Guide
Grain storage targets: moisture, cooling and the dates to hit them by
Updated
These are the numbers your storage kit exists to achieve. Any spend that does not help you hit them is decoration.
Moisture targets
- Food safety risk increases above 14.5% MC in cereals.
- Stored below 13% MC, moisture uptake is considerably limited, effectively preventing mites breeding.
- Dry grain at or above 18% MC immediately.
- Commercial grain is commonly traded at moisture contents of 15% and above.
- For oilseed rape, dry to 7.5-8% MC as soon as possible for prolonged, stable storage, and do not dry below 6%, the lowest acceptable moisture content.
All five are AHDB's published targets (grain storage moisture targets). AHDB also flags a monitoring rule worth writing on the store wall: if moisture content increases in a localised area by 2% or more in a week, find the cause, whether that is condensation, a leak, a hotspot or insects, and act.
Moisture and temperature work together
Storage safety is not moisture alone. AHDB explains it through equilibrium relative humidity: grain exchanges water with the surrounding air until a balance is reached, and as temperature falls, so does the ERH. Their worked example: at 5°C, wheat at 14.5% MC has an ERH of 56%; the same grain stored at 25°C at the same MC has an ERH of 66%. And crucially, mould growth and mite reproduction stop below 65% ERH.
That is why cooling capacity buys you tolerance on moisture. Cold grain at a given moisture is safer than warm grain at the same moisture, which is the argument for spending on ventilation rather than only on drying.
Cooling targets and the dates
| By when | Target | Why |
|---|---|---|
| Within 2 weeks of harvest | Below 15°C | Reduce insect activity, for example preventing saw-toothed grain beetles completing their life cycle |
| Within 3-4 months of harvest | Below 12°C | Stop insect activity, for example preventing grain weevils completing their life cycle |
| By the end of December | Below 5°C | Help kill adult insects and prevent mites increasing |
AHDB's temperature bands explain why those dates matter: above 40°C most insects die within a day, at 33-25°C most insects breed rapidly, at 25-15°C mycotoxin formation is most likely, at 15-12°C most insect species stop breeding although grain weevils may still reproduce slowly, and below 5°C insects stop feeding and mites stop increasing (AHDB cooling targets).
One exception worth knowing before you specify controls: AHDB notes that stored malting barley is usually not cooled below 10°C, because it can increase the risk of inducing secondary dormancy, and that for long-term storage malting barley is ideally dried to about 13% MC.
What that means for the kit you buy
- Start cooling as soon as ducts are covered. AHDB recommends low-volume aeration at 10 cubic metres per hour per tonne to cool grain from the moment ducts are covered, which also evens out temperature gradients and reduces moisture translocation.
- Duct spacing follows grain depth. AHDB notes that at greater depths the distance between ducts needs to be reduced, and that 4-8 m apart is often suitable for an average flat store.
- Blowing beats sucking for distribution. AHDB's comparison has blowing improving air distribution and cooling 20% more grain than sucking.
- Differential thermostats save fan hours. AHDB says a differential setting of 4-6°C provides the most rapid, cost-effective and carbon-efficient cooling, and reduces fan hours against manual control.
- Measure where cooling takes longest, furthest from the fan in blowing systems, usually 0.5 m beneath the surface and centrally between ducts.
Figures on this page are AHDB's published grain storage guidance as at this page's updated date, quoted with links above. Check the current version before designing a system around them, and follow supplier recommendations for specific products.