Agro-Climate Blog


 Growing Degree Days Update 

Dr. Eric Hunt

As we enter the middle of July and the critical stages for development of corn, it is instructive to see how the crop is progressing compared to recent years. One way of doing that is looking at the statewide numbers of the Crop Progress Report, which are compiled by statewide offices throughout the major corn growing region of the country. A quick peek at yesterday’s report (See Table 1) shows that the corn is starting to silk over much of the Corn Belt region, with percentages that are generally above to well above the 5-year average. 


Table 1. From the July 9th Crop Progress Report. Numbers courtesy of the USDA

Another way to determine development relative to normal is to look at the accumulated growing degree days (GDD’s) by each crop reporting district. For a quick refresher, a growing degree day is essentially the amount of “heat” accumulated over a base average temperature threshold that is determined by crop. For crops like corn, a base average temperature of 50F is used. As an example, on a day when the high is 86F and the low is 66F, your daily average temperature is 76F, which is 26 GDD’s for the day. More information on growing degree days can be found here from the University of Illinois.

Figure 1 shows the map of accumulated GDD’s since April 15 for the crop reporting districts (CRD’s) in the U.S. Corn Belt. As usual, the highest accumulations are in Missouri and southern Illinois and the lowest accumulations are in the northern and western fringes.


 Figure 1. Accumulated GDD’s for all 60 crop reporting districts in the Corn Belt. Numbers courtesy of the Midwest Regional Climate Center (MRCC) in Champaign, IL. 

However, with the exception of outstate Nebraska these accumulations are not only above a long-term normal (Fig. 2), in many cases they are some of the highest in the past 40 years (Fig 3). This is particularly true in northern Missouri, the southern three-quarters of Illinois and Indiana, and Ohio. Elsewhere, the GDD accumulations are generally above normal but not necessarily at record accumulation pace. Note that the current numbers were obtained from the MRCC and the numbers used to derive the averages came from gridded averages of Climate Forecast System Reanalysis (CFSR) data, but agreement between the datasets is very high. 

GDD departure from normal

Figure 2. GDD departure from normal compared to 1979-2013 Climate Forecast System Reanalysis (CFSR) data. 


Current GDD rank

Figure 3. GDD accumulation rank from April 15th- July 8th of 2018 when compared to the same period from 1979-2017. 

 Next update: My thoughts on how the season has progressed to this point as well as a preview of the next Crop Yield forecast report, which will be available online for a modest price. 


For further questions email Eric at