By Ken Hellevang
Figure 1. High moisture conditions are leading to high moisture soybeans, complicating drying and storage efforts.
A challenging soybean harvest is creating many questions related to storage and drying, according to Ken Hellevang, agricultural engineer with the North Dakota State University Extension Service.
Soybeans at 11% moisture have storage characteristics similar to wheat or corn at about 13.5% moisture, so 16% moisture soybeans might be expected to store similarly 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°F and 70% relative humidity is about 12%. With this air condition drying should occur with soybeans above 12% moisture. 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% moisture 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.
Figure 2. Extensive rainy, wet conditions in areas of Nebraska are leading to soybean pods popping open, further complicating this year's harvest.
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 is 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°F and 30°F 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 bushels of soybeans will require about 1.0 horsepower of fan. Achieving an airflow rate of 1.25 cfm/bu will require about 1.6 horsepower per thousand bushels and an airflow rate of 1.5 cfm/bu will need about 2.5 horsepower per thousand bushel.
The type of fan greatly affects the airflow provided per horsepower, so use a fan selection software program such as the one developed by the University of Minnesota. It is available on the NDSU Grain Drying and Storage website.
Soybeans can be dried in a high-temperature dryer, but the plenum 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.
One study found that with a dryer temperature of 130°F, 50% to 90% of the skins were cracked and 20% to 70% of the beans were cracked. Another study found that 30% of the seed coats were cracked if the drying air relative humidity was 30%. Roughly with each 20-degree increase in drying temperature, the air relative humidity is reduced to one-half. When air at 50°F and 80% relative humidity is heated to 70°F, the relative humidity drops to about 40%. Monitor the soybean seeds coming from the dryer and manage the dryer temperature based on the amount of damage occurring.
Caution
There is a risk of fire when drying soybeans. Soybean pods and other trash can accumulate in the dryer and become combustible. Clean the dryer frequently to ensure trash does not build up and to reduce the potential for debris becoming combustible. Also, assure that soybeans continue to flow in all sections of the dryer. Monitor the dryer continuously to limit fire potential.