Information Management to Maximize Your Yields

Improving water quality with variable rate N applications in corn

R.M. Vanden Heuvel, Ph.D., CPSS
VH Consulting, Inc.

January, 2015

Over the past ten years, VH Consulting and Max Yield Cooperative (SciMax N) have been promoting variable rate N applications (VRT-N) for continuous corn and corn in rotation with soybean. Over 60,000 acres have been sampled for this program in northern Iowa. The goal of the program was to refine N recommendations and optimize N rates. A growing university database across the Corn Belt has clearly shown that producers have been over applying N for corn production. It is also very clear that N needs vary across individual fields, but VRT-N programs across the Corn Belt remain in their infancy. Recommendations for SciMax N have reduced N rates by an average of about 35-40 lbs/a thus far, compared to standard farmer practice. While unusual, some fields have seen average reductions of 100 lbs/a. With these improvements, questions are being asked as to the anticipated water quality improvements that should result from the program. Such water quality data from universities are not common as the measurements needed to be taken are very time consuming and expensive. This article hopes to provide some estimates water quality improvements with improved N application rates from data that does exist from our area.table 1

Recent water quality data from the University of Minnesota at Waseca, MN measured nitrate that moved out of the soil profile with various N rates under field conditions (Table 1). The soil was a
Webster clay loam, typical of the area and also of northern IA. Nitrogen application rates were from 0 to 180 lbs/a in 30 lb increments. Yields were determined and nitrate-N (measured with special instrumentation to extract samples of the soil solution) is given as the total amount measured for the growing season. The red colored value of 135 lbs/a was the calculated Economic Optimum N rate (EON), not an applied N rate.

The first important item to note is that when no N was applied, 28 lbs/a of nitrate-N was lost over the growing season. This is not unusual as mineralization occurs with tillage. It also points out that a corn crop is not 100% efficient at using N in the soil profile, even when no fertilizer is applied. Some losses are always going to occur.

As the N rate increases it can be seen that nitrate-N also increases, reaching 51 lbs/a at the 180 lbs N/a, the highest application rate. The EON was 135 lbs/a, which produced an estimated 189 bu/a of grain. Ideally, this would be what a farmer would optimally apply at this site. But how well does this rate do for the environment by minimizing nitrate leaching? At the EON, the nitrate-N rate would have been 41 lbs/a, a value that is only 13 lbs/a above that of the 0 N application rate. So, moving from 0 to 135 lbs/a of applied N only increased nitrate-N by 13 lbs/a (28 to 41 lbs/a, Table 1)! This is important to note as it illustrates that optimum N rates serve to minimize nitrate leaching in a high production environment. Over application above EON will only serve to increase the rate of leaching. This is shown in the last column of Table 1 as nitrate-N increased to 23 lbs N/a above the check, at the highest N rate of 180 lbs/a. Using an optimum rate of N (the EON, of 135 lbs/a) reduced nitrate-N leaching by over 40% (from 23 lbs/a to 13 lbs/a). This degree of reduction is significant!

A nitrogen program that zeros in on optimum N rates for your fields will minimize environmental contamination. It won’t eliminate leaching, but it will minimize it. Over application of N accelerates the loss of N, costing farmers money and unnecessarily degrading water resources. The 180 lb N/a rate in the university study can be viewed as a reasonably common rate used in our region in traditionally managed fields. SciMax N has reduced N rates to below this number. In many cases, well below this amount of N. If the leaching data in the study above is an indication of what is happening in northern IA, then VRT-N recommendations in SciMax N are having a very significant impact on reduced nitrate-N levels.

As we continue to improve our recommendation system, only more improvements for reduced nitrate loads will occur. Optimum yields are required for profitable farming, and fortunately the N rates required to achieve them (EON) will go a long way to improve water quality.