Late planting and cooler conditions have delayed corn harvest this year. However, some fields are starting to be harvested as of last week. Grain moisture loss is typically linear from physiological maturity which is around thirty percent down to 20 percent moisture, and then the rate of moisture loss slows. The exact rate depends on hybrid, average daily temperature and humidity. High temperature and low humidity allow moisture losses to accumulate quickly compared to cool wet and high humidity conditions. For example, the average daily dry down rate in late August may be 0.8 percentage point per day for corn grain, but in late September, because of cooler temperatures, the rate may decline to 0.4 percentage points per day.
Some growers choose to field dry corn allowing moisture to decline. Field drying of corn is best balanced when judging stalk quality which will insure that the crop will remain harvestable compared to the drying costs or dockage that can occur. Drought stressed corn in areas of the field will dry down earlier than other parts of the field. Drought and disease can cause the corn plants in the area to form a kernel black layer earlier, leading to plants to dry ahead of others within the field. There will be a lot of spatial moisture variability across the field during harvest. This complicates grain drying. When selecting different hybrid maturities, Dr. Bob Neilsen, Purdue Corn Specialist, indicates that the seed industry uses hybrid maturity ratings to predict the grain moisture content at harvest, but the maturity ratings may vary among companies. Typically, a general rule of thumb is for two hybrids that vary in one day maturity often will vary about one-half percentage point of grain moisture, if planted on the same day.
When good drying conditions exist, there is not much difference in hybrid drying rates. However, under poor drying conditions, kernel and husk characteristics become influential in drying rates. Characteristics include the pericarp of the kernel, thickness of the husk, tightness of the husk and whether the ear is upright or declines, to name a few.