Post-harvest soybean quality preservation: best practices
The soybean harvest marks the end of the growing season, but certainly not the end of the work required to preserve its quality. Once harvested, the grain remains a living product that reacts to its environment. It can absorb moisture, develop mold, dry out, crack, become rancid, and eventually heat up.
For Quebec producers, one of the challenges stems from the fact that the soybean harvest takes place during a period of rapidly changing weather conditions. A hot, dry day may be followed by a very humid night, then a cold spell a few days later. Under these conditions, a silo filled with soybeans at 12 or 13% moisture content is not necessarily “ready for winter.”
Maintaining the quality of soybeans essentially depends on three factors: managing their moisture content, controlling their temperature, and avoiding unnecessary damage during drying. This is where aeration serves as the link between these three objectives.
Soybean quality starts at harvest
You can’t really improve the quality of a grain after it’s been harvested. However, it’s very easy to degrade it if you don’t pay close enough attention. The first goal, therefore, is to harvest soybeans when their moisture content is within a range that minimizes field losses, mechanical damage, and ensures grain cleanliness.
Rapid drying in the field
Under dry fall conditions, mature soybeans can go from about 15% moisture content early in the morning to nearly 11% by midday, before reabsorbing moisture overnight. However, drying in the field must be monitored carefully. This is because, on some occasions, the uniformity of grain moisture content can be very misleading. That’s why we recommend letting your grain sample rest in an airtight bag (such as a Ziploc bag) for at least three hours before performing your final test.
Why? Because some moisture meters fail to measure moisture at the center of the grain. This may be the case for you if you have a 919/3.5-inch moisture meter.
Keep in mind that newer UGMA-type moisture meters, such as the Perten AM5200-A, use higher frequency ranges and allow for accurate measurement of grain moisture uniformity. As a general rule, this is the type of moisture meter you’ll find at grain buyers or grain elevators.
Harvesting when the grain is too dry
Harvesting when the grain is very dry increases field losses and also increases mechanical damage. This is particularly important with soybeans because their seed coat becomes brittle when the grain is too dry. The seeds split more easily in the combine, augers, elevators, and conveyors.
NDSU recommends starting harvest at around 14 to 15% moisture content when conditions permit, to prevent a significant portion of the field from dropping below 11% as harvesting progresses. When soybeans become very dry, losses due to pod shattering increase rapidly. (NDSU Agriculture)
This obviously does not mean that you should put soybeans at 15% moisture content into a bin and forget about them until spring. Rather, it means that it may be economically and qualitatively preferable to harvest at a slightly higher moisture content and complete conditioning under controlled conditions rather than waiting in the field until the grain becomes excessively dry.
13%, 16%, or 11%: What moisture content should you actually aim for?
This question warrants some nuance. According to the Canadian Grain Commission’s current tolerances, soybeans are considered dry when they contain less than 14.1% moisture. From 14.1% to 16%, it is considered “damp”; from 16.1% to 18%, “wet” (Canadian Grain Commission).
But “dry grain” in the context of commercial grading does not automatically mean “optimal moisture content for any storage period.” This is where storage duration and temperature become important.
NDSU recommendations generally suggest a moisture content of around 13% for winter storage, while soybeans that will remain in bins during warmer periods should be closer to 11%. In some cases, storing soybeans at a moisture content below 13% may be beneficial to prevent them from becoming rancid.
At 13% moisture content and a temperature below approximately 16 °C, soybeans can normally be stored for several months under good conditions. As the temperature rises or storage is prolonged, the safety margin decreases. (NDSU)
In practice, the objectives can be summarized as follows:
Condition | Approximate moisture content |
Harvest when possible | Approximately 13 to 15% |
Commercial winter storage | Around 13% |
Extended storage into summer | Approximately 11 to 12% |
Seed soybeans | Approximately 13 to 14.5% |
Food-grade soybeans / IP | Follow the contract requirements first |
*These values are management targets and are not intended to replace the specific requirements of your buyer or your contract.
This distinction is particularly important for producers of food-grade soybeans, identity-preserved soybeans, or seed soybeans. For these markets, preserving the integrity of the seed coat, color, and, in some cases, germination capacity can be just as important as achieving the correct moisture content.
Too dry soybeans also cost money
There’s a lot of talk about the risks associated with wet grain, but over-drying soybeans isn’t free.
First, you sell less weight. Soybeans harvested at 9% or 10% moisture content obviously contain less water than grain sold at nearly 13%. After all, 1% moisture equals 1% weight!
Second, dry soybeans become more fragile. The number of split seeds and damaged seed coats increases during handling.
The NDSU gives the example of a yield of 40 bushels per acre. Harvesting soybeans at 9% moisture content rather than 13% results in a weight loss of about 1.8 bushels per acre—and that’s before even considering the additional losses associated with pod shattering and mechanical damage. If you do the math, that amounts to 4.5% in-field losses.
In other words, intentionally storing soybeans at very low moisture levels “to ensure they keep well” is generally not a good strategy. The goal should be to bring them into a safe storage range.
Dry gently: Soybeans are not corn
When soybeans arrive too moist for storage, you need to act quickly. But it’s also important to remember that soybeans tolerate drying less well than corn.
Air-drying is particularly beneficial when the initial moisture content remains reasonable. The Ontario Ministry of Agriculture states that natural drying is generally suitable for soybeans harvested at less than approximately 16 to 18 percent moisture, provided there is adequate airflow and enough time to complete the drying process before the grain deteriorates. (Gouvernment of Ontario). Generally, this approach is feasible for small bins—20 feet tall or less. For large bins, the grain must be processed through a dryer.
The problem in Quebec is that the natural air-drying season for soybeans is sometimes too short. This is particularly true when the grain is harvested very late and under cold weather conditions. As fall progresses and temperatures drop, the air’s ability to hold moisture decreases. Ventilation that worked very well at 15 °C may be much less effective at 5 or 8 °C.
CAUTION!
It is therefore important to distinguish between two types of aeration: conditioning venting (light drying) and cooling. These are not the same thing at all!
A aeration system designed primarily to maintain the temperature of a bin generally uses much less air (about 0.1 CFM per bushel) than a system intended for drying (1.0 to 2.5 CFM per bushel).
Heating: watch out for split seeds
When soybeans are too moist or outdoor conditions no longer allow them to dry quickly enough, it may be necessary to apply heat. However, the hotter and drier the air becomes, the greater the risk of damaging the seeds.
CAUTION!
Research compiled by NDSU indicates that a relative humidity of less than about 40% during drying can cause a significant increase in split beans. The phenomenon is easily explained: the surface of the bean dries quickly, while the interior still contains more moisture. The resulting internal stresses can cause the seed coat to crack.
For standard commercial soybeans, some column dryers can operate at around 120 °F (approximately 49 °C), but the quality of the grain as it exits must be closely monitored, and the temperature should be reduced as soon as the proportion of split seeds increases.
For soybeans intended for seed, NDSU recommends keeping the drying temperature below approximately 110 °F (43 °C) to protect the embryo and maintain germination capacity. Requirements for food-grade soybeans may be even more restrictive, depending on the market.
RECOMMENDATION
To minimize damage during soybean drying, it is best to limit drying to 2–3% moisture content. Therefore, if your soybeans have a moisture content above 16%, it is best to dry them in two passes.
After drying, it must be cooled
A common mistake is to think that moisture content is the only indicator of stability. Consider two silos of soybeans at 13% moisture. In the first, the grain is at 25 °C. In the second, it is at 5 °C. Even though the moisture content is identical, the biological risk is not the same. Keep in mind that temperature directly affects grain respiration, mold growth, and insect activity.
The Ontario Ministry of Agriculture notes that insect activity increases when the grain temperature exceeds about 17 °C. It also recommends using aeration to quickly remove heat from the field or heat left over from the dryer. (Gouvernment of Ontario)
With our northern climate, fall actually provides us with a very effective tool: increasingly colder air to ensure proper storage.
After harvest, the goal is generally to gradually lower the grain temperature as outdoor temperatures change. A temperature difference of more than 7 degrees Celsius is enough to cause condensation on the inner wall of the bin. That is why it is essential to ensure that the grain is not warmer than the outside air.
When condensation forms on the bin wall, that is precisely when conditions become favorable for mold growth.
Why can a bin heat up even if the average moisture content is good?
Because a bin is never perfectly uniform. One truckload might arrive at 12.2%, the next at 15.2%, and the third at 13.5%. Your average moisture content may be satisfactory for the entire harvest, but the bin may contain areas that are much wetter.
Impurities can also accumulate in the center of your bin. Broken seeds, weeds, and fine materials increase resistance to airflow. As a result, the center of the bin is not cooled, contains very moist weeds, and promotes heating. Combined with a temperature above 15 Celsius, all the conditions are in place for a thermal runaway.
Furthermore, it’s not uncommon to harvest soybeans early in the season when temperatures are around 20 °C. Once stacked, natural heating can cause the temperature to rise to 25–30 °C. Be vigilant!
Moisture migration: the pitfall of Quebec’s fall
Another phenomenon deserves special attention: moisture migration. Imagine a bin filled with soybeans at 15 °C in late September. A few weeks later, the outside air drops to 0 °C, but the center of the bin remains much warmer.
The grain near the wall cools first. Small convective movements then begin to form within the mass. Air sinks near the walls, moves toward the center, and then rises through the warmer grain. As it rises, this air carries moisture. When it reaches the cooler top of the bin, this moisture can condense.
This results in exactly the situation we wanted to avoid: a pocket of moist grain at the top of the bin, even though the average moisture content of the batch is adequate.
In fact, Quebec has already experienced instances where bins of soybeans harvested at only 10 to 12% of moisture content developed mold or sprouting near the walls and on the surface due to exceptionally high relative humidity and condensation. A warning from the Phytosanitary Alert Network at the time emphasized the importance of inspecting and ventilating silos.(Agri-Réseau)
Aerate, yes. But not just anytime.
Running a fan does not automatically improve the condition of the grain. The grain exchanges moisture with the air passing through it. At any given temperature and relative humidity, there is a corresponding equilibrium moisture content.
If the air is dry enough, it draws moisture from the grain. If it is too humid, the opposite occurs: the soybeans can absorb moisture. This is particularly relevant for soybeans, as their porosity allows for rapid changes in moisture content.
A good aeration strategy, therefore, isn’t simply to say, “It’s cold, so let’s turn on the fan.” It involves determining what effect that air will have on the grain: cooling, drying, or rehumidification.
This is precisely where automated systems that use temperature, relative humidity, and the grain’s equilibrium conditions become valuable. They allow you to select the times when outside air actually produces the desired effect rather than simply accumulating operating hours.
Avoid damaging the soybeans
Repeated cycles of drying and rehumidification are also undesirable. The grain swells when it absorbs water and shrinks when it loses it. These variations place physical stress on the seed coat.
It is therefore preferable to set a clear goal from the outset and gradually condition the bin towards that goal rather than continually alternating between drying and rehumidification.
CAUTION!
Repeated cycles of drying and rehumidification can affect the vigor of a seed kernel. That is why it is best to vent within precise and stable moisture ranges. Not only will you ensure proper storage, but you will also preserve the full value of your seed.
Monitor the bin throughout storage
Once the soybeans have cooled, the workload decreases, but monitoring should never stop. Grain that is in good condition today may develop a problem several weeks later.
In particular, monitor temperature changes, odors, condensation under the roof, and the appearance of the grain. An unexplained increase in CO2 concentration is often one of the first signs that biological activity is developing somewhere within the grain mass. Monitoring CO2 concentration allows you to detect the onset of activity several weeks in advance and take action before damage sets in.
Be extra cautious when temperatures change suddenly—such as a rapid drop in temperature in the fall, a spell of rain during the winter, or following a heavy downpour or snowstorm. Detecting leaks early will save you a lot of headaches.
The special case of soybeans intended for seed or human consumption
For these markets, maintaining quality goes beyond preventing mold. In seed soybeans, germination capacity must be preserved. A seed may appear perfectly normal yet have been exposed to enough heat or mechanical damage to reduce its vigor.
Natural air drying or low-temperature drying therefore becomes particularly advantageous. This method is especially well-suited because it reduces the risk of stress cracks and heat damage. In these specialized markets, saving a few hours during drying is rarely worth a reduction in grade or germination rate.
In food-grade soybeans, physical integrity, color, and appearance can directly influence the product’s acceptability. So, if you plan to dry your grain in your dryer, be careful and dry it on a very low heat setting!
A winning strategy for a Quebec grower
Preserving high-quality soybeans doesn’t necessarily require more interventions. Rather, it’s a matter of taking the right action at the right time.
At harvest, avoid as much as possible letting the entire field dry down to 9 or 10% to eliminate the need for conditioning. Slightly moister soybeans can be handled with fewer losses and less damage.
When loading into the bin, know the exact moisture content and be wary of averages. A moist batch hidden within an otherwise dry mass can trigger overheating. Test each load.
Also be vigilant about the grain’s temperature during filling. Even dry, hot grain can still experience thermal runaway. You must break the heating cycle and cool the newly stored grain.
When drying is necessary, prioritize a gentle approach tailored to the grain’s intended use. With seed or feed soybeans, caution is even more critical.
Next, take advantage of the natural drop in fall temperatures to gradually cool the grain while preventing moisture migration. Then, throughout winter and spring, continue to monitor the bin temperature.
In summary, a high-quality soybean bin is not simply a bin filled with grain that is “dry enough.” It is a bin containing uniform grain, with a moisture content appropriate for the storage period, kept cool, and subjected to as little stress as possible.
That is often the difference between simply storing your soybeans and actually preserving their value until delivery.
Sources
Canadian Grain Commission — Moisture content for Canadian grains. (Canadian Grain Commission)
Canadian Grain Commission — Protection of farm-stored grains, oilseeds and pulses from insects, mites and moulds. (Canadian Grain Commission)
Canadian Grain Commission — Soybeans Grading factors. (Canadian Grain Commission)
Ontario Ministry of Agriculture, Food, and Agri-Business — Grain Aeration. (Government of Ontario)
Ontario Ministry of Agriculture, Food, and Agri-Business — Natural grain drying. (Government of Ontario)
North Dakota State University Extension — Soybean Drying, Handling and Storage. (NDSU)
CRAAQ — Ventilation et conservation des grains à la ferme (French).
