Safely Storing Fall Harvested Grain Through the Following Summer
The risky nature of grain storage has led many growers to sell early, thereby sacrificing potential profits. They have sold early to avoid grain going out of condition during spring warm up or during extended summer heat. However, higher returns, because of a narrowing basis can pay handsomely when aeration strategies are implemented to manage these risks so grain can be held up to the next harvest or beyond without loss.
Maintaining grain in cool, dry condition is the accepted norm for safely storing grain. An example of an extreme strategy is to run fans during the coldest winter weather and not again. Another is to dry several points lower than market levels to be safe. Both strategies avoid additional aeration and are high stakes games.
These practices often see this grain go out of condition or have insect infestations in part of a bin. So cool and dry are not sufficient to properly store grain. What is the answer? First is to realize kernels of grain are seeds and they are designed to preserve themselves. All that is necessary is to keep them from being eaten. Rodents and birds want to eat these seeds. Insects and fungi also do but there is a big difference between animals and birds and insects and fungi.
Big cats – a tiger for instance — will eat just to fill its stomach. Rodents and birds will eat to be full but also contaminate more than they eat with droppings, hair, feathers, urine, and saliva. Good, sound structures can protect stored seeds from rodents and birds, but not from insects and fungi.
Insects and fungi need an environment in which to multiply, otherwise they present little threat to live seeds. In a suitable environment they colonize, grow fast, release heat and water, and may even provide a new setting for cousins to colonize so a cascade effect begins. So the answer is to keep the seeds in an environment that does not support insect activity or fungal multiplication. We might call this a 'safe' bin state which means the grain mass in a bin is near equilibrium with a targeted temperature and a targeted moisture content that is known to be safe for the current season. This means all sections of a bin.
This 'safe' bin state can be maintained year round and for longer with proper aeration. What is necessary is to aerate frequently enough with fresh air to prevent a stale situation from developing anywhere in the bin. When the grain temperature and moisture content are kept below the threshold for insect or fungal activity nothing happens and the grain is safe.
This fresh air is ambient air of some combination of temperature and relative humidity suitable for each season. When fans are operated, the air should be near the same combination as at earlier fan run times. This prevents counter productive fronts which complicates keeping a safe bin state. The changes from season to season are small, averaging about 2° F per week from spring to summer in the US Midwest.
Ideally some runtime would occur each day for a short while but weather does not deliver this ideal. The remedy is to select bursts of runtime when outside air is close to the ideal. As time lapses without air close to the ideal it becomes better to loosen up the parameters to accumulate some run time than to wait longer with none. The time to wait for weather to change or to loosen parameters is closely related to the CFM per bushel available.
Greater differences between grain temperature and the average outside temperature accelerate moisture migration. Other effects such as the sun warming one side of a bin also starts to move conditions away from a safe equilibrium to an unsafe disequilibrium. Insects and molds are waiting for this to pounce. They do not pounce like a cat but once the threshold is passed the escalation can seem like it. A little prevention to stop them from starting works very well and requires fewer fan hours and is much better than trying to stop them after they get going.
The air combination varies by season and year. In order to avoid a storage crisis it is essential to be within a range of safe particulars. This is very important to understand because it exposes rules like do not warm grain higher than 50° F to prevent insect activity. This rule will restrict fan operation for an extended period of time in some locals many weeks and even months without bathing the grain mass with fresh air to pull the areas that had started to move toward conditions likely to support insect and fungal activity back to a safe bin state.
Insects do not like to nest where there is air movement, rather they seek protected areas. So frequent fan operation tends to minimize insect proliferation from a disturbance effect in addition to keeping the temperature and MC below a spoilage threshold. But to run fans frequently in summer requires the grain temperature to be increased to somewhere near the seasonal average so adequate runtime can be accumulated.
How warm can grain safely be stored during summer? The answer is it depends on many factors but the warmer the temperature the lower the moisture content has to be. Guidance can be found in an equilibrium moisture content (EMC) table:
Yellow corn (variations from these levels occur by variety, season, geography, and other factors)
| Temperature and MC guidelines | |
|---|---|
| 50° F | 16.3 EMC |
| 55° F | 16.0 EMC |
| 60° F | 15.7 EMC |
| 65° F | 15.3 EMC |
| 70° F | 15.0 EMC |
| 75° F | 14.8 EMC |
| 80° F | 14.5 EMC |
Each of these MC levels is at 70% equilibrium relative humidity (ERH) which equates to 0.7 water activity which food scientists have observed to be an upper limit to avoid fungal growth. This guidance is soft, not hard because other factors also need to be considered. Grain quality has a large impact on this threshold as does BCFM (broken corn and foreign matter). Other grains correlate to corn but with a different moisture content even though the water activity is the same.
[Sound seeds are alive and exhibit a natural resistance to spoilage. If no dead organic material is mixed with these seeds the MC guidance above can be cautiously exceeded. Hot spots demonstrate the increased likelihood of spoilage when fines are present in higher concentrations.]
The forces at work — moisture migration, the sun warming one side, wind currents, etc. — move regions of the grain mass to disequilibrium (of the targeted bin state) and the seeds become sitting ducks for insects and fungi in these areas. Once started they spread vigorously. Frequent fan operation nips these deviations in the bud and returns the bin state close to equilibrium. This prevents the problem from starting. It is not necessary to run the fans for many hours like when trying to move a front all the way through the bin. What is necessary is to interrupt the small start toward disequilibrium in susceptible areas.
Fines and foreign material are more inviting for fungal growth because they have no resistance like sound seeds. Once started they give off heat and water so soon the resistance of sound seeds to the attackers is overwhelmed due to moving too far from equilibrium for a safe bin state. A little fresh air (within parameters) periodically interrupts this progression — cleaning and leveling also help a lot.
Is keeping grain cooler a good practice? Yes, but only in relation to the climate, season, and where it will be shipped. Otherwise attempts to keep grain cool (or dry) for safety can backfire when the forces moving the grain mass to disequilibrium become dominant. The reason is moisture migration accelerates with greater differences in temperatures. The pressures toward disequilibrium are minimized when grain is warmed within a small margin of the average ambient temperature.
(It is not necessary to warm grain as high as the average ambient temperature. What is important is to warm it sufficiently to allow regular fan runtime when dips in temperature occur during hot spells.) This enables more frequent fan operation during extended hot periods which is crucial to bringing the bin state back near equilibrium.
Precision sensors enable operating with high confidence at closer tolerances to the soft thresholds that lead to spoilage. This usually means delivering grain closer to its most usable MC. MC affects total weight but regulation of MC so it is optimal for its use increases value more than just water weight gains. Less precise sensors require a buffer to accommodate sensor error.
Another management consideration is recovery (from a problem — not repairing damage already accumulated) if grain starts to go out of condition. Grain temperatures that differ a great deal from the average ambient temperature can severely complicate recovery because running fans can cause condensation and compound the problem. So relying on cool to get through the summer is a bit like playing with fire. You may not get burned every time, but severely burned sometimes.
A strategy that minimizes these risks more intelligently has high appeal. This more intelligent strategy balances temperature control with MC levels with typical weather for a region and then adjusts automatically for deviations in real time weather and quality variations.
Understanding why grain spoils and applying this to an aeration strategy can maximize returns from crops.