Precision Q&A – for Apr. 6, 2009


Should you have a yield monitor and mapping system for your combine if you are thinking about variable rate fertilization?


For the answer, Grainews turned to Terry Aberhart. Terry is an Agri-Coach with Agri-Trend Agrology and he farms near Langenburg, Sask. Here is Terry’s answer:

There are two answers to this question. Do you NEED yield mapping on your combines to do variable rate? No. Should you invest in yield mapping if you are moving to precision applications and technology on your farm? Absolutely.

Yield mapping is a very powerful tool and can provide valuable information — and not just for variable-rate (VR) fertilizer application. Whenever I hear a grower talk about getting into variable rate fertilizer, I usually ask these questions: How do you know that you need variable rate fertilizing in your field? And how are you going to measure what the return on investment from variable rate applications will be?

Maybe your fields do not need VR fertilizer to improve productivity. Maybe it needs drainage work or has areas of compaction or other productivity issues not related to soil nutrient levels. Either way, a successful precision program for your farm will rely on three things: the data you use to build your precision program, applying the right science and agronomic knowledge to that information, and having the tools to measure how your precision program is working.

This is where yield maps come in. Yield maps provide a great layer of information. They give you an image of what was happening in that field the year the map was made. If you are looking to make zones for VR application, this can be a great layer to help build those zones. But just like other layers of information we can use to create zones, I would never be very comfortable making zones based off of only one layer or year of data. Many times, different crops will show different patterns in a field. A wet year will look different than a dry year. Maybe one year you had problems with weeds in a certain spot or had chemical damage on one side of the field. Hundreds of factors can affect yield across a field. If you rely on one year’s yield map or on one NDVI image, you are not likely to get the result you are looking for. (NDVI is a satellite or aerial image that shows the variability in vegetative growth across the field.)

When you can start combining multiple years and layers of information, you can have a much greater level of confidence in where you need to take your precision program and will have a much higher chance of return on investment.


Putting a yield mapping system on your combine is yet another expense. And if you have multiple combines, you will need a yield monitor on each one. Your combine operators will need some training and knowledge on how to run the system. This is usually very easy, though. Then you need to calibrate the system, but this is not nearly as big of an issue as it use to be. On our farm, we only calibrate once for each crop. I don’t really care if the system is two or three bushels out. I just want to know which areas of the field are higher and lower yielding.

It use to be a big deal to make sure you had the crop and field entered properly and calibration had to be perfect on all the combines in order to get a proper map. With newer GIS programs and support, this is not an issue anymore. If the operator forgets to change the field or crop, this can be corrected later. Combine calibration factors can also be corrected after the maps have been downloaded. If you had beaver piles or stopped in the middle of the field, this would cause improper readings in parts of the field. But we can now use software to filter this data out. On our farm, we employ the services of Geofarm Solutions out of Calgary to fix up our data if needed.

Many times I get asked why one would invest in yield mapping when you can take a satellite or aerial NDVI image of the field? NDVI images are useful tools, but they can be skewed or affected by many of the same issues as a yield map.

If you where to take a NDVI image of your field every year, the cost could quickly exceed what any yield mapping system would cost over the long run. Once your mapping system is paid for, you can make maps every year at no additional cost. NDVI images show variations in vegetative growth across the field, but these are not always correlated to yield. In many cases, farmers who have been using yield monitors will tell you that heavy areas of vegetation are not always areas of higher yields. This may be due to lodging issues or crop growing too heavy and not leaving enough moisture or nutrients to fill the grain. Being able to utilize multiple GIS layers and data will help give you the best understanding about what is affecting yield and productivity across your fields.

This is where you need the best agronomic support. We work through Agri-Trend Agrology and the Precision Managment Process to help get the most value out of information and build the best precision program for each field.


Combine yield mapping systems record yield data, moisture level readings and elevation. If you have a high quality GPS signal on your system, you will be able to get very good elevation maps from your harvest operations. One of the best things about yield mapping is that if you are the one driving the combine, you can see what things are affecting your yield in real time across the field. This is very powerful information.

Yield maps can be compared with other layers of data to verify management zones in your field. Multiple years of yield maps can be normalized and combined into a composite map. This will help remove some of the variability from one year to another. This is another service that Geofarm Solutions provides for our farm and my clients.

Yield maps can show how management practices are affecting your fields over time, and help verify if your variable rate program is providing return on your investment. Yield maps are also a great way to measure field trails or crop damage. The more we work with yield maps, we see so much value that we can pull out of the information. Every year that you can pull another yield map off a field is another layer of information that can be utilized and archived in time.

I find it very surprising when I run into guys who are willing to pay a consultant $8 per acre or more a year for a variable rate program yet think an investment in yield monitors is too costly. This just does not make much sense to me. Not all guys I work with on variable rate programs have yield monitors, but I really prefer and encourage growers and clients to invest in this technology. The maps on this page are good examples of the useful information you can get from yield mapping.

You can reach Terry Aberhart at [email protected]or 1-306-743-7657.



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