRTN rate range. Growers who do not want the risk of running a little short of nitrogen and possibly losing some yield, should apply the rate at the high end of the range. Growers who are comfortable with the risk of lower than maximum yields, if the N rate is not quite high enough in some years, should use the rate at the low end of the range.

How will AGVISE deal with these new MN corn nitrogen guidelines?

We have incorporated these new corn N guidelines into our system for the fall of 2006. We have added three new crop choices based upon the ratio of nitrogen price divided by corn price as shown below:

Corn NP/CP = 0.10, Corn NP/CP = 0.15, Corn NP/CP = 0.20

No yield goal is used for the nitrogen guideline, but previous crop nitrogen contribution and a minor adjustment for organic matter are considered. Phosphorus and potassium guidelines are available as University, Broadcast build, Broadcast maintenance and Band or Band maintenance. A yield goal of 160 bu/a is used to calculate the P and K fertilizer guidelines.

The three new AGVISE corn N guidelines are similar to the MRTN nitrogen guidelines from the U of Minnesota. One difference is due to the small adjustment made based on the organic mater level in the soil. Table 4 shows the three new AGVISE corn nitrogen guidelines compared to the U of MN MRTN guidelines if the previous crop was corn. Table 5 shows this same concept except for a corn/soybean rotation.

If you have any questions on the new U of M corn N guidelines or the new AGVISE corn N guidelines, please give our technical support staff a call.