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  • Writer's pictureRachel Stevens

Understanding Field Variability

You’ve probably read it several times on our blog. “Your fields aren’t uniform, so don’t treat them like they are.” We wanted to spend some time today diving into that statement, what kinds of variability we are dealing with, how to quantify that variability, and finally how to manage that variability. So, let’s go.

Types of Variability

There are two main types of variability. Spatial and temporal.

Spatial variability is any change in an attribute across space. An example would be the changes in various soil types across your field. Or perhaps the change in water holding potential or nutrient availability across a field. Spatial variability is easier to quantify and measure. We actively map and try to understand our spatial variability.

Temporal variability is any change in an attribute across time. An example of this would be the change in weather patterns from year to year, or difference in total rainfall each growing season, or changes in temperature. We can also thing of temporal variability as changes in our yield maps from year to year. (While the change in yield across the field is spatial variability, the change in patterns of yield from year to year is temporal variability. Essentially, does your yield map look the exact same ever year? No. That is temporal variability.)

So now that we know the two types of variability, we can think of lots of specific instances that fit within those two major categories. Soil types, organic matter, water holding potential, nutrient availability, weather, even man-made variability.

And since I’ve mentioned it, lets talk manmade variability. While there is a lot of inherent variability in our fields, we also have to consider the impact we have on the variability. Think about any compaction that might show up, or planting multiple hybrids that performed differently, or patterns we may introduce with any nutrient applications. The variability we introduce is just as important to consider when we start talking identification and management. A little tip---look for straight lines in your field. If you have some straight lines show up on a yield map, or fertility map, or aerial imagery, you can bet that straight line came from some sort of man-made variability.

We’ve identified the two main groups of variability and a whole host of examples of types of variability. Let’s move on.

Quantifying Variability

We know that variability exists within our fields. The next step is quantifying or measuring that variability. It’s important to measure these changes so we know how appropriately manage the variability present. One of our most common ways of measuring field variability is through soil sampling. We can understand some of that variability by selecting some soil samples from random spots in the field, but our best way to understand the changes is through grid sampling. By collecting this data on a 2.5 acre grid, we get a much better look at distribution of nutrients, organic matter, pH, or soil textures across the field. In fact, the more dense of a grid, the better. However, financially it isn’t often practical to sample at a density less than 2.5 acres. A good substitute for that is utilizing a Veris for electrical conductivity data. Running a veris every 50-60 feet can help provide a much more spatially dense map and help you see the variability present within your field. In fact, Veris is now offering new systems that add on to your planter so no additional trips through the field are needed.

We can also quantify variability by using multiple years of historical yield and aggregating into a normalized yield map. The more years of yield maps, the better, as research has shown that 5 years of yield maps are not enough to create a stable yield zone map.

Aerial imagery and satellite images can also be very useful for quantifying variability, particularly with bare soil images. These let us see soil changes across the whole field. Late growing season imagery can also be telling. If you hit the window right, you may notice portions of the field starting to senesce at a different rate, which can help you understand water availability. Additionally, tools like ground sensing LIDAR can give you super accurate elevation maps showing very precise terrain.

These are just a few tools available to help you quantify and measure the amount of variability that is present in your field. Now lets talk how to manage that variability.

Managing Variability

One of the first steps people take to start managing field variability is to start with variable rate fertilizer. Many of you are already doing this. But for those that aren’t, we take the combination of grid sampling data, historical yield, and yield goals to make you precise maps that fit each acre. No more uniform maps. Some of you might want to begin to think of moving away from uniform N applications, and start incorporating soil type variability in the rates you apply.

Another step would be to start adjusting your seeding rates based on the variability present. This however requires a lot more supplemental data, like multiple years of historical yield data, so make sure you have the data layers necessary before starting any variable rate seeding. The same is true for multi-hybrid planting. A lot of information is needed to create the appropriate management zones for this type of application.

Tiling or field leveling can be ways to manage terrain and water availability in the field. This is an instance when a LIDAR layer might be very beneficial.

While not on the same sub meter scale as some of these examples, seed placement can also help manage variability. Perhaps in a field with two contrasting soil types or elevations, two hybrids could be selected to meet those field conditions. This is a much simpler version of multi-hybrid planting.

These are just a few ways we can begin to manage variability. So, lets think over your fields. What variability do you suspect is present? What do you think is the best way to measure that variability? Get a plan in place so you can manage that variability in 2021.

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