The 2017 corn harvest has begun in Iowa. This year’s corn maturity is running about five to 10 days behind normal. With field drydown occurring in late September and into October, there is potential for a later harvest of corn at a higher moisture content.

With corn reaching maturity a little later than usual, some farmers may have to choose between waiting later than they’d like to get into their fields or harvesting a crop with higher moisture content, which makes drying the grain even more important.

Schedule fields for harvest based on maturity
Results from new ISU research show the drydown of corn grain in the field can be expected to occur at a rate of 0.69% per day for the first 20 days following maturity. “This information can be used to help schedule the harvest of fields based on when your fields and hybrids reach maturity,” says Mark Licht, Iowa State University Extension agronomist.

The rule of thumb has been that corn dries down in the field at a rate of 0.5% to 1% per day in September, 0.25% to 0.5% per day in October, and almost no drying occurring in November. Of course, these rules of thumb can change with favorable or unfavorable weather conditions.

To update that thumb rule, Licht, along with Sotirios Archontoulis, an assistant professor of integrated cropping systems at ISU, conducted the recent study on drydown of corn hybrids in the field.

Consider how corn drydown occurs
Water loss from corn kernels is divided into two phases, Licht explains. The first phase is kernel moisture loss before maturity in which water loss is related to accumulation of dry matter in kernels. After corn reaches the black layer stage, moisture loss occurs through evaporation of water from the kernel surface. When the black layer forms in the corn kernel, that’s when the crop reaches physiological maturity or “black layer.”

From 2014 through 2016, ISU researchers collected corn ears from a central Iowa field that had four dates of planting and four hybrid maturities. Corn ears were collected on a weekly interval in September and October. The data indicates that the average drydown rate during the entire drydown period is 0.58% per day (see graph). However, this drying rate is not constant. During the first 20 days, moisture is lost at a rate of 0.69% per day, while the next 20 days it drops to 0.44% per day.

DRYING DOWN: Average grain moisture drydown (blue line) across four hybrid maturities, four dates of planting and three years near Ames in central Iowa. Horizontal dashed line represents 15.5% kernel moisture; open circles are actual data.

Corn hybrid, weather influence drydown
“We did not find significant differences in drydown rates among hybrid maturities during the three years of the study,” says Licht. “On the other hand, the kernel moisture at maturity, which also influences how fast harvestable moisture is reached, ranged from 28% to 38%. It is known that kernel moisture at maturity can be different depending on genetics and growing-season weather conditions. For instance, environmental stresses during the grain filling period can cause lower accumulation of dry matter and higher kernel moisture at maturity. The opposite may be true in high-yielding environments.”

Based on this data set, corn field drydown to 15.5% moisture may take up to 35 days when kernel moisture at maturity is high (38% to 36%), while when it is low (30% to 28%), it may take about 25 days. It is important to note that these estimates are meant to represent normal fall conditions, says Licht. Low temperatures (less than 40 degrees F) or high humidity (greater than 90% relative humidity) for longer than two to three days can significantly delay drydown.

Corn grain drydown in the field can be expected to be at a rate of 0.69% per day in the first 20 days following corn maturity, based on results of the study. This can be used to help schedule harvest of fields according to when fields and hybrids reach maturity.

As a final note, ISU specialists emphasize the importance of cooling and drying grain as soon as possible after harvest. Failure to do so opens the door for mold to set in. They recommend to be sure cooling fans on stored grain are running on days when dew points consistently drop into the 40s and below.