There are a tremendous number of double cropped acres of soybeans this season. Temperatures are rapidly dropping this time of year. Harvesting soybeans in various stages of growth presents a clear issue in dealing with proper management of the harvest and drying strategies for these soybeans. In the past mills could fairly easily deal with frosted beans by putting them on air however there are alot of important decisions to make to increase income and reduce discounts that growers might not be aware of. Here is an informative factsheet from Purdue University that details issues with frost damaged soybeans. The direct link is Harvesting, Drying, and Storing Frost-Damaged Corn and Soybeans Dirk E. Maier, Agricultural & Biological Engineering Samuel D. Parsons, Agricultural & Biological Engineering .
HARVESTING FROST-DAMAGED SOYBEANS
An early frost on soybeans can greatly diminish soybean yield, reduce bean size, and lower test weights because the beans may not have had enough time to fill completely. Given a cool September and an early frost, one can expect harvesting difficulties, and lower than normal protein and oil levels of the beans.
Whether early-frosted or not, late-planted beans in many fields will be quite "short" this fall - both in height and yield. And special consideration will be needed for combine operation and adjustment. Cutting as low as practical, as usual, will be necessary to get what little crop is there. Slow down if needed to avoid stones. Crisp, clean cutting is essential to minimize shatter and pod drop if bean and pod moistures are low. Keep cutterbar in tip-top shape at all times - knife sharpness, guard alignment and positive clip hold-down are needed.
Don't set and forget the reel! Adjust as conditions change, which may be needed in different parts of the same field - as plant height varies. Position the reel axis a few inches ahead of the cutterbar, with bats just low enough to catch bean tops and tip plants onto the platform. Bats should usually run 25-40% faster than ground speed depending on conditions - just fast enough to avoid pile-up on the cutterbar and excessive shatter and pod drop.
If beans are at "normal" harvest moisture content, keep cylinder speed to a minimum to avoid bean crackage - especially as beans dry out more in mid-afternoon. If beans are wetter than normal, more aggressive threshing action may be needed. If so, reduce concave clearance first (as with corn), then increase RPM until acceptable threshing occurs.
In addition to threshing, pay attention to other internal settings and adjustments for beans - including the separation and cleaning units. Bean loss out the back of the combine are usually not significant, but can be when conditions change, settings slip, or plugging occurs. Monitors can signal dramatic changes, but can't replace attentive, skillful operating and diagnostic expertise.
DRYING OF FROST-DAMAGED SOYBEANS
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 this fall especially in fields that were planted late and/or frost-damaged may require harvesting of soybeans at 16 - 20%. 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.
Too much heat while drying soybeans causes excessive seed coat cracking, which results in splits. 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, 50F outside air with 80% relative humidity can only be heated to 70F in order to maintain humidity above 40%. Thus, high temperature drying with air heated to 160-180F or above is not an option when drying soybeans.
Medium Temperature Drying
The heat input in column 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 40F rise will average about 20F rise if fired only 50% of the time. The same unit will average 10F 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 temperatures limited to 120 - 140F to avoid heat damaged beans can be used.
Low Temperature Drying
Natural air above 60F 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 10F.
Drying speed can be increased by reducing the depth in the bin (which increases the airflow per bushel), by adding more than 10F of heat, and/or by utilizing stirring devices. When adding supplemental heat, the 40% humidity requirement becomes the limiting factor. The limitation on drying capacity can be further reduced by only harvesting during the afternoon hours when moisture contents are closer to 16%.
Fans (and low heat burners) 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.
STORAGE OF FROST-DAMAGED SOYBEANS
Green soybeans contain chlorophyll that will cause oxidation of the oil, and thus greatly reduce shelf-life. Although the chlorophyll can be removed as part of the oil bleaching process, processing costs and refining losses increase.
Data from the University of Minnesota indicates that the surface color does not change significantly during storage. During a six month test of green versus normal yellow soybeans under safe storage conditions, monthly surface color readings changed little. However, the green beans appeared to be slightly mottled at the end of the six months. Others have observed that green beans will fade somewhat after 4-6 weeks of aerated storage. They will also fade in the field if the stem has not been killed. Problems with green beans are generally most noticeable to processors at harvest time, and tend to diminish with time. This may be due to the perceived lightening of the surface bean color, or due to increased co-mingling with other beans.
Storage and Discounting of Green and Immature Soybeans
A related study by the University of Minnesota revealed that when either green or yellow beans at 18% moisture were stored, no significant difference in the rate of respiration of green versus yellow soybeans was determined. Thus, green beans dried in the field or in dryers do not appear to present a greater storage risk. The general recommendation for storing clean, high quality soybeans in aerated storage for up to six months is to maintain moisture contents at 13% or lower. However, given the concern over high FM levels due to ineffective weed control this past spring in many soybean fields and the kernel-to-kernel moisture variations among beans, it is advisable to reduce storage moistures to 11-12% moisture or lower to be safe. It has also been reported that moisture readings will generally read low on immature ("butter beans") soybeans fresh out of the field. Readings should stabilize after a few hours of equilibration at room temperature in a closed container.
Green and immature soybeans are included in the total damage factor in the U.S. soybean grading standard. In order to assign an official grade, the kernels suspected of being damaged must be sectioned or cut open and compared to the appropriate GIPSA line slide (SB-3.0 for green beans, SB-6.0 for immature beans). Since the revisions to the grading standard in 1986 the line for "greenness" on these slides is much lighter (and thus the definition for the amount of green present much stricter). Thus, although the surface color of the beans may fade during storage, once they are cut open for grading the amount of greenness may not have changed significantly during storage.
Although elevators and processors set their own discount levels, a typical discount may be 2 cents for each percentage point of total damage between 2.1-5%, 4 cents per point between 5.1-8%, and 6 cents per point above 8.1% total damage. It has been reported that during the fall of 1995 a number of elevators in Indiana, Illinois, Iowa, and Missouri rejected green soybeans at a damage level above 7% during the harvest season. Thus, the worst plan is to harvest green or immature soybeans wet and market them immediately at harvest. To reduce the potential for discounts further, it may also be desirable to screen out small beans before binning or delivery.