Plants in two corn fields in different parts of the state, planted two weeks apart, could be exactly the same size. At the same time, plants in two neighboring fields, planted three days apart, could be completely different sizes. Why?
After putting on our agronomist hats, we know it could be a host of reasons - soil type, fertilizer, pesticides, water status, or potentially, simply GDDs.
What are GDDs and why do they matter?
GDD stands for Growing Degree Days (also called "heat units") and these units are ultimately used to calculate and predict crop growth. The basic idea of GDD is to estimate the "accumulated growth" of a plant by assigning base temperature and peak temperature thresholds. These thresholds will vary by crop, but always bound the temperatures where normal growth is possible for a particular plant.
Because my research is focused on corn, it will be my example and focus for this post. (No shame here!)
Corn's temperature thresholds are 50 °F (10 °C) and 86 °F (30 °C), which theoretically means that corn does not grow when temperatures are below 50 °F, and the maximum growth per day is reached when temperatures near 86 °F. When temperatures soar above 86 °F, the plants can be negatively affected by the excessive heat (this phenomena is captured by calculating EDD, or Extreme Degree Days).
For the method I will be discussing, we use average daily temperatures in our calculations. That is, if we had a day with a high temperature of 70 °F and a low temperature of 45 °F, we would record a value of (70 + 45)/2 = 57.5 °F.
Time for the 4-letter word. Let's do some M-A-T-H
Considering the previous example with the knowledge of our corn peak threshold of 86 °F, if the high temperature would have been 90 °F, we would adjust this particular day's high temperature to 86 °F (because we know that 86 °F is the peak for GDD accumulation for our crop).
This would give us an adjusted average temperature of (86 + 45)/2 = 65.5 °F.
This 65.5 °F is not our final GDD accumulation however, because we have not accounted for our base temperature of 50 °F. This step is done AFTER we calculate our daily average temperature, so our final calculation would be to subtract the base temperature of 50 °F from our daily adjusted average of 65.5 °F.
This would give us a final degree day accumulation of 65.5 - 50 = 15.5 (°F per day).
As another example, which is representative of what most of the state of Kansas experienced in the last few weeks, what if we suppose a day with a high temperature of 51 °F and a low temperature of 42 °F?
Because the high is less than 86 °F, we do not need to adjust the high temperature, and our daily average would simply be (51 + 42)/2 = 46.5 °F. After subtracting our base temperature, we end up with a final degree day accumulation of 46.5 °F - 50 °F = -3.5 (°F per day).
Because "negative growth" is not possible, we assign all negative values as zero. That is, for this daily example, the corn theoretically would not have grown at all. This situation was the case for our corn tillering plots across the state of Kansas from May 7 - May 13. (See below)
This graph shows the daily accumulation of GDD (in °C) based on the calculation we discussed above. The point furthest to the left for each color indicates the date of planting, and the color corresponds to a given location (shown in the key on the right). As you can see, the lines all level off for about a week before resuming their climb, indicating a period when the corn was in a "stagnant state" of growth.
Based on the estimates provided by this graph, we would guess that although Goodland (GLD) and the first Colby plots (COL1/2) were planted more than a week prior to our second round of Colby plots (COL3), the plants would be at similar stages in their growth and development. (Just emerging or "Stage VE" on or around May 20.)
Following growing degree days is very important (particularly if you are tracking crop progress in multiple locations with different planting dates, etc). As a graduate student, I would be lost in trying to target specific stages in each of these locations without checking GDD accumulation.
As mentioned in my previous post, Kansas Mesonet is a great resource for those in Kansas interested in tracking GDD accumulation in their fields. Click here to check it out! There are also a lot of other great apps/resources out there that will do the dirty work for you.
If you are interested in trying to calculate GDD on your own or are looking for codes to automate the process of pulling this information directly from Mesonet, I have plenty of resources to share, so shoot me an email or leave a comment here!