As more thunderstorms dumped more rain on Indiana crop acreage this past week, many Indiana corn growers continue deliberating on the decision of when to switch from fuller season corn to shorter season corn. Some are wanting to forego corn planting and switch to soybeans as planting is further delayed.
The reason, of course, why we worry about delayed planting of corn is not so much the yield losses associated with delayed planting, but more so the ultimate question of whether a corn hybrid will even mature before a killing fall frost. While I've talked about this issue several times already this spring, it's deserving of one more discussion since it is becoming obvious that quite a few Indiana acres are going to planted extremely late.
The traditional relative maturity rating of a hybrid (i.e., 'days to maturity') does not help us determine whether a hybrid will beat a killing frost. The 'days to maturity' rating of a hybrid is simply a method to compare relative grain moisture contents of hybrids at harvest and does not refer to absolute calendar time from planting to maturity.
The best, albeit not perfect, characteristic of a hybrid that helps us make this decision is the estimated GDD required for a given hybrid to reach physiological maturity (kernel black layer). The term 'GDD' is the abbreviation for Growing Degree Days and refers to the amount of heat accumulated over a period of time. Related terms used by the seed industry include Heat Units (HU) and Growing Degree Units (GDU). Historical weather records maintained by the Midwest Ag. Weather Service Center at Purdue give us the ability to estimate the available GDD from particular planting dates to average fall frost dates for each Crop Reporting District in Indiana.
One of the reasons hybrid GDD ratings are not perfect is that seed companies do not all necessarily follow the same procedures for determining GDD to maturity. Another reason is that GDD from planting to maturity varies somewhat from year to year AND as planting is delayed.
Peter Thomison (Ohio State University) and I have been evaluating this issue for the past four years at several locations in Indiana and Ohio with a group of Pioneer brand corn hybrids. Based on that field research, the following recommendations can be made regarding delayed planting and hybrid maturity selection. The recommendations are listed by Crop Reporting District (CRD) in Indiana.
Important Note: The hybrid maturities are defined in terms of the rather nebulous 'days to maturity' that most seed companies use to describe corn hybrid maturity. Since seed companies don't always agree on rating hybrid GDD to maturity, the following recommendations are couched in terms of Pioneer brand hybrid maturity ratings (CRM). I am NOT promoting Pioneer Hi-Bred International, I am simply using these examples for comparison sake in order for growers to identify those hybrids from other seed companies that may be of similar maturity.