September 28, 2007

Whether you're storing wet grain for a short time or dry grain for a long period, it's important to control grain temperatures, a University of Nebraska-Lincoln agricultural engineering specialist said.

Both wet grain and molds respire and give off heat, said David Shelton, extension agricultural engineer at the Haskell Agricultural Laboratory near Concord.

"Aeration is key to keeping the grain cool and to slow mold growth," Shelton said. "Properly aerated grain can be held safely about four times longer than grain that is not aerated."

Aeration is the movement of low volumes of air through stored grain. The primary objectives of aeration are to keep the grain at a seasonally cool temperature, which is within 10 to 15 degrees of the average monthly ambient air temperature, and to maintain a relatively uniform temperature within the grain mass, he said. This should be no more than about a 10 degree difference in temperatures from one part of the bin to another.

Temperature differences within the grain mass create convection currents that can move and concentrate moisture in the top center of the bin.

"The first indication of trouble usually is damp or tacky feeling kernels at the grain surface, followed by the formation of a crust," he said.

Grain mass temperature needs to be controlled throughout the year. It is especially critical to cool grain from warmer harvest or summer storage temperatures.

Grain should be cooled early in the fall, held at 35 to 40 degrees through the winter, warmed in the spring, and then held below 60 degrees as long as possible into the summer to help minimize insect activity.

A cooling or warming zone moves through the grain in the same direction as the airflow, Shelton said.

"The rate of movement depends on the airflow rate and the length of fan operation," he said. "When the fan is turned off, movement of the zone stops. Movement resumes when the fan is turned back on. When changing grain temperatures, run the fan continuously until the cooling or warming zone has been moved completely through the grain."

Upward airflow within the bin is preferred because the top of the grain mass is the last area to change temperature. This makes it easier to determine if the zone has moved completely through the grain.

Uniform airflow distribution is necessary for the most satisfactory temperature control, Shelton said.

Uniform airflow requires clean grain, a level grain surface, and a well-designed aeration system, preferably with a fully perforated floor.

"A common concern is running aeration fans during rainy or humid weather," he said. "Rewetting grain normally is not a problem during the short time it takes to move a cooling or warming zone through the grain. Effects of operating a fan during damp conditions usually are more than offset by the time the fan is operated under more favorable conditions."

Shelton recommends routinely monitoring grain conditions to verify that the desired temperature control is achieved.

"Generally, grain should be inspected at least once a month during the winter and every two weeks at other times of the year," he said. "Measure and record the temperature at several locations within the grain mass.

Without records, it is difficult to determine whether elevated temperatures are caused by normally occurring outside temperatures or by heating due to mold activity. Use a grain probe to locate any moisture pockets where molds will develop rapidly as temperatures warm."

Extreme caution should be exercised when checking grain, Shelton said.

"Bridged grain is very dangerous and collapse of the bridged area can result in suffocation. Use a safety harness, lifeline, and grab rope; and have a second person outside the bin in case of an emergency," he said.

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