Tracking GDD Just Got Easier With New Tool

Tracking GDD Just Got Easier

By Mark Rothermich, Product Specialist

Growing Degree Days or GDD are the accepted method of tracking the growth process of many crops, most notably corn. While we are all familiar with the term, tracking GDD and using them as a resource for evaluating crops has not been fully utilized by most of us. I wanted to dive into the subject with you and explain a tool we have developed to help you make use of them.

There are two main methods for tracking your corn’s growth – staging and GDD. Both options have good attributes but also some challenges that make them less than perfect.  I feel that combining the two is the best method.

Staging is the practice of counting collared leaves and monitoring grain development to get an estimate of your crop’s progression toward maturity. While it is probably the most used method, it is not without some challenges. 

In my experience, I can ask five people to stage a plant and will often get multiple answers depending on their understanding of a fully collared leaf.  

For example, on a larger plant how many leaves have sloughed off?  We also know that there can be variations from plant to plant that muddies the water. Staging does not lend itself very well to predicting the timing of the next stage. 

If we are trying to time an application to a particular time in the plant’s life, it gets pretty tricky using the V or R stages.

GDD is a calculation based on the actual high and low temperatures each day experienced by the plant adjusted to the maximum and minimum temperatures the plant will grow in.  One downfall with GDD is that there are other environmental factors that impact the plant’s growth and we are only monitoring this one. 

Also, I would contend that a growing degree day in mid-March, when the warmer part of the day is shorter, compared to one in mid-May when it is longer, will have different impacts especially on germination and emergence. 

In the overall scheme of the entire growing season, the GDD method still has good applicability. One benefit of tracking GDD is the ability to look at the weather forecast for the next few days and predict when your crop will likely be at a particular stage for an application.

Last year, with the help of a friend, I developed a GDD chart that turned out to be a very helpful tool. It works especially well when combined with doing ear assessments right before harvest.  The chart gives us the ability to look back accurately at what conditions we had during different stages of the crop’s life. 

Rather than trying to remember when that hot spell hit, or whether it was cold during the ear initiation phase, we can get an explanation of why our ears only have 14 around, or why we have tip back. 

Armed with this type of information we can begin to predict how weather events or application timings will impact the crops performance. In the future, with this information, we can look at the weather forecast and perhaps make applications or apply practices that will help the crop bridge the gap through a period of stress.  

We can become PROACTIVE rather than REACTIVE in our approach.  But we can only do that once we have a couple years of data and experience which includes making time to observe and record conditions.

The chart is an example of how one producer used the GDD chart to track a great deal of information. By entering the high and low temperature each day, the spreadsheet automatically makes adjustments to temps outside of the growth range and then calculates the daily and accumulated GDD units. 

Under the notes section the grower entered information regarding rainfall, cloudy days, growth stages, normal GDD for his area, and the disparity. 

Click here to download a GGD chart sample.


The red number 1’s are automatically filled in when a temperature is outside of the ideal range. In this example you can see all of the days, early in the season, that were below the 50-degree threshold. Conversely, later in the season, it will place a red 1 behind the adjusted high temperature when it gets above the set temperature range.  

This makes it very easy to look back and recognize those periods. With these notations you can see that during this cold spell the grower went from a +43 GDD position to a -9 compared to his 5-year average, including a frost.  

These conditions are not really ideal for a new seedling trying to decide what kind of environment it is growing in and how many seeds it can maintain.

It doesn’t have to take a great deal of time to gather this information, but it does take a little dedication.  Having a weather station of your own, or one nearby would be great. 

Spending a couple of minutes every day to enter the information would be ideal since you have a real time feel of what is going on. Doing this allows you to keep tabs on when you might need to be prepared to do foliar or other applications.

Some producers have a notebook that they record the daily information into and then transfer it periodically. Some will utilize a service such as Climate that While this is a functional idea for end of the year assessments, the real power is in keeping it current every day or at least every week.

I have made some adjustments to this year’s version of the tool that has separate columns for rainfall, weather events and other items.

If you have ever pulled out of a field with the combine and thought “Wow, where did all of that crop come from!” or “Wow, where did the crop I thought I had disappear to?”, this is a great tool to revisit the season your crop had and start learning why.

If you have an interest in understanding how the weather is impacting your crops, I strongly encourage you to commit to using this tool. If you are interested in using the tool, please contact me using the form here.

And remember – “You can’t control something you don’t understand!!”

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