Reconditioning low-moisture soybeans in storage can damage the grain bin, cautions Ken Hellevang, the North Dakota State University Extension Service’s grain-drying expert.
Warm, dry fall weather can result in soybeans being harvested well below the market moisture content of 13 per cent. Hellevang says he has heard reports of harvested bean moisture contents as low as six to nine per cent.
The lost value of soybeans with a moisture content of seven per cent rather than the market standard of 13 per cent is a loss of about 97 cents per bushel for beans with a value of $15 per bushel and 66 cents per bushel if they are at nine per cent moisture. This lost value will encourage producers to recondition the beans to 13 per cent.
However, reconditioning causes the beans to expand, which can damage the grain bin’s bolted connections or even cause the bin to rupture from the increased pressure on the bin wall.
“The forces on the bin increase much more rapidly than by the percentage of moisture content increases,” Hellevang says. “Therefore, a moisture content increase of more than a point or two can be problematic. The bin warranty may be voided if damage occurs while reconditioning grain.”
One way to reduce the damage is to use a negative pressure system to pull humid air down through the soybeans and remove the soybeans from the top of the bin as they are reconditioned. Grain flows from the top of the bin in a funnel shape as it is unloaded from the centre bin sump.
Another way to reduce that pressure is to use a vertical-stirring auger to mix the beans frequently. However, soybeans become more fragile at lower moisture contents, so stirring may damage the beans.
Producers need to operate fans during weather with an average relative humidity of about 70 per cent if they want to recondition soybeans to 13 per cent during normal fall temperatures of -1 C to 15 C. The moisture doesn’t change gradually throughout the entire bin during reconditioning, Hellevang notes. Instead, a rewetting zone develops and moves slowly through the bin in the direction of the airflow.
Too much reconditioning also can be a problem. Soybeans will be reconditioned to a moisture content exceeding 13 per cent if the humidity of the air entering the soybeans is too high. If that happens, the wet soybeans may deteriorate in storage or be discounted when sold.
One way to control reconditioning is to use a humidistat to turn the fan on any time the humidity is above about 60 per cent. Hellevang recommends adding a second humidistat to stop the fan when the relative humidity reaches very high levels to prevent excessive moisture contents.
Another method is to install a microprocessor-based controller that monitors temperature and humidity, and runs only when air conditions will bring the crop to the desired moisture content. Running the fan only at night when the humidity is higher will recondition the soybeans but does not provide any control options.
“Reconditioning time primarily depends on the airflow per bushel and weather conditions,” Hellevang says. “Reconditioning occurs the fastest when the airflow rate is high and the air is warm and humid.”
Reconditioning will be the most successful in a drying bin that has a fully perforated floor and a fan that can deliver at least 0.75 cubic feet per minute of airflow per bushel. Even with this airflow, moving a rewetting front all the way through the bin may take more than a month of fan operation. Using lower airflow rates to move the rewetting front through the bin will take several months.
For more information about reconditioning, drying, handling and storing soybeans, visit the NDSU Extension Service’s soybean production guide at http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a1172.pdf and NDSU’s grain drying and storage website at http://www.ag.ndsu.edu/graindrying.