Godsey Ag

Steps of the Process

General Approach

Figure 1 outlines the general process that takes place for each field we evaluate. This approach is not limited to corn or irrigated acres, success can be achieved with other crops such as peanut, wheat, grain sorghum, cotton, soybean, etc.  You can also utilize other methods for basing prescriptions on such as soils maps, and producer knowledge. The best method depends on each individual field.

Below is an actual example of how historic yield data can be used to incorporate variable rate seeding and nitrogen prescriptions in an irrigated corn field.

Figure 1. Outline of the process that goes into making variable rate decisions.

Step 1:

The first step is to analyze yield maps from each year (Figure 1). This example uses 3 years of yield data. In some cases you can get by with as few as one but ideally the more years you have the better your decision making will be in year 1 in varying rates. In some cases soil maps or producer knowledge can be used to draw different zones.

Figure 1. Three years of yield history with basic uniform management. Yields lowest to highest are brown, yellow, light green, dark green, and blue.

Step 2: Determine temporal stability and yield potential

Once you have processed the data you need to determine temporal (year to year) stability and determine historic yield potential in all areas of the field (Figures 2 and 3). In this example we are using 60’ by 60’ grids. Therefore, in all grids we are determining the variability in yield between years and determining what the yield potential is in each cell. To determine if yield is stable from year to year we use coefficient of variation (CV) which is a statistical tool that provides an indication of variability between years. Low CV’s indicate stable yields from year to year, while high CV’s indicate unstable yields from year to year. Once you have determined that yield is stable between years and you have historic yield potential you can proceed to generating prescriptions based on this information. If yield is not stable more years of yield data may be needed before you can alter management.

Figure 2. This is a measure of the temporal stability or the year to year stability in each area of the field. The statistical measure used is coefficient of variation (CV). If CV is less than 10 we can assume yield is stable. If greater than 10 may want to include more years of yield data. Yellow and brown have CV less than 10.

Once we have performed the above steps we are ready to vary seeding rate and N rate based on Figure 3.

Figure 3. This is a figure illustrating the yield potential throughout the field. It is normalized yield. We can use this historic yield potential to base seeding and nitrogen rates recommendations on.

Step 3: End of Year Evaluation for Irrigated Corn Field

In this year variable rate seeding and variable rate N prescriptions were used on this field. A total of $12.82 per acre was saved just in seed and fertilizer input costs (Table 1).

†Savings based on a uniform seeding rate of 32,000 seeds per acre and $80 per acre seeding cost.
‡Savings based on a uniform rate of 85 lb N/ac and $0.60/lb N, applied as UAN.

Yield Data Summary:

Several methods are used to evaluate data at the end of the year. The first method is comparing the change in the yield for the year the variable rate treatment was applied to the historic yield. In each year yield data was normalized to make comparison across years. To normalize the data, the entire field average is calculated and then each cell (60 ft by 60 ft) is divided by the field average. This normalized data determined for 2006, 2008, 2009, and 2010. The long-term normalized averages for each cell were determined by taking the average of 2006, 2008, and 2009. These long-term averages were then compared to normalized yield data from 2010. Analysis on the yield data indicated that on average we observed a 6.9% increase in yield when compared to the long-term yield. You can further look at specific zones (i.e. low yielding, high yielding, etc) to see if the response was greater in certain zones. This will help guide you for next year. The next method is to look at the entire field average and compare it to the long-term yield for the entire field.  This shows a 17.1 bu/ac (2240 total bushels for the field) increase when considering the entire field average. The biggest increase was observed in the historically higher yielding areas of the field by planting more plants and applying more N.

†Savings based on a uniform seeding rate of 32,000 seeds per acre and $80 per acre seeding cost.
‡Savings based on a uniform rate of 85 lb N/ac and $0.60/lb N, applied as UAN.
¶Increased revenue at $4.00/bu selling price.