Regular moisture testing can help farmers estimate how quickly corn is drying and decide whether waiting or harvesting is the better option.
As corn reaches physiological maturity, growers face a familiar but important decision: allow the crop to continue drying naturally or begin harvest and remove the remaining moisture mechanically.
The answer depends partly on how quickly grain moisture is declining. Although general drydown estimates can provide a starting point, the most useful figure is often the rate measured in the individual field.
Weather, hybrid characteristics, maturity timing and crop condition can all change the pace of moisture loss. That means a rate observed during one week, in one field, should not automatically be applied to the remainder of the farm.
Start at physiological maturity
Corn reaches physiological maturity when kernel dry matter accumulation is complete. A black layer at the base of the kernel is commonly used as an indicator of this stage, although severe stress can sometimes cause black layer formation before normal grain fill is completed.
The experts at Pioneer advise kernel moisture at physiological maturity is commonly around 30 to 35 per cent, but it can vary according to the hybrid and growing conditions. After maturity, further moisture loss occurs primarily through evaporation from the kernel.
This distinction is important. Before physiological maturity, a decline in the percentage of kernel moisture reflects both water loss and continued accumulation of dry matter. After maturity, moisture readings provide a clearer measurement of field drydown.
Calculate moisture loss per day
A straightforward way to determine a field’s drydown rate is to collect grain moisture readings on two different dates and divide the change in moisture by the number of days between samples.
The calculation is:
- Drydown rate per day = initial moisture percentage minus later moisture percentage ÷ number of days
For example, assume a representative field sample tests at 28 per cent moisture. Eight days later, a second sample tests at 24 per cent.
28 − 24 = 4 moisture points
4 ÷ 8 days = 0.5 moisture points per day
At that observed rate, the field lost an average of half a moisture point per day during the sampling period.
The result should be described as percentage points of moisture per day, rather than simply a percentage. A decline from 28 per cent to 24 per cent is a four-point reduction in grain moisture.
Use representative and repeatable samples
For the calculation to be useful, both moisture readings need to represent the field as accurately as possible.
Samples should be collected from several areas rather than from a single edge, headland or low spot. Avoid relying only on the earliest or latest plants in the field. If different hybrids, planting dates, soil types or areas of crop stress are present, testing those areas separately may provide a more realistic picture.
The same sampling and testing approach should be used each time. Representative ears can be shelled and tested with properly calibrated moisture-testing equipment.
In an Ontario field study, ears from multiple hybrids were sampled repeatedly, hand-shelled and tested with benchtop equipment or a calibrated moisture tester.
Recording the date, moisture level, hybrid, field location and recent weather creates a simple field history that can improve estimates as harvest approaches.
Growing degree units offer another measurement
Farmers can also express drydown in terms of growing degree units (GDUs) required for each point of moisture loss.
The calculation is:
- GDUs per moisture point = accumulated GDUs ÷ moisture points lost
If a field loses four moisture points while accumulating 60 GDUs, the result is:
60 ÷ 4 = 15 GDUs per moisture point
General guidance indicates that drying corn from 30 to 25 per cent moisture may require about 30 GDUs per point according to Pioneer. Moving from 25 to 20 per cent can require approximately 45 GDUs per point. These figures illustrate why drydown commonly slows as fall temperatures decline.
However, GDU accumulation does not explain every change in grain moisture. An Ontario study conducted at 14 field-trial locations found that corn lost an average of one moisture point for every 13.6 GDUs over the entire study. Drydown was much faster during one week than another even though the two periods accumulated similar GDUs, demonstrating that temperature was not the only influence.
Weather can quickly change the rate
Temperature, relative humidity, sunshine, wind and rainfall all influence field drying.
Warm, sunny, windy weather with low humidity generally supports faster moisture loss. Cool, cloudy or rainy weather can slow drydown considerably and may produce days with little or no measurable change.
Research summarized by Purdue University indicates that average moisture loss over the drydown period can range from approximately 0.8 percentage points per day for grain nearing maturity in late August to around 0.4 points per day for grain nearing maturity in mid- to late September. Under favorable conditions, moisture loss can exceed one point per day, while cool and rainy conditions may produce no drydown.
Drydown also slows as grain moisture approaches approximately 15 to 20 per cent. Field moisture loss is relatively linear above that range but tapers as grain approaches equilibrium with the surrounding air.
Growers should therefore treat a calculated rate as a recent field measurement, not a guaranteed forecast.
Hybrid and ear characteristics matter
Two fields exposed to the same weather may not dry at the same rate warns Pioneer.
Hybrid characteristics associated with faster drying include thinner or more permeable kernel pericarps, fewer or thinner husk leaves, earlier husk senescence, looser husks and less coverage over the ear tip. Ears that turn downward can also shed rainfall more effectively than upright ears, which may capture moisture inside the husk.
Of course, planting date and relative maturity can also influence harvest moisture. In the Ontario research, each one-week delay in planting increased the average moisture of the widely represented 93- to 94-day comparative relative maturity hybrids by 1.7 points. Longer-season hybrids also remained wetter at the end of the sampling period than several shorter-season hybrids.
Convert the rate into a harvest estimate
Once an average drydown rate has been established, farmers can make a rough estimate of the time needed to reach a target moisture.
The calculation is:
- Estimated days to target = current moisture minus target moisture ÷ daily drydown rate
If corn is at 24 per cent moisture, the target is 20 per cent and the measured rate is 0.5 points per day:
24 − 20 = 4 moisture points
4 ÷ 0.5 = approximately eight days
This estimate assumes that drying conditions remain similar. A shift toward colder, wetter weather can extend the timeline, while warm and windy conditions may shorten it.
For those farmers in Iowa, the Iowa State University Corn Drydown Calculator provides another option. It uses an initial date and grain moisture reading along with temperature and relative humidity data to estimate drying potential after physiological maturity. Its projections should still be combined with current field measurements, particularly where kernels have been affected by frost or pests.
Do not make the decision on moisture alone
Waiting for additional field drying can lower mechanical drying costs, but the potential savings should be weighed against crop condition.
Fields with stalk rot, poor standability, ear rots, insect damage or a risk of ear drop may need to be harvested before reaching the preferred moisture level. A field that is drying slowly but standing well presents a different risk calculation than one losing stalk strength.
The most practical approach is to combine repeated moisture measurements with weather forecasts, hybrid information and regular crop scouting. Updating the drydown calculation every few days can help growers respond to changing conditions rather than relying on a single seasonal rule of thumb.
Corn drydown is not perfectly predictable. However, consistent sampling can turn a general estimate into a field-specific measurement, giving farmers better information for balancing drying costs, harvest capacity and the risk of leaving mature corn standing too long.