Originally published in the Chat 'n Chew Café (5 May 1998)
Also published in the Purdue Pest Management & Crop Production Newsletter (8 May 1998)

Don't Pull the Trigger Yet on Hybrid Maturities

The fearmongers among us are already worrying about the need to switch to earlier maturing corn hybrids as corn planting continues its slow pace in 1998. Some are even speculating how many intended acres of corn will have to be replaced with soybean to accommodate the shortened growing season. My response to this as of early May?………..Phooey! (or words to that effect!)

Recent research conducted by former graduate students Greg Brown (currently with Asgrow Seeds) and Tony Halter (currently with Pioneer Hi-Bred) under the direction of myself and Peter Thomison (Ohio State Univ.) provides insight into the often challenging question of when to switch from "full season" hybrids to early maturing ones as planting is delayed. Greg and Tony investigated the effects of delayed planting on the relationship between corn hybrid development and thermal time. The bottom line of their research conducted at 12 environments throughout Indiana and Ohio was that pollination and especially grain maturation occurred in less thermal time as planting was delayed.

The practical consequence of this research is that a hybrid will require fewer GDD to mature when planted later and later. In fact, the rate of decrease in season-long GDD is about 6.5 GDD per day of delayed planting beyond May 1. For example, a 2700 GDD hybrid planted May 31 (i.e., 30 days beyond May 1) will likely mature in 2505 GDD (30 x 6.5 = 195 fewer GDD).

Based on this research, we can more accurately determine when to "pull the trigger" and switch to earlier maturities as planting is delayed. Table 1 lists "normal" full-season hybrid maturities (my opinion) for geographic areas of Indiana and their corresponding approximate GDD ratings from planting to kernel black layer (see my accompanying article, A Primer on Hybrid Maturity Ratings). The hybrid CRM (comparative relative maturity) and GDD ratings illustrated in the table correspond closely with those used by Pioneer Hi-Bred Int'l, Inc.

The first conclusion that can be made using the results of our research is that there is no need to consider switching to earlier maturing hybrids anywhere in Indiana until early June plantings. Hybrids with relative maturities and GDD listed in Table 1 will likely mature safely when planted anytime during the month of May in Indiana.

Once planting is delayed into June, the ever-increasing reduction in growing season length (measured by available GDD) finally begins to surpass the full-season hybrids' abilities to adjust their developmental GDD needs. Table 2 lists the hybrid relative maturity values that would be appropriate for planting during the first half of June. These values are estimated according to the hybrids' GDD adaptations to delayed planting and the available length of growing season between the planting period and the average first killing fall frost.

If planting is delayed into early June, the northern and eastcentral areas of the state are the first to justify switching to earlier maturing corn hybrids (Table 2). By the second week of June, hybrid maturity switching would be advised for all of the northern two-thirds of the state.

Note: Be aware that there are no agreed upon standards within the seed industry for assigning relative hybrid maturities. The hybrid CRMs listed in Table 2 correspond most closely to those used by Pioneer Hi-Bred Int'l, Inc. Given Pioneer's large market share in seed corn, almost every other seed corn company can likely tell you which of their hybrid maturities correspond to Pioneer hybrids with the CRMs listed in Table 2.

For other information about corn, take a look at the Corn Growers Guidebook on the World Wide Web at http://www.agry.purdue.edu/ext/corn/

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