Most corn producers are aware of growing degree days as a way to both monitor the accumulation of heat units favorable to growth and estimate when a crop may reach important developmental milestones. Briefly, to calculate the GDD value for a particular day, the daily high and low temperatures (between and including a low of 50 and a high of 86 degrees) are added together, divided by 2, with 50 subtracted from the total.
In this age of both "big data" and the ready availability of both free and fee-based web-tools (including GDD tools), some corn producers and agronomists have begun more closely monitoring GDD accumulation. One topic of conversation this growing season has been about how corn development has seemed to lag behind what one would have expected based on GDD accumulation.
Table 2.1 in Chapter 2 of the Illinois Agronomy Handbook lists the approximate GDDs that a hybrid that requires 2700 GDDs to reach physiological maturity needs in order to reach different developmental growth stages. The GDD-growth stage approximations listed there are based on a 'normal' planting time.
Each year Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) personnel sow a corn planting date trial. We can use the plants in this trial to follow growth and development of the crop over the 2015 growing season, making comparisons to Handbook Table approximations.
This year the two corn hybrids planted in this trial have relative maturities of 109 and 112 days, requiring 2630 and 2760 GDDs, respectively, to reach physiological maturity (black layer). While the four planting dates were April 1, April 21, May 12 and June 4.
These planting dates are meant to bracket, on both the early- and late-side, the typical planting dates in the Monmouth area. In a typical year, April 1 is considered a little too early to plant corn as the risk of a killing frost is high. Indeed, corn planted April 1 was exposed to below-freezing temperatures on both April 21 and 23. April 21 is a more typical planting time for this area, while the May 12 and especially the June 4 planting dates are considered later than recommended.
According to Purdue University Corn Agronomist Dr. Bob Nielsen, leaf stage prediction is most reliable up to about the V6, or six-leaf-collar growth stage when calculating GDDs using soil temperatures. This is because up to this growth stage the growing point remains below ground. One would then use air temperatures to calculate daily GDDs after V6, when the growing point has moved above ground.
Plants were staged in this trial on May 18, May 29 and June 8. A comparison can be made between the actual growth stages of the plants versus GDD-approximated growth stages listed in the Handbook table (Table). While the growth of the April 1 planted corn is three leaf stages behind the Handbook table approximations, this is not that surprising. Not only was this outside the 'normal' planting time, frost-damaged plants had to regrow from below-ground. What might be a little more surprising is that the corn planted on April 21 is two leaf stages behind Handbook table approximations. Perhaps some physiological damaging developmental delay may have taken place even in those plants that appeared to be outwardly unaffected by the freezing temperatures. Perhaps we just had one too many cloudy days in May.
Regardless, the more data that is collected by following crop development in these sorts of trials, the more will be learned about the relationship between GDDs and crop development.
Illinois Agronomy Handbook - Chapter 2: Corn - By Dr. Emerson Nafziger
Use Thermal Time to Predict Leaf Stage Development in Corn –April 2014. Dr. Bob Nielsen, Purdue University Agronomist