Stored grain needs to be cool and dry during the summer, a North Dakota State University Extension Service grain drying expert says.

“Cold or cool grain has been safely stored through the summer for many years,” notes Ken Hellevang, an Extension agricultural engineer.

That means grain should not be warmed to average outdoor air temperatures during the summer. The goal is to keep the grain as close to 40 degrees as possible in northern regions of the U.S. and as close to 50 degrees as possible in southern regions. Grain at the top of the bin and along the walls will be warmer than that, but the goal should be to keep the bulk of the grain cool.

One reason for keeping grain cool is that insect infestations and mold growth are more likely at warmer temperatures. The optimum grain temperature for insect activity is approximately 70 to 90 degrees. Reducing grain temperatures below 70 degrees will lessen insect reproduction and activity. Also, warming the grain using aeration may increase the moisture content of the grain slightly. Typically, the increase will be less than 0.50 percentage point.

Despite some people’s belief that condensation will occur, particularly near the bin wall, if the grain is not warmed to near average outdoor air temperatures, that is not the case, Hellevang says. Condensation forms on cool or cold surfaces when warm, moist air comes in contact with the cool surface. An example of this is condensation on a glass or container of cold liquid.

In the case of stored grain, warm, outdoor temperatures heat the bin wall during spring and summer, so conditions for condensation on the interior of the bin wall will not exist. The bin wall is warmer than the grain and the air in the stored grain.

“There also has been concern that moisture will move from the warm grain near the bin wall into the cooler grain away from the bin wall,” Hellevang says. “However, when 16 bins of grain were monitored through a summer, no statistically valid change in grain moisture content occurred within 4 feet of the bin wall.”

Many grain storage problems that have been blamed on leaving grain cold during the spring and summer actually are the result of condensation during the fall and winter. Condensation will form on the interior of the grain bin wall when warm grain is placed into a bin with a wall that is being kept cool by cold outdoor temperatures. Reports of grain sticking to the bin wall and possibly deteriorating likely are due to condensation in the fall causing the grain to increase in moisture content and then deteriorate during the spring and summer due to mold growth occurring when temperatures grow warmer.

Aeration fans should be covered to prevent wind and a natural chimney effect from warming the grain. Wind blowing into uncovered fans or ducts will move air through the grain in a way that is similar to operating an aeration fan.

One problem during the summer is that a galvanized bin roof absorbs large amounts of solar energy, heating the air above the grain. Convection currents in the grain flow up along the bin wall and down into the grain near the top middle of the bin, drawing this heated air into the grain. Ventilating the space between the grain and the bin roof can reduce the amount that the grain near the top of the bin is warmed.

Natural ventilation to cool this space can occur if the bin has openings near the eave and peak; these openings work in like the vents in an attic of a building. The heated air rises and exits near the peak, drawing in cooler air near the eave. This natural ventilation will not occur unless the bin has adequate openings at the eave and peak. Roof exhaust fans also can be used to draw the heated air out of the bin if it has openings to allow air into the area above the grain.

Cool the grain near the top of the bin by operating the aeration fan about every three weeks to reduce the potential for insect infestations during the summer. Using positive pressure aeration to push air up through the grain enables the cool grain in the bottom of the bin to cool the air, which then cools the grain near the top of the bin.

Only run the fan long enough to cool the grain near the top surface. That may require running the fan for a few hours during a cool, dry morning for a couple of days. Running the fan more than necessary will warm more grain at the bottom of the bin, increasing the potential for storage problems.

If the air dew point is warmer than the grain temperature or if the air relative humidity is high, some moisture will condense onto the grain during fan operation. Condensing moisture will release heat that will warm the air slightly, reducing the effectiveness of the aeration and increasing the amount of warming occurring in the grain at the bottom of the bin. Therefore, selecting mornings when the air is cool and dry is important.

Check the grain moisture content to assure the grain is dry enough for storage at summer temperatures. The recommended long-term grain storage moisture contents are about 13.5 percent for wheat, 12 percent for barley, 13.5 percent for corn, 11 percent for soybeans, 13 percent for grain sorghum, 8 percent for oil sunflowers and 10 percent for confectionary sunflowers.

Also measure the stored grain temperature at several locations near the top surface, along the walls and several feet into the grain. Temperature sensors are an excellent tool when monitoring stored grain, but remember that they only measure the temperature of the grain next to the sensor. Because grain is a good insulator, the grain temperature may be much different just a few feet from the sensor. Record the measured temperatures. Increasing grain temperature may be an indicator of an insect infestation or mold growth.

Mold growth and insect infestations occur rapidly at summer temperatures, so stored grain should be checked every two weeks. An insect infestation can go from only a few insects to a major infestation in less than a month. Using insect traps or placing grain samples on white material helps you look for insects.

NDSU Agriculture Communication - May 11, 2015