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Cafe. Even though much of the state has experienced some drying out after the significant rainfall of recent weeks, there will still likely be a fair amount of corn acreage not planted until early June. Growers contemplating such delayed planting are also wondering 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 why we worry about delayed planting of corn, of course, 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.
Hybrid Maturity Rating Systems. 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).
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.
Delayed Planting and Hybrid GDD. Peter Thomison (Ohio State University) and I recently finished a four year study at several locations in Indiana and Ohio evaluating the effects of delayed planting on GDD needs of a group of Pioneer brand corn hybrids. The results of that study suggest that a typical corn hybrid's GDD requirements decrease about five GDD per day of delayed planting from late April - early May through at least the early part of June. This means that a 30 day delay in planting may result in a hybrid maturing in 150 fewer GDD (30 days times 5 GDD per day).
Recommendations. Historical weather records maintained by the agricultural climatology folks 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. Using these estimates of the length of the growing season plus the effects of delayed planting on hybrids' GDD needs, I offer the recommendations in Table 1 relative to delayed planting and hybrid maturity selection. The recommendations are listed by Crop Reporting District (CRD) in Indiana.
| Table 1. Approximate safe hybrid maturities for corn planted during several planting periods in areas of Indiana. | |||||
| Planting period | |||||
|---|---|---|---|---|---|
| Area of state | Average frost date | May 26 to June 1 | June 2 to June 8 | June 9 to June 15 | June 16 to June 22 |
| Hybrids no later than CRMs of.... | |||||
| Northwest | Oct. 6 | 103 | 97 | none | none |
| Northcentral | Oct. 6 | 103 | 97 | none | none |
| Northeast | Oct. 6 | 103 | 97 | none | none |
| Westcentral | Oct. 13 | 115 | 113 | 106 | none |
| Central | Oct. 13 | 115 | 110 | 106 | none |
| Eastcentral | Oct. 6 | 103 | 97 | none | none |
| Southwest | Oct. 20 | 118 | 118 | 118 | 113 |
| Southcentral | Oct. 13 | 118 | 115 | 113 | 106 |
| Southeast | Oct. 13 | 118 | 115 | 113 | 106 |
| Important Note: Since seed companies don't always agree on rating hybrid GDD to maturity, these recommendations are conveyed in terms of Pioneer brand comparative relative 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. | |||||
| Note: Where the word 'none' occurs in the table, growers are advised to consider switching to crops other than corn because suitable hybrid maturities are either inappropriate or unavailable. | |||||
| Caution: When considering planting hybrid maturities earlier than about 106 CRM in southern Indiana, be aware that such early maturity hybrids often do not have suitable tolerance to diseases often encountered in southern Indiana. | |||||
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