Most years, fall weather conditions in the upper Midwest will dry soybeans to 11% to 13% moisture in the field. But that's not likely this fall. This year, many growers will choose to artificially dry soybeans.
Bill Wilcke, University of Minnesota ag engineer, says most drying recommendations for soybeans are based on limited experience or are extrapolated from corn-drying recommendations. However, in most cases, dryers that were designed for corn can be adapted for use with soybeans.
"When it comes to natural-air drying, using unheated air to dry soybeans usually works well, but it is a slow process, taking about 2 to 6 weeks, depending on initial moisture, airflow and weather," Wilcke says.
He says bins used for natural-air drying should have full-perforated floors and fairly large drying fans. Fan power requirements depend on desired airflow and depth of beans. For example, Wilcke says delivery of 1.0 cubic feet of air per minute per bushel (cfm/bu) of beans through an 18-foot depth of soybeans would require about 0.6 horsepowers per 1,000 bushels of beans in the bin, while delivery of 1.5 cfm/bu through 18 foot of beans would take about 1.6 horsepowers per 1,000 bushels of beans.
"Management of natural-air soybean dryers is similar to that for natural-air corn dryers, except that soybean moisture values need to be about 2 percentage points lower than those recommended for corn," Wilcke adds. "The further north you're located, higher airflow is needed since fewer days are available for drying in the fall."
Because natural-air drying is a slow process, it will be difficult to use one bin to dry both beans and corn in the same year, Wilcke says. Don't plan on having the beans dry before corn harvest unless the soybeans are only slightly wetter than 13%, or unless you use a shallow drying depth.
While early in the fall, especially in years with warm, dry weather, it's possible to dry soybeans to less than 13% moisture with no supplemental heat. However, late in the fall or in years with cool, damp weather, soybeans might not dry to 13%. Then, it might be helpful to add a small amount of supplemental heat to the air in natural-air dryers, says Vance Morey, University of Minnesota ag engineer.
"Do not heat the air more than 3 to 5 degrees, or you will overdry the beans and cause an increase in splitting," Morey says. "Research has shown that exposing soybeans to relative humidity values of less than 40% can cause excessive splitting.
"For every 20 degrees that you heat air, you cut its relative humidity approximately in half, so it doesn't take very much heat to produce relative humidity values less than 40%."
Some alternatives to adding supplemental heat to natural-air drying bins include:
* Turning off the fan when weather gets cold in the fall, keeping beans cold during winter and resuming drying when average temperatures climb above freezing in the spring.
* Installing bigger fans so that you can finish drying earlier in the fall when weather is better.
* Using manual or automatic control to turn off the fan during periods of high humidity. Fan control will increase the amount of time required for drying, but it will result in drier beans.
Many kinds of gas-fired corn dryers can be used to dry soybeans, but be careful, says Ken Hellevang, North Dakota State University ag engineer. Soybeans split easily if they are dried too fast or are handled roughly.
"Set the drying air temperature lower than you would for corn and avoid dryers that recirculate the crop during drying," Hellevang says. "Column-type dryers can often be operated at 120 to 140 F without causing too much soybean damage, although some trial and error might be required to set dryers properly.
"Examine beans leaving the dryer carefully and reduce the temperature if you're getting too many splits. If the soybeans will be saved for seed, keep drying temperatures under 110 F to avoid killing the embryo."
Hellevang reminds growers that crops dried in gas-fired dryers must be cooled within a day or so to remove dryer heat. This can be done in the dryer or in aerated storage bins. Stored beans should be aerated again later in the fall to cool them to 20 to 30 F for winter storage.
In years when frost kills soybean plants before the seeds are fully mature, the ag engineers say you should remove as much chaff and green plant material as possible before binning the beans. Immature beans can be stored without significant molding, but concentrations of green chaff can lead to heating in storage.
Although it is commonly stated that green soybeans will eventually turn yellow in storage, Wilcke says the color change observed in a university laboratory study was minimal.
"It might still be worthwhile to store green soybeans for a few months after harvest to avoid the high discounts that are applied in years when large quantities of green beans are delivered during harvest," Wilcke says. "Just make sure that any green beans going into storage are clean, evenly distributed throughout the bin and cooled as soon as possible after harvest.