Harvest timing can have a huge impact on soybean shatter losses, according to North Dakota State University Extension Service agricultural engineer Ken Hellevang.
“Field losses, splits and cracked seed coats increase as moisture content decreases,” he says. “Shatter losses have been shown to increase significantly when seed moisture falls below 11 per cent or when mature beans undergo multiple wetting and drying cycles.”
Because harvest losses increase dramatically when the moisture content is below 11 per cent, harvesting during high humidity such as early morning or late evening or damp conditions may reduce shatter loss, Hellevang notes.
Many times, the discount for delivering beans with a moisture content in excess of 13 per cent may be less than the discount for shatter losses from harvesting overly dry soybeans. He recommends that producers begin harvesting at 14 or 15 per cent moisture to reduce the amount harvested below 11 per cent.
Moisture content can increase by several points with an overnight dew or it can decrease by several points during a day with low humidity and windy conditions. Avoid harvesting when beans are driest, such as afternoons, to maintain moisture and reduce shattering losses.
“Unfortunately, there has not been adequate research examining if immature green soybeans will change colour in storage,” Hellevang says. “Limited studies indicate that green soybeans will tend to stay green in storage. They do not lose their internal green colour caused by chlorophyll, although the surface colour may lighten or mottle somewhat after weeks or months in storage.”
Field losses need to be balanced against the discounts for green seeds in determining when to harvest. Another possibility is harvesting some of the field and leaving the portion with the green soybeans unharvested, he says.
Equalizing moisture content
Soybean moisture variation may lead to storage and marketing losses. Operating an aeration fan will help move moisture from wet beans to drier beans. Air going past wet beans picks up moisture, and that moisture will transfer to drier beans as the air goes past them.
Moisture movement will be minimal without aeration airflow. Hellevang suggests initially running the fan longer than is required to cool the grain to even out the moisture content. The moisture will not be all the same, but it should become more uniform.
Soybeans at 11 per cent moisture have similar storage characteristics as wheat or corn at 13.5 to 14 per cent moisture, so an allowable storage time (AST) chart for cereal grains can be used to estimate allowable storage times for soybeans.
For example, soybeans at 16 per cent moisture content would be similar to cereal grains at about 19 per cent moisture, so soybeans would be expected to have an AST of about 70 days at 50°. The AST is reduced to 35 days at 60° and extended to about 140 days at 40°.
The recommended maximum moisture content for air-drying is about 16 per cent moisture, with an airflow rate of at least one cubic foot per minute per bushel (cfm/bu.) during October. The amount of natural-air drying that will occur in late October and November is limited in northern states.
The equilibrium moisture content of soybeans for air-drying at 40 F (4.5 C) and 70 per cent relative humidity is 13.7 per cent, but even with an airflow rate of one cfm/bu., drying soybeans with 16 per cent moisture will take about 70 days. Adding supplemental heat to raise the air temperature by 5 F (2.4 C) will permit drying the soybeans to about 11 per cent moisture in about 55 days.
Only about one-half of the beans would be expected to dry by mid-November, when outdoor temperatures become too cold to dry efficiently. 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° for winter storage and complete drying in the spring. Hellevang recommends starting to dry when outdoor temperatures are averaging about 40°.
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, the rule of thumb is that each 1,000 bushels of soybeans will need about one horsepower of fan to achieve an airflow rate of one cfm/bu. Achieving an airflow rate of 1.5 cfm/bu. will require about 2-1/2 horsepower per 1,000 bushels, and an airflow rate of two cfm/bu. will need about five horsepower per 1,000 bushels.
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 (https://www.ag.ndsu.edu/graindrying).
Soybeans can be dried in a high-temperature dryer, but the 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 non-food soybeans is about 130 F (54.5 C). Even at that temperature, some skins and beans will be cracked.
One study found that with a dryer temperature of 130 F (54.5 C), 50 to 90 per cent of the skins were cracked and 20 to 70 per cent of the beans were cracked. Another study found that 30 per cent of the seed coats were cracked if the drying air relative humidity was 30 per cent, and 50 per cent of the skins and about eight per cent of the beans were cracked at 20 per cent relative humidity.
The relative humidity is reduced by one-half for each 20° that the air is warmed. Therefore, if air at 40 F (4.5 F) and 80 per cent relative humidity is warmed to 60°, the relative humidity is reduced to 40 per cent, and if it is heated to 80°, the relative humidity is reduced to 20 per cent. Monitor the amount of damage occurring during drying and regulate the temperature to obtain the acceptable amount of damage.
Most dryer fires occur due to trash accumulating in the dryer. Monitor the grain flow in the dryer and periodically clean the dryer to reduce the potential for a fire.
Food soybeans and seed beans must not have damage to the seed coat, so natural-air or low-temperature drying is the preferred drying method, Hellevang says.
For more information, do an internet search for NDSU soybean drying.