Corn harvest is fast approaching. This year’s corn maturity in Iowa is about 5 to 10 days behind normal. With field dry down occurring in late September and October this year, there is the potential for a later harvest of corn at a higher moisture content. The rule of thumb has been that corn dries at a rate of 0.5 to 1.0% 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.

Water loss from corn kernels is divided into two phases. The first phase is kernel moisture loss before maturity in which water loss is related to accumulation of dry matter in kernels. After black layer, moisture loss occurs through evaporation of water from the kernel surface.

From 2014 through 2016 we 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. Our data indicates that the average dry down rate during the entire dry down period is 0.58% per day. 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.

We did not find significant differences in dry down rates among hybrid maturities during the three years of the study. 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 dataset, corn field dry down 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. Low temperatures (<40 degrees) and/or high humidity (>90% relative humidity) for prolonged periods (>2 or 3 days) can significantly delay dry down.

Corn grain dry down in the field is expected to be at a rate of 0.69% per day in the first 20 days following maturity. This can be used to help schedule harvest of fields based on when fields and hybrids reach maturity.