NDSU Extension - Griggs County


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October 19

By Megan Vig

                Harvest came to an abrupt halt with the recent moisture.  As we look forward and hope for better weather conditions, I share information from Ken Hellevang, NDSU Extension Agriculture Engineer on the challenging soybean harvest. 

                Soybeans at 11% moisture have similar storage characteristics to wheat or corn at about 13.5% moisture, so 16% moisture soybeans might be expected to store similar to about 19% moisture corn.  It is important to be able to aerate the soybeans to keep them cool.

                The amount of natural air drying that will occur in late October and early November is limited.  The equilibrium moisture content of soybeans for air at 40 degrees and 70% relative humidity is about 12%, so drying of soybeans above 12% would be expected with this air condition.  However, the drying rate will be slow at typical in-bin drying airflow rates.  An airflow rate of 1 cubic foot per minute per bushel (cfm/bu) is expected to dry 18% moisture soybeans in about 60 days.  With an airflow rate of 1.5 cfm/bu the drying time is reduced to about 40 days.  The drying time for 16% moisture soybean is slightly less.  The drying time of 16% moisture soybeans is about 50 days.  Adding supplemental heat to raise the air temperature by 3 to 5 degrees will permit drying the soybeans to about 11% in about 40 to 45 days.  Increasing the airflow rate proportionally reduces the drying time.

                The moisture holding capacity of air is reduced at lower air temperatures.  As average air temperatures approach 35°F, natural air drying becomes inefficient and not economical.  Adding heat would cause the beans on the bottom of the bin to be dried to a lower moisture content and it would increase drying speed only slightly.  Cool the soybeans to between 20 and 30 degrees for winter storage and complete drying in the spring.  Hellevang recommends starting drying in the spring when outdoor temperatures are averaging about 40°F.

                Increasing the airflow rate will increase the drying speed.  However, the fan horsepower required to achieve the higher airflow rate becomes excessive unless the grain depth is very shallow.  For a soybean depth of 22 feet, each 1,000 bu of soybeans will require about 1.0 horsepower of fan.  To achieve an airflow rate of 1.25 cfm/bu will require about 1.6 horsepower per thousand bu and airflow rate of 1.5 cfm/bu will need about 2.5 horsepower per 1,000 bu.  The type of fan greatly affects the airflow provided per horsepower.  University of Minnesota has a fan selection software program and a link can be found on the NDSU grain drying and storage website.

                Soybeans can be dried in a high-temperature dryer, but the dryer temperature needs to be limited to minimize damage to the beans.  Refer to the manufacturer’s recommendations for maximum drying temperature.  Typically, the maximum drying temperature for nonfood soybeans is about 130°F.  Even at that temperature, some skins and beans will be cracked.  Monitor the soybean seeds coming from the dryer and manage the dryer temperature based on the amount of damage occurring.

                There is a risk of fire when drying soybeans.  Soybean pods and other trash can accumulate in the dryer and become combustible.  Assure that there is not an accumulation of trash in the dryer that becomes combustible.  Also, check that the soybeans continue to flow in all sections of the dryer.  Monitor the dryer continuously to limit fire potential.  Clean the dryer frequently to reduce the potential for debris becoming combustible.

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