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Corn+Soybean Digest

Natural-Air Dry Grain Now

Now is the time to start spring natural-air drying grain.

"Start natural-air drying corn when the average air temperature is about 40° F," says Ken Hellevang, North Dakota State University Extension Service agricultural engineer and grain-drying expert. "This will be when the nighttime temperature is at least in the upper 20s and the daily high temperature is at least in the 50s. The moisture-holding capacity of colder air is limited, so the rate of drying is slow and drying efficiency is poor at colder temperatures."

The maximum initial corn moisture for natural-air drying using an airflow rate of at least 1 cu. ft./minute/bu. (cfm/bu) is 20% this year due to the increased potential for deterioration with damaged or immature kernels.

The expected drying time for corn is about 45-50 days using an airflow rate of 1 cfm/bu. Drying time is proportional to the airflow rate, so at an airflow rate of 1.25 cfm/bu, the drying time is about 35-40 days. Adding heat will change the final corn moisture but will change the drying speed only slightly.

Ensure that the fan's airflow rate is adequate by checking fan charts or estimate airflow by using the fan selection program available at

The average temperature and relative humidity for April in eastern North Dakota is about 40° F and 70%, respectively. The air will be warmed 3-5° as it goes through fans being used for drying corn at a static pressure of 4-6 in. After warming due to the fan, the air is about 45° and relative humidity is about 60%. This air will dry corn to about 14-14.5% moisture.

This year, early April temperatures have been warmer and drier. The temperature is averaging about 45° and the relative humidity is averaging about 60%.

If temperatures cool to an average of about 30°, the fans can be stopped. Wait until the temperature again averages at least about 40° before starting the fans. Cool the grain by operating the fans at night or other cool periods before shutting off the fans to extend the storage life of the grain.

Some people are concerned about shutting fans off because that leaves a drying front in the grain. The drying front is the area in the grain mass where the drying is occurring. The dry air comes in contact with wet grain at the bottom of the drying zone, picks up moisture until it comes into equilibrium with the grain in the drying zone and then carries that moisture through the wet grain above the drying zone and out of the bin.

"There is nothing magical about the drying front or zone," Hellevang says. "The grain and the drying zone will be in the same condition several days later when the fan is started again."

Fans should be shut off when rain falls for more than a few hours and during fog. Drying is not occurring during these times, so the only benefit of operating the fans is to cool the grain. Some grain rewetting will occur if operating the fans during wet conditions. The fans can be started again when the air is drier.

Here are Hellevang's recommendations for drying other crops:

Use an airflow rate of at least 1 cfm/bu to natural-air dry up to 16% moisture soybeans. The expected drying time with this airflow rate will be about 50 days. The allowable storage time for 18% moisture soybeans is only about 40 days at 50°, so use a minimum airflow rate of 1.5 cfm/bu to natural-air dry 18% moisture soybeans.

The air will be warmed about 5° as it goes through fans being used to dry wheat at a static pressure of about 6 in. After warming due to the fan, the average April air is about 45° and relative humidity is about 60%. This will dry wheat to about 14% moisture.

May is typically warmer and drier than April, so wheat will dry to about 13% moisture by mid-May. The estimated time to dry 17% moisture wheat using an airflow rate of 0.75 cfm/bu is about 40 days at 47° F. Because this year's air temperature has been warmer and the relative humidity lower, now is the time to start natural-air drying wheat.

Operating the fans during the warmer and drier daytime hours will permit drying the wheat to a lower moisture content. For example, if the temperature is 55° F and the relative humidity is 55%, the wheat will dry to about 13% moisture in about 30 days. However, if the warmer and drier air is obtained by operating the fans from 7 a.m. to 9 p.m., then drying the wheat will take about 50 days due to the reduced fan operation time each day.

Waiting until mid-April, when temperatures are warmer and the air is drier, to start drying wheat generally is advisable. Adding supplemental heat that warms the air 3-5° permits drying at a higher humidity but will approximately double the cost of drying.

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