Grain and Forage Center of Excellence, University of Kentucky

Warmer, drier air will dry corn grain faster than cooler and/or wetter air whether the corn is in the bin or still in the field. The past two seasons are demonstrating those extremes. In 2019, corn dried quickly in the field. In fact, it dried so quickly that most farmers did not need to run grain driers. In contrast, 2020 has been a very different season in that respect. Corn is drying slowly in fields across Kentucky this year.


Farmers are asking whether nitrogen application, foliar fungicide application, husk tightness and/or hybrid genetics are causing this corn to dry so slowly. While each of these could be factors, the overwhelming factor in corn drying this year is the weather. Once black layer or physiological maturity occurs, water loss occurs from evaporation through the kernels. So, simple physics dictates how quickly kernels will dry in the field. Warmer, drier air will help the kernels evaporate water more quickly. Bright sunshine and gentle winds further enhance field drying. The farmer uses the same principles when applying heated air for grain drying. That air is usually above 100°F, dry and blown at a high rate of fan capacity. We do not run cold, wet air through a grain drying system. The same relationship is true for mature corn in the field. The limits of drying in both situations is dictated by the equilibrium moisture content, which is shown for different temperature and relative humidity levels in Table 1.


From September 1, 2019 to October 8, 2019 at Spindletop Farm in Lexington, Kentucky, there were 29 days where the maximum temperature was at or above 85°F. This year, over that same time span, there were zero (0) days with a maximum temperature over 85! The contrast is even greater. In 2019, there were 10 days with a maximum temperature over 95°F. In addition, in 2020, Spindletop Farm received almost 1.5 inches more rainfall. So, 2020 has been cooler AND wetter than 2019 … and the grain drying process reflects those differences.

At Spindletop Farm, we watched corn dry 0.7 points in one week (25.7 to 25.0% in seven days) because of the weather. Then, it dropped to 21.3% in four days because of warmer, drier weather. The following Monday, it was back up to 23.0% because of rainfall over the weekend. Corn grain moisture in the field can drop or rise depending on the weather. If humidity levels remain high (greater than75%) for extended periods in October when the average temperature is 60, corn will not dry in the field or bin below 16% (Table 1).


Farmers face a challenge. Do we wait for this corn to dry in the field? Do we harvest now and spend more on drying the corn than we expected? If the bins are full, do we harvest now and take a price dockage at the grain buyer? If corn stalks are weak, then those fields should be harvested. If stalks are strong, then the question is harder to answer and depends on the farmer's risk tolerance. Are you willing to let it ride a few weeks and see if moisture drops? Then, leave it in the field. Would you rather get the corn harvested when you can and avoid the risk of more crop loss? Then, get it harvested.


Whatever your risk tolerance, make sure you know the risks. Today is probably the best quality that mature corn will be. Corn in the field will not improve in quality the longer it stays in the field. Alternating weather that dries grain and then adds water back to the grain usually results in lower test weights, more shattering and poorer grain quality. Temperatures are likely to progressively get cooler. While there are fluctuations in temperature day to day, historically, we are getting into cooler temperatures. In addition, day length is getting shorter, so there will be less sun to help with the drying. Each day is trending towards slower and slower rates of grain drying in the field.

The forecast this week involves some temperatures above 70°F, but the odds of getting days above 70 in future weeks are less and less. Perhaps farmers give corn a few more days this week to dry and then harvest as many acres as possible before the next rounds of rain. Waiting for all the acres of corn to reach 16% moisture or less is probably not likely to occur without severe degradation of grain quality.

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