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Storing grain is not ‘set and forget’ management

Chris Newman, Technical Officer, Department of Agriculture and Food, Forrestfield

Abstract

Management of farm stored grain is sometimes referred to as ‘crisis management’—finding solutions when things start to go wrong!
To produce a successful and profitable outcome from stored grain, management must commence as soon as the grain is loaded and continue at regular intervals until outturn.
Control of stored grain insects is pivotal to retain quality, and the infrastructure built into the grain store will determine the success of the program. Sealing and aeration may appear to be an additional charge attached to the capital cost of the silo but in fact are integral to successful management plans and should be regarded as part of the initial investment in large grain silos.
Investment in grain storage is a long-term strategy and the equipment installed will ensure lower cost management operations in the future.

Key points

Plan to expand the enterprise in the future—expansion is not compulsory but lack of a future plan may make expansion more difficult.
Consider the quality of the storage before the size—better to have a grain store that will give complete control over the quality of the product than a larger store in which pests and grain condition cannot be controlled effectively.
Complete control over the quality allows access to a broader range of markets.
Consider sealing and aeration as part of the capital cost instead of an ‘optional extra’.
Fumigation cannot be effective in an unsealed store and most likely will lead to increasing resistance to the fumigant used, compromising future insect control and posing a threat to the export stream.
Large grain storages need gas recirculation devices to ensure elimination of all insects present.
Monitoring the fumigation is essential to be sure of success and to comply with quality assurance (QA) requirements.
Maintenance of the seal is an annual task and the silo cannot be declared sealed until it has been tested and proved.
Be aware of the hazards of handling quantities of phosphine in large grain stores.

Plan for success in postharvest grain marketing

Investment in grain storage must be viewed as a 15-year investment program and, within that term, marketing of the product may change and the product stored may also change. To encompass change, careful planning of the grain pathway and silo infrastructure will provide long-term flexibility for storage of a variety of grains to a range of markets. Modifying or adding equipment in the future is usually a more expensive exercise.
Future planning will never be easy but some consideration should be given to expansion of an enterprise so that investment can be staged. Crop yields will vary annually and unless you plan to store the entire crop it is better to consider storage with sufficient capacity for your current market with the surplus sold direct from harvester. Then add to existing storage as you develop your market.
Large silos are attractive because of the lower costs per cubic metre of storage if you intend to store large tonnage of a specific grade. However it may be worth considering installing smaller storages at a lower capital outlay which can be expanded as profitability permits. Smaller storages have the advantage of greater flexibility to segregate, allowing access to very specific markets, and quality control is more precise.

Plan to deliver to Quality Assured standards

Construct a smaller amount of high-quality storage to ensure a quality outturn instead of opting for a lower cost larger system without essential infrastructure and the ability to control quality in the long term. If your markets are within the food chain then the product will most likely be QA, which means within the quality bands proscribed by the buyer and insect free. If the storage is set up to deliver to the highest standards this opens up a full range of markets. If the storage does not have adequate infrastructure to deliver to QA standards then market opportunities will be limited.

Storage infrastructure to achieve QA

Cooling grain and keeping it in a cool condition is the best way to preserve the grain qualities that arrived in store from the field. Grain stored warm will gradually lose some of its baking qualities and germinability over a 12-month storage period. In addition, insect and microbial activity will continue unchecked leading to more serious quality losses. In a warm bulk of grain the temperature gradient between the walls and core drive air currents which can assist the transfer of moisture to the headspace and which may reach the point where moulds and bacteria become active. A temperature-equalised grain bulk will not have air currents operating at the same rate and so moisture in the headspace is less likely.

Aeration

Grain can be successfully cooled using an ambient air aeration system. This is a relatively low cost installation, adding 10–15 per cent to the capital cost, but provides great benefits when marketing. Insect control will be enhanced considerably by keeping grain at 20 °C or lower and these temperatures are achievable even in the warmer parts of the grain growing areas.
Aeration can be considered part of an integrated insect management program to reduce the reliance on phosphine by reducing the frequency of fumigation.
Ambient air passing through a bulk of grain alters the temperature in the interstitial space which causes the grain temperature to move towards equilibrium with the change. If the selected air has a lower relative humidity than the air surrounding the grain this action will be quite rapid due to the evaporative cooling effect.
The aeration can be controlled manually by observing local conditions but this has limitations and requires some effort to be successful. Automatic controllers remove the guesswork from the management process and reduce the labour costs and ‘inconvenience’ factor.
The larger the grain store the greater the necessity for aeration to prevent moisture migration to the headspace and it should be considered part of the initial investment plan when setting out the contract details. The initial budget may not permit additional cost but if plans are made to install aeration in future, then the groundwork can be laid at very little extra expense. When the concrete pad is laid it is a small extra cost to have duct trenches installed which can be fitted with mesh in future years and there is also a small additional cost to install the transition section in the silo wall that will eventually take the air duct from the fan. Additionally, installing the venting system in the roof at ground level as it is assembled would prevent the need to work at heights when the silo is operational.

The key to insect control is to exploit their dependence on temperature for development. The cooler the conditions the longer the period from egg to adult in any of the stored grain insect species (Table 1).

Table 1 Insect development at 70% Relative Humidity (RH)

Lesser grain borer @ 70% RH (periods in days)
Temp °C	Egg	Larvae	Pupa	Egg to Adult
34	5	17	3	25
30	6	22	7	35
28	8	25	4	37
25	11	39	5	55
22	15			84

Temperatures colder than 20 °C will further extend the life cycle but will not kill the insects until the temperature is sustained below 10 °C, so if insects must be eliminated, a fumigation procedure will be essential. This will require the silo to be sealed making it necessary that all aeration equipment is fitted with a facility to enable it to be sealed for the period of the fumigation (Figures 1 & 2). An alternative strategy may be to store all grain under aeration in unsealed storage and have one or two smaller ‘hospital’ silos into which grain can be transferred for fumigation prior to sale. This plan may fit well, for example, with an enterprise supplying mainly to feedlots and surplus grain fumigated for sale into a QA market.

Figure 1

Small aeration fan with movable seal plate on the intake fitted to a cone base silo.

Figure 2

Large aeration fan fitted to large flat floor silo, removable seal plate shown

Aeration management strategy

Remove the initial harvest heat by running the fans continuously for a period depending on the size of the grain store. The larger the store the longer the initial run period but for small silos run the fans for at least the first 24–48 hours and for large silos for about seven days. A temperature probe of some type installed in the peak of the grain stack will provide the best information but also simply sniffing the air coming out of the top of the stack is a useful determinant. The air coming out of the top of the silo will feel warm and moist and smell musty until the cooling front moves through to the top of the stack when the air will feel cool and smell pleasant (normal grain smell). Selecting cool air is not necessary for the initial period but after the initial period run the fan in the cooler part of the day. An automatic controller is a better option for long-term storage.

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Sealing and fumigation

The only fumigant available for domestic grain trading is phosphine, which can be used by unlicensed personnel. If the grain is to be eventually exported, Methyl Bromide (MB) can be used but its long-term future is in question. The Montréal Protocol requires that MB will eventually be removed from use due to its ozone-depleting characteristics. It is currently available for use in Australia only as a quarantine pre-shipment procedure. Alternative fumigant gasses to MB will be available in the future for quarantine pre-shipment and for use in the domestic market but it is most likely that these gasses will be available for use only by professional fumigators.
Carbon dioxide is an effective controlled atmosphere to eliminate all stages of the insect but the exposure time is longer, requiring a CO2 atmosphere of 35 per cent for 15 days. To retain it at lethal levels for sufficient time, the structure must be sealed to a very high standard. It is more expensive than phosphine and used mainly for higher value organic/biodynamic products.

Fumigations will not be successful unless the gas can be retained within the grain bulk for sufficient time to control all stages of the insect. The cooler the grain the slower the insect metabolism and therefore the longer the exposure period to the fumigant gas to ensure complete elimination of all life stages of the insect (egg, larvae, pupae and adult).
The egg and the pupae are the most tolerant of phosphine because they are in a less active, slow respiration stage. It is critical for success that these stages are eliminated (Figure 3).

Figure 3

The less active stages of grain insects, egg and pupa, are more tolerant of phosphine

Fumigation requires two factors for success, a threshold concentration of the gas and a time for which this concentration must be maintained. To eliminate all stages of the insect a concentration of phosphine must be maintained above 100 ppm, in the entire grain bulk, for seven days at greater than 25 °C commodity temperature and for 10 days between 25 and
15 °C.
Carbon dioxide must be held at 35 per cent for 15 days to control all insect species. To achieve this in a farm silo the gas is injected at the base of the silo to purge the atmospheric air. The gas cylinder is turned off and the silo sealed when the concentration in the headspace reaches 60–80 per cent. Sorption into the product will account for the excess CO².

Sealing will ensure that you will be able to fumigate successfully if your prospective markets demand insect-free grain.
In an unsealed—or poorly sealed—silo, the gas will be stripped out by the action of the wind on the surface causing a low pressure zone above the silo. This is also known as the chimney effect and is a cause of fumigation failure (Figure 4).

Figure 4

Chimney effect – air blowing over a silo will draw the air (and fumigant gas) upwards and out of the silo

Sealing a bolted silo to retain the gas can be more economically accomplished during construction and in fact may add about 5 per cent to the construction costs (Figures 5 & 6).
A grain store can be effectively sealed from the outside after construction but this will be more expensive to accomplish and the maintenance of the seal will be greater because of the exposure of the seal coat to the elements (Figure 7).

Figure 5

Rubber backed washers seal bolts used in construction of large silos

Figure 6

Polyurethane mastic injected into the overlaps of sheet steel as the silo is being constructed

Figure 7

Acrylic membrane sealing paints being applied to a silo after construction

Gas distribution

In silos smaller than 200 tonnes the distribution of the gas to the threshold concentration through the grain profile is normally successful without intervention and occurs within three days, at which point the fumigation period commences. In silos larger than 200 tonnes and especially in flat-floor silos the gas distribution will be slower and in some very large silos may not be achieved in all parts of the silo. It is recommended that a recirculation fan be installed in large silos and in some very wide silos a second fan be fitted. Figure 8 shows a recirculation fan that draws the gas down a tube from the headspace and blows it through the lower wall of the silo.
The aim of fumigation is to achieve the threshold lethal value in all parts of the silo as quickly as possible so that the time elapsed is consistent and there are no low value zones in the grain profile where insects may survive and be selected for resistance.

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Figure 8

Fumigant recirculation fan draws air and gas from the headspace of a silo and forces into the base

Grain insect resistance to phosphine

The elimination of stored grain insects is needed to prevent damage and loss of value to the product but also to prevent spread of insects to uninfested premises. The other very important reason is to prevent the development of strong resistance within the local insect population. Weak resistance is widespread in Western Australia but in the eastern states strong resistance to phosphine has been detected in farm and central storage. This is the result of selection pressure in low concentrations of phosphine and the mating of two weak resistant strains. The control of strong resistance adds to the challenge and cost to fumigate effectively and threatens the export stream so it is desirable that this added problem does not occur in WA.
Fumigating with the correct dose rate in a fully sealed and tested silo will eliminate all stages of stored grain pests.

Pressure testing farm silos

When a silo is delivered to farm direct from the factory it is most likely to be sealed to the recommended standard. When a silo is sealed as it is constructed on-farm, a demonstration of the sealed standard should be regarded as part of the contract. Subsequently the pressure test should be conducted annually—after inspection and replacement of damaged seals—to ensure the storage is capable of holding the gas for the required period.

The test appears to be a daunting task in small silos and a huge task in large silos. However the procedure can be accomplished quite easily with a standard farm compressor. The pressure required is very low, around 250 Pascals, but you will need large volumes to complete the test. This can be achieved with a venturi-type airgun which draws in a large volume of air while using a relatively small volume of air from a compressor, or use a vacuum cleaner set to blow (Figure 9). Inject the air through a PVC fitting inserted into the wall of the grain store. Alternatively, a tubeless tyre valve can be inserted into the wall of the silo to conduct the air. Remove the valve and direct connect an air line to speed up the operation.

Compress the silo to a pressure of 25 mm of water gauge and then time the decay of pressure. The silo should be able to retain half of the original level (down to 12 mm) for three minutes or longer (Figure 10).

Figure 9

Venturi air gun being used to pressure test a silo. Pressure is recorded on the simple manometer made from clear plastic tubing

Figure 10

Silo pressure test

From an engineering perspective, this standard is assuming minimal atmospheric influence on the pressures within the silo and it is recommended that the test be conducted under stable conditions if possible—for example, full sun or full cloud and light winds and midday temperatures.
From a practical perspective the test on-farm is more likely to be undertaken in less than perfect conditions. A full silo under strong wind or declining late afternoon temperatures is more likely to demonstrate a longer halving pressure due to the buffering effect of the grain bulk against a falling temperature. An empty or partially full silo could demonstrate a test considerably shorter than three minutes under the same conditions due to the internal air contracting more rapidly. However the seals may be adequate to retain the gas for an effective fumigation.
The test gives a reasonable indication of the quality of the seals in the candidate silo, and a degree of operator judgement is needed to factor the ambient conditions into the result.

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Phosphine monitoring and Phoscards

The test of a successful fumigation is to measure the concentrations in different parts of the grain store. Electronic measuring devices give an instant readout of the gas value in that zone and daily measurements chart the progress of the fumigation and allow decisions to prescribe a ‘top up’ if gas values are falling too quickly. Hand-held units are relatively expensive (around $2300) but are a small cost compared to loss of value of product in a large grain store.
A very low cost alternative is to use a small piece of copper placed in the grain store in the farthest accessible point away from the gas introduction port. The most convenient form of copper is a Phoscard® (Fig 11) developed by the Department of Agriculture and Food and supplied by CBH. Exposing a piece of copper to phosphine gas for the recommended fumigation period and concentration will cause it to turn green/black signifying a successful fumigation. This can only be accessed at the conclusion of the exposure period and if it has failed to change colour a re-fumigation after re-checking the silo is indicated.

Figure 11

Phosphine formulation

The most common formulation of phosphine is the solid, aluminium phosphide tablet form which has been available in Australia since the 1950s. The use of this formulation in large silos presents a logistical and safety challenge to spread the tablets thinly and apply them to the headspace many metres above ground level. The tablets need to be well spaced on a tray to allow atmospheric moisture to activate the phosphine gas and prevent excessive heating. Failure to spread them out can result in incomplete gas release as the aluminium hydroxide residue from the tablets seals off the underlying layers, prevents atmospheric moisture reaction and—in a worst case scenario—creates sufficient heat to self combust. The aluminium hydroxide powder retains 1 per cent phosphine that will only release in the presence of moisture. This could be in the stomach of an animal or breathed in if the operator is not wearing a face mask. It is important to prevent this powder entering the grain or being blown around.

A more recent formulation is the blanket or bag chain (Figures 12 & 13) where the phosphine in powder form is contained in joined paper sachets. Once the container is opened atmospheric moisture penetrates the sachets and releases the phosphine. On retrieval after the fumigation is complete the powder is retained and cannot blow around and injure the operator.

Figure 12

Phosphine in blanket formulation is a safer more convenient option when fumigating large silos

Figure 13

Phosphine in Bagchain formulation for fumigating silos up to 75 tonne

Insect monitoring

Insect monitoring is part of an integrated pest management program, providing information to initiate control. If aerating, pitfall traps installed in the peak of the grain will give early warning of infestation. When fumigation is complete it is also wise to monitor the grain for insect activity in the event the fumigation was unsuccessful or if the silo has been opened for part outloading. Aeration of grain store may also continue after fumigation leaving it vulnerable to invasion.
The most likely point of entry for grain insects is through the upper vents when the aeration is turned off and the easiest place to install insect traps is at the peak of the grain.
Commercial insect traps are not easy to obtain in Western Australia but homemade traps can be made from receptacles such as drink cans with some fly mesh taped across the top. Push these in level with the surface of the grain and inspect them monthly for signs of insect activity.

Conclusion

The establishment of a grain storage complex requires consideration of the markets you will be supplying and how you will achieve the quality required. It will never be cheaper to install the infrastructure required than at the time of building the complex. It must be given high priority to ensure success of the enterprise.

Reviewed by Rob Emery, Senior Stored Grain Entomologist, Department of Agriculture and Food, Western Australia

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Page updated : April 2008