Late planting and weather that continues to be cooler than normal into August has many wondering if the corn and soybean crops will reach maturity and harvest moisture within a reasonable time this fall. Crop conditions remain good for both crops, but crop development, including pod formation and filling in soybean and grain fill in corn, remains well behind normal. Corn is 10 days to 2 weeks behind normal, and soybeans are 2 to 3 weeks behind normal. The number of days behind will “stretch” as the weather cools, so late crops get even later. Ten days behind in mid-August will be become 15 or 20 days behind in mid-September, even if temperatures are normal.
We have often pointed out that late-planted corn tends to require fewer growing degree days to reach maturity than does early-planted corn. That’s often reflected in lower yields, as the crop experiences stress during the high temperatures and dry weather we often experience in mid-summer. This year, however, with temperatures generally below normal in recent weeks, we do not expect to see this accelerated development. Instead, we are seeing that corn development is following closely the normal number of GDDs required to reach each stage. This means less chance of premature death and so a better chance to fill grain completely. But for late-planted corn it also means late maturity.
How late can we expect the corn crop to mature? If we assume normal GDD accumulations in August and September, corn planted in the Champaign area on May 1, May 15, and May 31 will accumulate about 3,020, 2,850, and 2,530 GDD, respectively, by the end of September. If we further assume that a mid-season (111-day RM) hybrid needs 2,700 GDD from planting to maturity regardless of planting date, we can expect corn planted on these dates to reach maturity (black layer, about 32% grain moisture) about September 5-10, September 15-20, and mid-October, respectively. The crop seems to be on course to do this: corn planted in early May is at stage R3 (milk stage) now while corn planted in late May or early June is just finishing pollination. This reflects ongoing cool temperatures so far in August, and if these continue, maturity dates will be even later.
With a 50% frost date of about October 20 here, we would expect a mid-season corn hybrid planted in early June to mature before frost, if frost does not occur before its normal (50%) date. But drydown slows quickly as we move into October, and even early-planted corn will dry slowly after maturity unless September is unusually warm. Corn planted in mid-June (some fields in central Illinois are still not pollinated) is unlikely to mature before frost if temperatures are normal and frost comes at its normal time.