Railcar shortages and bottlenecks at local grain elevators have resulted in a big increase in on-farm grain storage in recent years. With the large wheat crop that much of South Dakota enjoyed, and the prospects of a good row crop in the field, many of those new bins will be used this fall. Other than small bins for special purposes, of which newer structures are often hopper bottom models, most new bins are quite large. As grain bin capacity goes up, the risk associated with grain going out of condition increases dramatically, and actually the potential for grain stored in them can be much greater.
The foundation of safely storing grain is moisture content, and growers are usually familiar with the recommended levels for the crops they raise. For short-term storage (less than 6 months), the following moisture contents are the maximum recommended: corn – 15.5%, edible beans – 15%, millet – 10%, sorghum/milo – 13.5%, soybean – 13%, non-oil sunflower – 11%, and oil sunflower – 10%. For long-term storage (longer than 6 months), the maximum levels are lower: corn – 13%, edible beans – 13%, millet – 9%, sorghum/milo – 13%, soybean – 11%, non-oil sunflower – 10%, and oil sunflower – 8%.
The other factor that determines how long grain can be stored safely is temperature, and the combination of the two determines “allowable storage time.” Allowable storage time is defined as the length of time that grain with normal harvest damage can be stored with no more than 0.5% dry matter loss. If harvest damage is greater than normal, grain can spoil 2 to 5 times faster, i.e. allowable storage time for damaged grain can be half or less of the time listed for undamaged grain. Shelled corn at 20% moisture and 60 degrees F can be stored without excessive storage loss for 28 days. If that corn is cooled to 50 degrees F, one could expect to store it safely for 50 days. Dry the corn to 18% moisture and at 60 degrees F, it could be stored for 57 days.
Cooling is often accomplished with aeration fans, and can be done with airflow as low as 1/10 cfm (cubic feet per minute) per bushel or less if allowable storage time is great enough. At 1/10 cfm per bushel, it will take about 120 hours to cool grain with a 20 degree temperature differential (60 degrees F to 40 degrees F). Doubling the airflow rate to 1/5 cfm per bushel will cut the time required in half, etc. Airflow rates below 1 cfm per bushel should not be expected to dry grain, only cool it. Airflow rates of 1 cfm per bushel or higher will dry grain to the equilibrium moisture content of the air temperature and the relative humidity without additional heat, i.e. natural air drying. For example, with an average daily temperature of 50 degrees F and an average relative humidity of 65%, corn will ultimately dry to 14.8%, but this also takes time.
There is often debate as to whether one should push air up through a bin or suck air down. If aerating, suction will cool off the top of the grain mass first and reduce insect activity in the top layer of the grain mass due to the lower temperature. If trying to accomplish natural air drying, it is important to note that air will be warmed 4 to 5 degrees as it passes through the fan if operating at a static pressure of 6 to 7 inches. Warming the air 5 degrees reduces the relative humidity about 10%, allowing the grain to be dried to a lower moisture content. For more information, visit the NDSU website: www.ag.ndsu.edu/graindrying.