It seems pretty straight forward. Counting the growing degree units during the season should be a simple math equation and not controversial at all. It’s not, though. The big question is WHEN do you start counting? Do you count from emergence to black layer? Should you count from planting to black layer instead? It doesn’t seem like a big difference, but when you consider it takes roughly 100 to 120 GDUs for most corn plants to emerge, that could make the same hybrid look like it needs 2650 GDUs when it really needs 2770 GDUs to mature. This is especially important in a late-planted situation where maturing before frost hits is non-negotiable.
The other challenge here is seed companies often only track hybrids for a few seasons before they are put on the market. After that, there is so much data to collect, breeders often spend their time on disease ratings and not necessarily on maturity. In my experience, the proof is evident in the fact that hybrids that have been on the market for two or three years RARELY see their GDU ratings change. So if a hybrid comes out in a cool, cloudy year like 2019 versus during a warm, sunny year like 2021, there is bound to be a difference in how many GDUs it takes for that hybrid to reach black layer.
In summary, GDUs are a flawed method to compare hybrids, but this system is about the best the industry has to offer. In the future, there will be far better ways to account for the time from planting to maturity, including measurements of sunlight and temperature variance throughout the day. Do you think a day that was cloudy and cool until 5 p.m. with a temperature reaching 86 degrees for 5 minutes would push corn hybrids to maturity as well as a day that was 86 degrees for 8 hours and sunny the whole time? Not a chance.