Another May, another planting season! And with putting those seeds in the ground comes a host of things to keep track of - a main one being CROP DEVELOPMENT. If you're planning on adding fertilizer, spraying a pesticide, or selecting the best time to irrigate, understanding where your crop is development-wise will be really important!
Agonomists track crop growth with units called "Growing Degree Days" (GDD)
Every crop has a minimum temperature that it needs to grow, and every crop also has its own maximum temperature, when plant growth is reduced by the heat. Somewhere between these numbers is the optimal temperature, which is the point at which the plant achieves maximum growth potential.
With these three temperatures (minimum, maximum, and optimal), we can build our "cardinal temperatures", similar to our "cardinal directions". This provides a map of how the corn plant is developing based on the temperatures it is exposed to. We use this window to calculate GROWING DEGREE DAYS. GDD can be described as the cumulative number of temperature degrees the plant experiences between its minimum and optimum limits each day.
Each crop has its own cardinal temperatures, but here we will focus on corn which has a minimum temperature (also called "base") of 10 °C or 50 °F, and an optimum temperature of 30 °C or 86 °F. This (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.
For the method I will be discussing, we use average temperatures in our calculations.
Example: Today, we had a high temperature of 70 °F and a low temperature of 50 °F.
The daily temperature would be (70 + 50)/2 giving us an average of 60 °F.
Go time - let's do some MATH
Now is when we need to remember our cardinal temperatures from before. For corn, that was a minimum of 50 °F and an optimum of 86 °F. This means that corn plants will not grow at all below 50, and max out their growth at 86.
Any temperature below 50 °F will have the same effect on GDD as 50 - NONE. Zero GDD added. In the same way, if the temperature goes above 86 °F, we are not gaining any more GDD than we would have at 86 (these temperatures above 86 actually can be used in an EDD or Extreme Degree Day calculation to approximate heat stress on our corn crop). Because of this, the maximum adjusted GDD that corn can accumulate is 36 °F per day. (86 - 50 = 36)
Let's do an example, with temperature adjustments this time!
Example: Today, we had a high temperature of 92 °F and a low temperature of 49 °F.
Before we can take the average, we need to adjust these numbers based on our cardinal temperatures!
Remember that our optimum temperature of 86 is understood as the maximum for growth. We have to adjust our daily maximum temperature to reflect the fact that this higher temperature is not gaining us any more GDD. The 92 °F is changed to 86 °F.
Now we have an adjusted high of 86 °F and a low of 49 °F.
With these numbers, our adjusted daily average temperature is (86 + 49)/2 = 67.5 °F.
We subtract our base temperature of 50 from the adjusted average to provide our daily GDD accumulation with these temperatures. 67.5 - 50 = 17.5 °F per day.
Today, our corn crop accumulated 17.5 GDD.
Let's do another example!
Example: Yesterday, we had a high temperature of 90 °F and a low temperature of 77 °F.
Before we can take the average, we need to check these numbers with our cardinal temperatures!
We still have to adjust our daily maximum temperature because it is higher than our optimum. The 90 °F is changed to 86 °F.
Now we have an adjusted high of 86 °F and an approved low of 77 °F.
With these numbers, our adjusted daily average temperature is (86 + 77)/2 = 81.5 °F.
We subtract our base temperature of 50 from the adjusted average to provide our daily GDD accumulation with these temperatures. 81.5 - 50 = 31.5 °F per day.
Today, our corn crop accumulated 31.5 GDD.
Sometimes, doing these calculations can be annoying or time consuming. Because GDDs are to key to track development in my corn tillering research fields across the state, I use programming/coding to simplify this process.
I have also created some resources to simplify it for you too!
A great tool for Kansas farmers is the Kansas Mesonet, which is a network of weather stations across the state maintained by Kansas State University. The stations take daily and even hourly data including temperature, precipitation, soil moisture, wind direction and speed, etc. As a special feature of their website, this data can be downloaded and used freely! Check out their website here: https://mesonet.k-state.edu/
I have created an online app (CornCalc) that uses your location coordinates to find your nearest weather station, download weather information based on your planting date, and calculate cumulative GDD and precipitation. Access it by clicking the link here: https://rveenstra.shinyapps.io/GDDTracking/ or scan this QR code with your smartphone camera!
I have also created two tutorials in conjunction with this - one to walk you through using the tool I've made, and a more in-depth coding tutorial on how I am downloading the Mesonet data, doing these calculations, and creating an app in the R programming environment. Check them out!