February 18, 2003

Ag Engineer Offers Tips for Grain Storage and Drying During Winter and Spring

High-moisture grain placed into storage this past fall and early winter may need to be dried before temperatures moderate, a North Dakota State University agricultural engineer says.

"Corn at 24 percent moisture content has an allowable storage time of about 130 days at 30 F, but only about 40 days at 40 F, and 15 days at 50 F. Corn at 24 percent moisture content or higher will need to be removed from the bin and dried before the top or sidewalls of the bin are heated by the sun to temperatures that will lead to spoiled grain," says Ken Hellevang of the NDSU Extension Service.

Spring drying of corn using a natural-air or low-temperature system will take about 35 to 40 days using an airflow rate of 1.25 cubic feet per minute per bushel (cfm/bu) starting in early April, when outside air temperatures average about 40 degrees or warmer, Hellevang says. The maximum corn moisture content that should be dried using an airflow rate of 1.25 cfm/bu is 22 percent. The allowable storage time of 22 percent moisture corn is about 60 days at 40 F and 30 days at 50 F.

Hellevang notes that natural-air and low-temperature drying is not efficient at temperatures below about 40 F because of the small amount of moisture picked up by cold air. "The water-holding capacity of air is related to the air temperature. A 20 degree reduction in temperature cuts the water-holding capacity of the air in half, which doubles the drying time," he explains.

Using the moisture-holding capacity of air at 70 F for comparison, air at 50 F will hold or pick up 48 percent as much moisture as air at 70 F, and at 30 F the air will only pick up about 22 percent as much moisture. The estimated drying time for 21 percent moisture corn using an airflow rate of 1.25 cfm/bu is 36 days at 47 F and 70 days at 27 F.

"The average relative humidity during November to March is about 75 percent, so corn will only dry to about 19 percent using a natural-air system," Hellevang says. "Adding heat to the system will reduce the relative humidity, which reduces the final grain moisture content, and reduces the drying time some."

The average March temperature is 24 F. Warming air by 5 degrees with an airflow rate of 1.25 cfm/bu will reduce the final corn moisture content to about 14.5 percent and reduce the drying time from about 70 days to about 50 days. The drying time will be almost two months in length.

Heating the air by 10 degrees will reduce the final corn moisture content to about 12.5 percent, and reduce the drying time to about 41 days. "Because corn is usually marketed at 15.5 percent moisture, the corn is over-dried just by warming the air 10 degrees. The drying time is only reduced from 50 to 41 days," Hellevang says.

"It is best for the grain to be at room temperature to accurately measure grain moisture content. Electronic meters are affected by grain temperature, so a temperature adjustment must be added to the moisture reading to get an accurate measurement," he says. The adjustment must be done manually, unless the meter automatically measures the temperature and makes the adjustment.

At a grain temperature of 40 F, the temperature correction may be about 2.5 percent. If the meter reading indicates a moisture content of 20 percent, the adjusted moisture content is 22.5 percent. Meters will not be accurate with grain temperatures near or below freezing. "Warm the sample to room temperature in a sealed container to obtain the most accurate value," Hellevang says.

"Grain coming from a high-temperature dryer will be drier on the exterior of the kernel than on the interior. Since many moisture meters will be affected by the exterior moisture content of the kernel more than the entire kernel, the sample should be allowed to equilibrate in a sealed container for at least 12 hours before the moisture content is taken," he says. "The difference between this reading and that coming directly from the drier can be used to estimate the amount that the meter is being fooled. Also, remember to apply the temperature adjustment if the grain sample is warmer than the meter standard. A reduction in moisture content of more than 1.5 percent may be needed if the grain temperature is near 100 F."

For more information, a presentation on drying and storing corn and soybeans is available on the Web at http://www.ag.ndsu.nodak.edu/abeng/postharvest.htm "

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Source: Ken Hellevang, (701) 231-7243, kjh-eng@ndsuext.nodak.edu
Editor: Tom Jirik, (701) 231-9629, tjirik@ndsuext.nodak.edu