It’s been a wet week for much of Nebraska, further delaying planting and causing growers to consider whether they should move from a full-season corn hybrid to a short-season. The short answer is: Not yet. Not until late May. The longer answer, as late May nears, depends on several considerations (including potential for killing frost in the area and yield/disease/stress potential of hybrids), which are well outlined in a recent article by Bob Nielsen, Extension corn specialist at Purdue University.
Writing in “Hybrid Maturity Decisions for Delayed Planting,” Nielsen notes that “because hybrids appear to decrease their GDD needs with delayed planting, one can plant adapted, full-season hybrid maturities later than otherwise expected.” Research by Nielsen and Peter Thomison of Ohio State University indicates that yields for full-season hybrids may be similar for planting dates up to late May and sometimes into very early June, after which growers may want to switch to a medium maturity hybrid.
Nebraska research also supports staying with a full-season hybrid until late May.
In Nebraska, UNL researcher Angela Bastidas compared yields of early to late maturities planted in mid-May and early-June in Nebraska and found the highest corn yield was with a full-season maturity planted in mid-May. When planting shifted to early June, medium season CRM (97-105 days) hybrids performed best in this study. Short season CRM (80-86 days) hybrids showed the lowest yields of all tested when planted either mid-May or early June.
In Iowa, M.E. Baum et al. 2019 looked at hybrid maturity and planting dates from mid-April to mid-June. They found that planting full-season hybrids mid-April through mid-May resulted in higher yields than shorter RM hybrids; however, with planting dates after mid-May, RM choice did not affect yields significantly.
Corn growth is driven by heat units (GDDs). Nielsen’s research indicates that when planting in warmer conditions in May, corn may require fewer total GDD to reach maturity. In Indiana, they estimated the reduction at 6.8 GDD per day for every day after May 1 that corn was planted. For example, when a hybrid requiring 2700 GDD to reach maturity isn’t planted until May 30, the plant adjusts, reaching maturity at 2,496 GDD, or approximately 200 fewer GDD than if planted in April.
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Nielsen offers a calculator to estimate adjusted GDD requirements for late planting various hybrid GDD requirements.
When considering this information, also consider typical first freeze dates for Nebraska, paying particular attention to dates for 28°F freezes. (Nielsen notes that while lows of 32°F can cause significant leaf injury, “the corn plant usually remains alive and capable of remobilizing non-structural carbohydrates from the stalk tissues to the immature grain. A temperature of 28°F for several hours is considered lethal for corn plants.”)
Growers planting corn into late May also may consider whether to increase the seeding rate to compensate for the later-than-normal date. Research at Midwest universities indicates an increase is not necessary and sometimes the rate can be decreased without affecting yields as soils are warmer and conditions may be more conducive to quicker and more consistent emergence.