The storability of grain depends on the grain's quality, moisture content and temperature, according to Ken Hellevang, agricultural engineer with the North Dakota State University Extension Service.
Grain moisture content must decrease as grain temperature increases to store grain properly. For example, the allowable storage time (AST) of 15% moisture corn is about nine months at 60° F, five months at 70° and only three months at 80°. The AST is reduced by approximately one-half for each 10° increase in grain temperature. If grain can be kept cooler, then it can be stored at higher moisture contents.
The recommended long-term grain storage moisture content normally is associated with the equilibrium moisture content (EMC) for the grain at a relative humidity of about 65% to limit mold growth at summer storage temperatures. The EMC at 80° and 65% relative humidity is about 13.5% for wheat, 12.2% for barley, 13.1% for corn, 11.2% for soybeans, 7.6% for oil sunflowers and 9.9% for confectionary (non-oil) sunflowers. These EMCs correspond to the general recommended long-term storage moisture contents of 13.5% for wheat and corn, 12% for barley, 11% for soybeans, 8% for oil sunflowers and 10% for confectionary sunflowers.
"Grain with mechanical damage is more susceptible to mold growth and has a shorter allowable storage life than undamaged grain," Hellevang says. "Corn harvested at moisture contents of 25% and higher likely has a higher-than-normal amount of mechanical damage and will have a shorter allowable storage life than corn with limited mechanical damage."
Immature or poor-quality grain generally has a lower test weight and is more susceptible to deterioration in storage. The allowable storage life of low-test-weight corn may be only about one-half of that expected for mature, good-quality grain. Also, allowable storage time can vary among corn hybrids, so the numbers in AST charts should be considered only estimates.
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. Hellevang recommends using insect traps or placing samples on white material to aid in looking for insects.
Check the grain moisture content to assure the grain is dry enough for storage at summer temperatures. 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, but remember that they only measure the temperature of the grain next to the sensor," Hellevang says. "Since grain is an excellent insulator, the grain temperature may be much different just a few feet from the sensor and not affect the measured temperature."
Record the measured temperatures. Rising grain temperature may be an indicator of an insect infestation or mold growth.
"The goal for summer storage should be to keep the grain as cool as possible to limit insect activity and reduce the potential for mold growth," Hellevang says. "Insect reproduction is reduced at temperatures below about 65-70°."
Grain should not be warmed using aeration during the spring and summer. Aeration fans should be covered to prevent wind and a natural chimney effect from warming the grain. Grain near the top of the bin should be cooled every two to three weeks by operating the aeration fan for a few hours 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 and exhausts the warn air from the grain out the top of the bin.
Only run the fan a few hours, or just long enough to cool the grain near the top surface. Running the fan more than necessary will warm more grain at the bottom of the bin, increasing the potential for storage problems. Cover the aeration fan when it is not running.
A galvanized bin roof absorbs large amounts of solar energy during the summer, 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 during the summer, 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, Hellevang says.
Natural ventilation to cool this space can occur if the bin has openings near the eave and peak in a manner similar to ventilating the 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 both 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.
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 warming of the grain, than the cylindrical shape of leveled grain.
For more information about storing grain, visit http://www.ag.ndsu.nodak.edu/abeng/postharvest.htm.