Canadian Grain Commission
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Protection of farm-stored grains, oilseeds and pulses from insects, mites and moulds



4. Protecting stored products

Types of storage

Well-constructed, weather-proofed bins are essential to prevent infestations and to preserve crop quality during long-term storage. Bins on high, well-drained land protect the crop from heavy rainfall and spring floods. Steel bins, when empty, provide fewer places for insects to breed than empty wooden granaries, but residual insects can be present in dust and chaff under perforated floors.

Erect steel bins on steel-reinforced concrete slabs to prevent cracks and moisture transfer through the floor. Use a caulking compound to fill both cracks in the floor and open joints between the floor and wall. Shape the concrete pads slightly convex to shed water. Fill bins no higher than the top ring, leaving ample head room above the surface of the grain for inspection and sampling. Install aeration systems that cool the stored crop and reduce moisture migration to minimize the risk of spoilage and insect or mite infestations during storage.

When yields are above average, crops are often stored in machinery sheds or barns. Take extra care to prevent spoilage when using these types of storage. Fill cracks in concrete floors with a caulking compound.

For temporary crop storage, plywood sheets can be used to construct circular cribs. Locate cribs on high, dry land and cone the ground under the grain so that rain and melted snow water can drain away. Clear away grass or straw so that mice are not given shelter around the crib. Cone the grain as high as possible at the centre to shed rain and snow and to avoid a space between the top edge of the crib and the grain surface. Avoid walking on the grain as depressions will collect water. If a plastic sheet is placed over the coned grain, tie down the sheet with fish netting or place several old tires on top of the sheet to prevent it from flapping and tearing in the wind. Corner-vented sheets are designed to permit the escape of moisture but may allow in more snow than unvented sheets.

Prevention of spoilage

Most spoilage begins near the top centre of the bulk where moisture contents can increase due to moisture migration and snow blowing into the bin (Fig. 1). The quality of grains and oilseeds can be maintained economically by forcing air through bulk-stored crops. Air is blown in or drawn out by means of a fan (Fig. 2) attached to a bin equipped with either perforated ducts or a perforated floor. When air is blown in, the last part of the bulk to cool will be the top layer. Check from the top to determine whether the whole bulk is cooled or whether spoilage has begun. If the air is drawn out by reversing the airflow, the last part to cool is the bottom layer. In this case, spoilage may occur at the bottom of the bin where it is much more difficult to control or monitor.

Moisture migration in an unventilated bin during autumn and winter.

Fig. 1. Moisture migration in an unventilated bin during autumn and winter.

Aeration unit.

Fig. 2. Aeration unit.

Producers have three options for moving air through bins as a means of preventing spoilage:

  • aeration to cool crop.
  • aeration to dry crop.
  • aeration with heat to dry crop.

Aeration

Purpose

The purpose of an aeration system is to preserve dry stored grain by cooling the grain and preventing moisture migration. A properly designed and operated aeration system requires only small, inexpensive fans but the airflow rate is too low to dry the stored crop. Aeration helps to conserve the quality of malting barley without pesticide residues, fungal odours, or germination damage.

Airflow rate

In aeration systems the usual airflow rates per cubic metre of grain or oilseed are about 1 to 2 L/s.

Fan size

Usually, relatively small fans are required for cooling purposes. A cylindrical bin, 6.4 m in diameter and with a 6.8-m eave height, filled with 215m3 (6000 bu) of wheat may require a fan of only 250 to 600 W (exact size depends on the actual performance of the specific make and size of fan).

When the main purposes of the ventilation system are to cool the crop and prevent moisture migration, the nonuniform airflow patterns developed by ducts placed on or in the floor are usually acceptable. A completely perforated floor produces a uniform air flow throughout the bulk, and reduces the chance of unventilated spoilage pockets developing. Even with a completely perforated floor, less air flows through the centre of the bin (Fig. 3). Install trap doors in perforated floors and ducts to facilitate removal of crop residue build-up that can harbour pests.

The movement of cooling and drying fronts through crops ventilated with air during autumn.

Fig. 3. The movement of cooling and drying fronts through crops ventilated with air during autumn.

Grain harvested on sunny days can go into storage at a temperature 8°C above the ambient temperature. Stored grain temperatures can also be high if the grain was inadequately cooled after passing through a heated-air drier. When upward aeration is started to cool this hot grain moisture will condense on the cold bin roof and drip onto the grain surface. Such condensation will decrease as the roof warms up. Adequate exhaust opening in the roof will reduce this problem and continuous, uniform airflow will remove the added moisture on the top grain surface.

Fan operation

Forced movement of cool, outside air through grains or oilseeds causes a cooling front to move through the bulk from the air entrance to the air exit. Aeration per cubic metre of grain or oilseed at an airflow rate of 1 L/s requires about 240 h or about 10 days of continuous operation to pass a cooling front completely through the bulk. Do not turn off the fan until the seeds on top have the same temperature as the outside air. During initial cooling after harvest the moisture content of the cooled crop may be reduced by about 0.5 to 1.0%.

Normally the best management strategy is to run the fan continuously after harvest until the temperature of the stored crop has been cooled down below 20°C. When the outside air temperature has further dropped to about 5°C below the crop temperature, operate the fan again continuously until the new cooling front has passed through the bulk. In winter, repeat aeration until the outside ambient temperature and grain have reached a minimum.

In-bin drying with outside air

Drying process

Moisture can be removed from stored crops by passing outside air through the bulk. In Western Canada, except for the winter months, outside air can be used with heat added only by the fan and motor. Grain in a ventilated bin begins to dry where the air enters the bulk, usually at the bottom of the bin. A drying front develops and moves slowly upward through the bulk. Below the drying front the grain is at the temperature of the incoming air and at a moisture content in equilibrium with the incoming air. For example, incoming air at 70% relative humidity will result in a moisture content of about 14 to 15% for wheat or 8 to 9% for canola. The grain above the drying front will remain at a moisture content within about 1% of its initial storage condition. To be effective, the drying front must be moved completely through the bulk before spoilage occurs. The rate of movement of the drying front is mainly affected by the airflow rate per unit mass of grain or oilseed.

To dry all the stored crop in the least possible time requires a uniform air pattern throughout the bulk. The airflow pattern in a bin equipped with a completely perforated floor and leveled grain surface is uniform unless a centre core of densely packed grain and dockage develops under the filling spout. Poor transitions from the fans to the plenum under the floor reduce the airflow through the bulk near the fan entrances.

Airflow selection

To obtain the lowest equipment and operating costs, the lowest acceptable airflow should be selected. Minimum airflows for in-bin drying are chosen so that the crop dries just before it undergoes unacceptable spoilage in the worst drying years. Farmers should contact their local provincial biosystems or agricultural engineers for airflow and equipment recommendations.

Bin selection

For a given diameter, taller bins require larger fans and hence more energy. The reduced cost of drying shallower grain or oilseed bulks must be balanced against the increased costs of steel and concrete as bin diameter is increased to store the same quantity of grain.

Fan operation

Run the fan continuously in the fall until either the crop temperature has been brought down to -10°C or the grain is dry. In the spring, if drying was not completed the previous fall and no spoilage has occurred, then continue drying when the air temperature begins to rise above 0°C. Even under humid or rainy conditions operate the fan continuously. Moist air will rewet the bottom slightly, but the main drying front will continue moving through the bulk. As long as the fan is continued in operation for a few days after the humid period, the rewetting will distribute through the bulk and will probably not cause spoilage. Rewetting can be an economic benefit if the grain at the bottom has overdried below the maximum allowed selling moisture content. Although it improves the storage quality, any drying below this regulatory value reduces the saleable mass, and thus, the monetary value of the bulk. Rewetting, however, causes the grains or oilseeds to expand and may cause structural failure of the bin walls.

In-bin drying with heated air

Increasing the air temperature by adding heat reduces the relative humidity of air entering the bulk. For example, increasing the temperature of 20°C air at 70% relative humidity to 25° C reduces the relative humidity to 50%. Wheat exposed to 70% relative humidity air will dry to 14 to 15% moisture content whereas at 50% relative humidity the wheat will dry to 10 to 11%. Although this grain will store much better than the 14-15% wheat, its saleable mass will have been reduced by about 4 to 5%, causing a similar reduction in its economic value under present marketing regulations. Thus, heat added by electric or propane heaters, furnaces and solar collectors may be uneconomical. Usually the extra heat is an economic benefit only when the relative humidity of the outside air remains high for many days, such as in parts of eastern Canada. During warm weather, adding heat requires a larger fan and more-rapid drying because the crop spoils more rapidly. Later in the fall a smaller, less-expensive fan may be used in combination with a heater.

Prevention of infestations

Insects

To prevent and control infestations we need to know where and when insects occur. Surveys have shown that most empty granaries are infested with low numbers of insects and mites. Animal feeds, trucks and farm machinery are other sources of insect infestations. Some insects can fly as well as walk, which increases their ability to infest stored crops. Take the following measures before the crop is harvested to prevent infestation and spoilage during storage.

  • Keep dockage to a minimum by controlling weeds in the growing crop; insects do not multiply extensively in stored crops that contain low amounts of dockage.
  • Clean granaries, preferably with a vacuum cleaner; burn or bury the sweepings.
  • Repair and weatherproof granaries before filling.
  • Do no allow waste grain or feed to accumulate either inside or outside storage structures.
  • Eliminate grass and weeds around granaries.
  • Do not store crops in bins next to animal feeds that are likely to be infested.
  • Spray the walls and floor of empty granaries with an approved insecticide about 1 week before crop storage.
  • Examine grains and oilseeds that have been binned tough every 2 weeks:
    • (1) push your hand into the surface at various points to feel for warmth or crusts; and
    • (2) insert a metal rod into the bulk to test for heating at various depths; after at least 15 min, preferably 60 min, withdraw the metal rod and test for warmth on the wrist or palm of the hand.
  • Store new grains or oilseeds only in clean, empty bins. Bins that contain old grain might be infested.
  • Try to sell high moisture grains first (as feed).
  • Remember that cool, dry grains or oilseeds seldom spoil.

Mites

Mite infestations can be prevented and/or controlled by the following procedures:

  • Keep the moisture content of cereal grain below 12% and that of canola below 8%.
  • Transfer the grain or oilseed to an empty bin to break up moist pockets, during winter.

Storage fungi (Moulds)

To prevent storage mould activity, give particular attention to the moisture and temperature of the bulk at binning, especially in unaerated bins. Monitor bulk temperatures at 1- or 2-week intervals. Dry high-moisture and cool high-temperature grains or oilseeds by aeration (see “Prevention of spoilage”). Use spreaders to disperse dockage (small, broken and shriveled kernels; weed seeds; chaff; and straw) throughout the bulk. Remember that the increased bulk density in the bin reduces the rate of forced airflow through the bulk. Remove windblown snow before it melts and provides a focus for mould development. To control heating or spoilage in progress, move the bulk to cool it and break up high-moisture pockets. Alternatively, aerate or dry the bulk. Have someone with you when climbing into or onto granaries. Wear a dust mask to prevent inhalation of mould spores either when breaking up a mouldy crust within a bin or when handling spoiled grains or oilseeds.