Persistent rains have caused major delays in corn planting across Ohio. According to the USDA/NASS as of Sunday May 1, corn was 1 percent planted, which was 60 percent behind last year and 32 percent behind the five-year average. The weather forecast for the next week indicates the likelihood of more rain so it probable that many soggy fields will not be drying out soon.

Given this outlook, is there a need to switch from full season to shorter season hybrids? Probably not — in most situations full season hybrids will perform satisfactorily (i.e. will achieve physiological maturity or "black layer" before a killing frost) even when planted as late as May 20-25, if not later in some regions of the state.

Results of studies evaluating hybrid response to delayed planting dates indicate that hybrids of varying maturity can "adjust" their growth and development in response to a shortened growing season. A hybrid planted in late May will mature at a faster thermal rate (i.e. require fewer heat units) than the same hybrid planted in late April or early May).

In Ohio and Indiana, we've observed decreases in required heat units from planting to kernel black layer, which average about 6.8 growing degree days (GDDs) per day of delayed planting. Therefore a hybrid rated at 2800 GDDs with normal planting dates (i.e. late April or early May) may require slightly less than 2600 GDDs when planted in late May or early June, i.e. a 30 day delay in planting may result in a hybrid maturing in 204 fewer GDDs (30 days multiplied by 6.8 GDDs per day).

There are other factors concerning hybrid maturity, however, that need to be considered. Although a full season hybrid may still have a yield advantage over shorter season hybrids planted in late May, it could have significantly higher grain moisture at maturity than earlier maturing hybrids if it dries down slowly. Moreover, there are many short-to mid-season hybrids with excellent yield potential. Therefore, if you think you may end up planting in late May, consider the dry down characteristics of your various hybrids. During the past two years we’ve seen a wide range of drying conditions. In 2010, some mid- to- full season hybrids had grain moisture levels at harvest similar to those of short season hybrids because of rapid dry down rates. However in 2009, cool, wet conditions after maturity slowed dry down and major differences in grain moisture at harvest were evident between early and full season hybrids.

Late planting dates (roughly after May 25) increase the possibility of damage from European corn borer (ECB) and warrant selection of ECB Bt hybrids (if suitable maturities are available). In past OSU studies, Bt hybrids planted after the first week of June consistently outyielded non-Bt counterparts even at low to moderate levels of ECB. Since many corn growers will be planting stacked hybrids this year, which include Bt resistance for ECB, this may be a non-issue unless there’s a need to switch to earlier maturing hybrids.

For more information on selecting corn hybrids for delayed planting, consult "Delayed Planting & Hybrid Maturity Decisions," a Purdue/Ohio State University Extension publication available online.

Nielsen, R.L. and P.R. Thomison. 2003. Delayed Planting & Hybrid Maturity Decisions. Purdue Univ. Cooperative Extension Publication AY-312-W.