Grain Quality Fact Sheet #16
Field and weather conditions in the fall are usually such that field drying is sufficient to reduce the moisture content in soybeans to a safe storage level. However, wet and cool conditions have already hampered some of the soybean harvest this fall especially in fields that are to be planted with winter wheat. Enclosed is a brief review of some of the basics to consider when the moisture content of mature soybeans remains at 16 - 20% for extended periods.
Provided that soil conditions support equipment and soybeans are sufficiently defoliated, high moisture beans up to 18% can be successfully harvested and dried. Essentially, all grain drying methods (see Grain Quality Fact Sheet 15) are adaptable with some restrictions on the use of heat and handling practices. Good quality soybeans should be dried to 13.5 - 14% for winter storage, and 12 - 12.5% if they are to be carried into the summer. If quality out of the field is poor, the moisture content should be reduced by an additional 0.5 - 1.0 percentage point.
Too much heat while drying soybeans causes excessive seed coat cracking, which results in splits. Although not a dockage factor below 10%, seed coat cracking destroys the integrity of the seed and its protection during storage and handling. The key factor in avoiding splits is to keep the relative humidity of the drying air above 40%.
This is a significant limitation on heat input and drying capacity. For example, 50°F outside air with 80% relative humidity can only be heated to 70°F in order to maintain humidity above 40%. High Temperature Drying The heat input in high temperature batch, continuous-flow and bin dryers can be restricted either by using short heat-on cycles, or changing the burner jets to a low-fire type. The resultant temperature rise from ON/OFF cycling is proportional to the percent of ON time.
For example, a burner with a minimum continuous fire rate of 40°F rise will average about 20°F rise if fired only 50% of the time. The same unit will average 10°F rise if fired only 25% of the total ON/OFF cycle time. Utilization of a proportional timer that allows calibration of the total cycle by turning a percent dial can be used to control the length of the fire cycle. If splits are not as much of a concern, drying air tempera-tures should be limited to 120 - 140°F to avoid heat damaged beans.
Natural air above 60°F and below 75% humidity will require no supplemental heat to remove 2 to 3 points of moisture from soybeans.
However, natural air and low-heat drying in deep bins are slow processes. For example, a 24 ft diameter bin filled to 16 ft depth with 18% moisture soybeans will require about 23 days to complete drying to 13% during an average weather year. This assumes a 7.5 HP fan delivering 1.4 cfm/bu and a temperature rise of 10°F.
Drying speed can only be increased by reducing the depth in the bin (which increases the airflow per bushel), and/or by adding more than 10°F of heat. When adding supplemental heat, the 40% humidity requirement becomes the limiting factor. The need for drying capacity can be further reduced by only harvesting during the afternoon hours when moisture contents are closer to 16%.
Fans should generally be operated continuously as long as the average 24 hour air conditions are below 70 - 75% relative humidity and soybean moistures are above 15%. Generally, only little rewetting occurs, and then only in the bottom 6 to 18 inches. The balance of good weather during the day or week more than off-sets short high-humidity periods during the night, or 1 to 2 days of drizzle. Additionally, heat generated by the fan motor reduces the outside air relative humidity by 10 to 20 percentage points.
For additional information refer to AE-84 Drying Soybeans with Heated and Unheated Air available from your local Cooperative Extension Service Office.
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