Grain storage structures
What good grain storage needs, traditional and improved stores, hermetic bags, silos, warehouses, aeration and fumigation, and capacity sums.
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Harvested grain must last until the next harvest, and often much longer as seed, reserves or for sale when prices rise. During storage it can be eaten by insects, rodents and birds, spoiled by moulds, contaminated with mycotoxins, or lose quality through heating and moisture. Losses in poor stores in warm, humid regions can reach a substantial share of the grain within months. Storage structures, from clay bins in a farmhouse to steel silos and warehouses holding thousands of tonnes, are designed to keep grain dry, cool, clean and protected.
Grain storage structures and their management are standard topics in post-harvest engineering, entomology and agronomy.
Requirements of a good store
- Protection from moisture: rain, ground water, condensation and humid air.
- Protection from insects, rodents and birds, and ease of cleaning and fumigation.
- Temperature control: cool grain stores longer; aeration with cool air limits heating and insect growth.
- Airtightness where hermetic storage or fumigation is used.
- Strength to resist grain pressure, wind and handling.
- Ease of loading, unloading and inspection, and suitable capacity and cost.
Grain must go into store clean and dry, at or below the safe moisture content for the storage period and climate, in a store that has been cleaned of old grain and insects.
Traditional and household storage
Farmers in many regions store grain in clay or mud bins, woven baskets plastered with mud, wooden or bamboo structures, underground pits, cribs for maize cobs, and jute or polypropylene bags. Many are well adapted to local materials and conditions, but are vulnerable to rodents, moisture and insects, and are hard to fumigate.
Improved household structures include metal bins and drums, improved mud-brick bins with damp-proof bases, and metal silos promoted widely in Central America and East Africa, which are airtight when sealed.
Hermetic storage
Hermetic (airtight) storage uses the respiration of insects, microbes and the grain itself to deplete oxygen and raise carbon dioxide inside a sealed container.
The best-known example is the PICS (Purdue Improved Crop Storage) bag, a triple-layer bag with two inner polyethylene liners inside a woven outer bag, first developed for cowpea in West Africa and now used for many grains and pulses. Other options include hermetic cocoons, super grain bags and sealed drums or metal silos. Hermetic storage works best with dry grain and intact, unpunctured containers.
Bulk storage: bins and silos
- Steel bins and silos, flat-bottomed or with hopper bottoms, with aeration fans, temperature cables and conveyors, used on farms and at commercial facilities.
- Concrete silos, large and long-lasting, at ports, mills and grain terminals.
- Silo bags: long, sealed polyethylene tubes laid in the field, holding about 200 tonnes each, widely used in Argentina and elsewhere for temporary storage.
Bulk storage needs aeration with ambient air at cool times to keep grain cool and uniform, preventing moisture migration and condensation caused by temperature differences.
Bag storage: warehouses and outdoor stacks
- Warehouses store grain in bags on pallets or dunnage, in stacks with gangways for inspection and fumigation. Good warehouses have damp-proof floors, rodent-proof walls, sealed eaves and good ventilation.
- Cover and plinth (CAP) storage: bags stacked on raised plinths and covered with waterproof sheets, used as low-cost, temporary bulk storage by public agencies; losses are higher than in warehouses if covers are not well managed.
- Warehouse receipt systems let farmers deposit grain in certified warehouses and borrow against it.
Pest control in store
- Hygiene: cleaning stores, removing old grain and residues.
- Drying and cooling grain.
- Hermetic and controlled atmospheres.
- Fumigation with phosphine (from aluminium or magnesium phosphide) in gas-tight conditions, for the full exposure time; poor fumigation has led to widespread phosphine resistance in stored-product insects.
- Grain protectants where approved, and inert dusts such as diatomaceous earth.
- Rodent control by proofing, traps and baits.
Silo capacity and bag stacks
A silo. A cylindrical steel silo has a diameter of 6 m and a usable grain height of 10 m.
- Volume = π × (6 ÷ 2)2 × 10 = π × 9 × 10 ≈ 283 m3.
- Wheat has a bulk density of about 780 kg/m3: capacity ≈ 283 × 0.78 ≈ 220 t.
- For maize (about 720 kg/m3), capacity ≈ 204 t; for paddy rice (about 580 kg/m3), ≈ 164 t.
A bag stack. A warehouse stack of 50 kg bags measures 6 m × 4 m in plan and 20 layers high. Each bag occupies about 0.9 m × 0.5 m lying flat, and each layer holds 6 × 4 ÷ (0.9 × 0.5) ≈ 53 bags.
- Bags in the stack ≈ 53 × 20 = 1,060 bags, or about 53 t.
Key points
- Good stores keep grain dry, cool, clean, and protected from insects, rodents and birds, and allow inspection and fumigation.
- Store only clean, dry grain in a cleaned store.
- Traditional structures use local materials; improved options include metal bins, metal silos and hermetic bags such as PICS.
- Hermetic storage kills insects by oxygen depletion without chemicals.
- Bulk storage in steel or concrete silos needs aeration; silo bags give temporary field storage.
- Bag storage uses warehouses or cover-and-plinth stacks; phosphine fumigation must be done gas-tight to avoid resistance.
- Silo capacity = volume × bulk density.
What the exams ask
- School-leaving and entrance papers: causes of storage losses, and types of storage structures.
- Degree and research-fellowship entrances: requirements of storage structures, traditional and improved bins, hermetic storage, silos and warehouses, aeration and fumigation.
- Agricultural engineering examinations: design and capacity calculations, grain pressure, aeration systems, and warehouse management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do hermetic bags need dry grain?
Wet grain respires and supports moulds even without oxygen-loving insects, and can ferment or develop off-odours. Hermetic storage works best when grain is at or below safe moisture.
Why does grain spoil near the top of a silo?
Temperature differences cause air currents that carry moisture from warm grain to cooler grain, where it condenses, often near the top surface. Aeration with cool air prevents this moisture migration.
Is fumigation enough to protect grain?
Only if the store is gas-tight and the full exposure time is used. Fumigation kills insects present at the time but does not prevent reinfestation, so hygiene and sealing remain essential.
Sources
- FAO. Grain Storage Techniques: Evolution and Trends in Developing Countries. FAO Agricultural Services Bulletin 109, 1994.
- Murdock, L. L. et al. "Death by desiccation: effects of hermetic storage on cowpea bruchids." Journal of Stored Products Research 49, 166–170 (2012).
- Brooker, D. B., Bakker-Arkema, F. W. and Hall, C. W. Drying and Storage of Grains and Oilseeds. Van Nostrand Reinhold, 1992.
- Hagstrum, D. W., Phillips, T. W. and Cuperus, G. (eds) Stored Product Protection. Kansas State University, 2012.
- Tefera, T. et al. "The metal silo: an effective grain storage technology for reducing post-harvest insect and pathogen losses in maize while improving smallholder farmers' food security in developing countries." Crop Protection 30, 240–245 (2011).
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