Figure 1. Stored grain temperature and moisture should be checked in multiple spots at least every two weeks and managed to ensure continued grain quality.
Stored grain needs to be cool and dry during summer storage, 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. "Keeping the grain as cool as possible should be the goal of summer grain storage."
Allowing grain to warm to average outdoor air temperatures during the summer can lead to insect infestations and mold growth. The optimum grain temperature for insect activity is approximately 70°F to 90°F. Reducing grain temperatures below 70°F will lessen insect reproduction and activity, and temperatures below 60°F will reduce insect activity greatly.
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. Hellevang warns that using aeration to warm the grain increases its moisture content by up to 1 percentage point.
Where to Measure Grain Temperature
Measure and record the stored grain temperature at several places near the top surface, along the walls and within the grain. Temperature sensors are an excellent tool when monitoring stored grain, but remember that they only measure the grain's temperature next to the sensor.
Because grain is a good insulator, the grain temperature may be much different just a few feet from the sensor. 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," says Ken Hellevang, an Extension agricultural engineer at North Dakota State University. "A situation with only a few insects can turn into a major infestation in less than a month. Using insect traps or placing grain samples on white material helps you look for insects."
Good Bin Roof Ventilation Important
One challenge to keeping the grain cool during the summer is that solar energy on the bin roof heats 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 like the vents in a building's attic. 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 controlled by a thermostat also can draw the heated air out of the bin if openings are available to allow air into the area above the grain.
When to Use Aeration
Hellevang recommends cooling grain in the upper portion of the bin by operating the aeration fan about every three weeks during a cool morning. 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.
Run the fan only 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 very 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. The grain moisture content increase is typically less than 1 percentage point because the grain warms and is no longer cool enough to cause moisture to condense onto the grain. Therefore, select mornings when the air is cool and dry.
Unload Some Grain
Some people unload some of the grain periodically during the summer to remove the warmed grain at the top. The grain unloads in a funnel shape in bins with a center sump, with the grain from the top flowing down to the unloading sump at the bottom of the bin.
Removing peaked grain reduces the potential for grain warming at the top of the bin. A cone-shaped peak has a larger ratio of surface area to grain quantity, which leads to more grain warming than occurs with leveled grain. In addition, because air takes the path of least resistance, the aeration airflow will be near the bin walls, with little airflow through the peak. This makes the peak difficult to cool.
Summer Moisture Recommendations
Verify that the grain moisture content is dry enough for storage at summer temperatures. The recommended long-term storage moisture contents are:
- wheat, 13.5%;
- corn, 13.5%;
- soybeans, 11%; and
- grain sorghum, 13%.
The market moisture content may be higher, but storing warm grain at higher moisture contents may lead to mold growth on the grain.
Make sure the moisture content measured by the meter has been adjusted for grain temperature. Confirm the accuracy of the measurement by warming the grain sample to room temperature in a sealed plastic bag before measuring the moisture content.
Grain Storage Time
Grain storage life depends on the grain's moisture content and temperature. The allowable storage time is cumulative, so if half of the life is used since harvest, only half remains for the rest of the storage period. This is particularly crucial for grain being stored into late summer or for more than a year.
Each 10 degrees the grain temperature increases reduces the allowable storage time by about half. For example, the allowable storage time for 15% moisture corn is only about 70 days at 80°F. The allowable storage time increases to 125 days at 70°F and 240 days at 60°F. This emphasizes the importance of keeping the grain as cool as possible during the summer.
The allowable storage time typically increases by 50-75% for each percentage point reduction in grain moisture content, depending on grain moisture and temperature. For example, the approximate storage life of 15% moisture corn at 70°F is about 125 days. The AST increases to about 200 days at 14% moisture.
However, that is only about 4.7 months, so if corn is stored at 14% moisture and 80°F during the summer, little storage time remains. This stresses the importance of controlling grain moisture content and temperature.