Grain growers are using silo aeration on their stored grain to gain harvest flexibility and more marketing options. Silo aeration can be used to cool grain and keep insect populations low. But it can also be used to dry the grain – allowing greater tolerance of moisture at harvest. But aeration drying makes the task of storing grain on farm even more challenging.
Fumigating a grain silo with phosphine.1 of 3 demonstration videos by DAFWA’s Chris Newman.
When it comes to controlling pests in stored grain — prevention is better than cure. Grain residues in storages or older grain stocks held over from last season provide ideal breeding sites. Meticulous grain hygiene combined with structural treatments, such as diatomaceous earth (DE), can play a key role in reducing the number of stored grain pests.
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Prevention: Successful pest management starts before grain goes into storage.
A bag of infested grain can produce more than one million insects during a year, which can walk and fl y to other grain storages where they will start new infestations. Meticulous grain hygiene involves removing any grain that can harbour pests and allow them to breed. It also includes regular inspection of seed and stockfeed grain so any pest infestations can be controlled before pests spread.
Removing an environment for pests to live and breed in is the basis of grain hygiene, which includes all grain handling equipment and storages. Grain pests live in dark, sheltered areas and breed best in warm conditions.
Common places where pests are found include:
Successful grain hygiene involves cleaning all areas where grain gets trapped in storages and equipment. Grain pests can survive in a tiny amount of grain, so any parcel of fresh grain through the machine or storage becomes infested
Straight after harvest is the best time to clean grain handling equipment and storages, before they become infested with pests. A trial carried out in Queensland revealed more than 1000 lesser grain borers in the first 40 litres of grain through a harvester at the start of harvest, which was considered reasonably clean at the end of the previous season. Discarding the first few bags of grain at the start of the next harvest is also a good idea. Further studies in Queensland revealed insects are least mobile during the colder months of they year. Cleaning around silos in July – August can reduce insect numbers before they become mobile.
The better the cleaning job, the less chance of pests harbouring. The best ways to get rid of all grain residues use a combination of:
Using a broom or compressed air gets rid of most grain residues, a follow-up wash-down removes grain and dust left in crevices and hard-to-reach spots. Choose a warm, dry day to wash storages and equipment so it dries out quickly to prevent rusting. When inspecting empty storages, look for ways to make the structures easier to keep clean. Seal or fill any cracks and crevices to prevent grain lodging and insects harbouring. Bags of left-over grain lying around storages and in sheds create a perfect harbour and breeding ground for storage pests. After collecting spilt grain and residues, dispose of them well away from any grain storage areas.
After cleaning grain storages and handling equipment treat them with a structural treatment. While most grain buyers accept small amounts of residue on cereal grains from chemical structural treatments, avoid using them or wash the storage out before storing oilseeds and pulses.
It is always safer to check with the grain buyer’s delivery standards for maximum residue level (MRL) allowances before using grain protectants. Diatomaceous earth (DE) (amorphous silica), commonly known as Dryacide®, can be applied either as a dust or a slurry to treat storages and handling equipment for residual control. DE acts by absorbing the insect’s cuticle (protective exterior), causing death by desiccation (drying out). If applied correctly with complete coverage in a dry environment, DE can provide up to 12 months protection — killing most species of grain insects and with no risk of building resistance.
DE requires a moving air-stream to direct it onto the surface being treated. Throwing it into silos by hand will not achieve an even cover so will not be effective.
For small grain silos and bins a handoperated duster, such as a bellows duster, is suitable. If compressed air is available it is the most economical and suitable option for on-farm use — connected to a venturi duster such as the Blovac BV-22.
The application rate is calculated at two grams per square metre of surface area treated.See table 1.
Although inert, breathing in excessive amounts of dust is not ideal, so use a disposable dust mask and goggles during application.
If safe, apply the DE dust from the top of the silo, otherwise open all outlets and apply via the ground access door. Moving the Blovac gun quickly, coat the roof, walls then base of the silo.
Finish by closing all outlets top and bottom to capture the remaining suspended dust and keep moisture out of the silo. If silos are fitted with aeration systems, distribute the DE dust into the ducting without getting it into the motor, where it could potentially cause damage. Machinery application Calculation of surface areas of machinery is not normally possible.
For augers, conveyors and grain handling equipment, use a Blovac to apply a steady dust stream into accessible openings, coating all the internal surfaces as much as possible. Continue until a dust stream emerges from the exit/discharge points of the equipment. For an average harvester the recommended quantity of inert dust is about 2.5 kilograms.
With the right equipment, DE can also be applied in a slurry form. A little more involved than applying dust, the slurry needs to be mixed in a mixing tank then sprayed on through a fl at fan nozzle capable of at least five litres per minute. Mix the DE with water at a rate of 10-20 per cent to form a slurry and apply at six grams per square metre (dry basis). The aim is to apply the slurry to give complete coverage but ensure it doesn’t run off the walls of storages and equipment. An inline filter with 1000 micron (one millimetre) mesh and a recirculation hose will help prevent nozzle blockages and keep the slurry mixed during application. Impeller pumps are most suitable — typically a fire-fighting pump with a 3.7 kilowatt (five horsepower) motor. Do not use positive displacement pumps, such as gear or piston pumps, as they will block easily.
If applying a lot of slurry regularly, use a designated, older pump as pumping slurry will reduce a pump’s working life. Apply the slurry in the same order as the dust — start at the top of the silo or storage and work down the walls applying an even coat, avoiding runs from spraying too close or too much slurry. A solid pipe extension on the application hose will enable a more even coating on hard-to-reach areas such as silo walls.
Grain kept for seed or stockfeed is a common breeding ground for pests so monitor all grain storages every two weeks during warmer periods of the year and at least monthly during cool periods of the year. Use grain insect sieves and traps to monitor for pests in all stored grain and regularly check grain handling equipment during the off season. Finding grain pests early allows them to be identified, treated appropriately and removed before they spread and become a much larger problem, which may be more difficult to treat. See Fact sheet, “Stored grain pests — identification” for more information.
Grain at typical harvest temperatures of 25–30°C and moisture content greater than 13–14 per cent provides ideal conditions for mould and insect growth (see Figure 1). There are a number of ways to deal with high-moisture grain — the key is to act quickly and effectively.
A Department of Employment, Economic Development and Innovation (DEEDI) trial revealed that high-moisture grain generates heat when put into a confi ned storage, such as a silo.
Wheat at 16.5 per cent moisture content at a temperature of 28°C was put into a silo with no aeration. Within hours, the grain temperature reached 39°C and within two days reached 46°C providing ideal conditions for mould growth and grain damage. Grain that is over the standard safe storage moisture content of 12.5 per cent can be dealt with by:
Blending is the principle of mixing slightly over-moist grain with lower-moisture grain to achieve an average moisture content below the ideal 12.5 per cent moisture content. Successful for grain moisture content levels up to 13.5 per cent, blending can be an inexpensive way of dealing with wet grain, providing the infrastructure is available. Aeration cooling does allow blending in layers but if aeration cooling is not available blending must be evenly distributed (see Figure 2).
Aeration cooling can be used to reduce the risk of mould and insect development for a month or two until drying equipment is available to dry grain down to a safe level for long-term storage or deliver. In most circumstances, grain can be stored at up to 14–15 per cent moisture content safely with aeration cooling fans running continuously, delivering at least 2–3 litres per second per tonne. It is important to keep fans running continuously for the entire period, only stopping them if the ambient relative humidity is above 85 per cent for more than about 12 hours, to avoid wetting the grain further.
Aeration drying relies on a high air volume and is usually done in a purpose-built drying silo or a partly filled silo with high-capacity aeration fans. Aeration drying is a slow process and relies on four keys:
It is important to seek reliable advice on equipment requirements and correct management of fan run times, otherwise there is a high risk of damaging grain quality.
Unlike aeration cooling, aeration drying requires high airflow, in excess of 15L/s/t, to move drying fronts quickly through the whole grain profile and depth and carry moisture out of the grain bulk. As air passes through the grain, it collects moisture and forms a drying front. If airflow is too low, the drying front will take too long to reach the top of the grain stack – often referred to as a ‘stalled drying front.’ Providing the storage has sufficient aeration ducting, a drying front can pass through a shallow stack of grain much faster than a deep stack of grain. As air will take the path of least resistance, make sure the grain is spread out to an even depth.
The way to avoid hot spots is with adequate ducting to deliver an evenly distributed flow of air through the entire grain stack. A flat-bottom silo with a full floor aeration plenum is ideal providing it can deliver at least 15L/s/t of airflow. The silo may only be able to be part filled, which in many cases is better than trying to dry grain in a cone-bottom silo with insufficient ducting.
Adequate ventilation maximises airflow and allows moisture to escape rather than forming condensation on the underside of the roof and wetting the grain on the top of the stack. The amount of moisture that has to escape with the exhaust air is 10L for every one per cent moisture content removed per tonne of grain.
For moisture transfer to occur and drying to happen, air with a lower relative humidity than the grain’s equilibrium moisture content must be used. For example, Table 1 shows that wheat at 25°C and 14 per cent moisture content has an equilibrium point of the air around it at 70 per cent relative humidity. In order to dry this wheat from its current state, the aeration drying fans would need to be turned on when the ambient air was below 70 per cent relative humidity.
Aeration drying fans can be turned on as soon as the aeration ducting is covered with grain and left running continuously until the air coming out of the top of the storage has a clean fresh smell. The only time drying fans are to be turned off during this initial, continuous phase is if ambient air exceeds 85 per cent relative humidity for more than a few hours.
By monitoring the temperature and moisture content of the grain in storage and referring to an equilibrium moisture table, such as Table 1, a suitable relative humidity trigger point can be set. As the grain is dried down the equilibrium point will also fall, so the relative humidity trigger point will need to be reduced to dry down the grain further. Reducing the relative humidity trigger point slowly during phase two of the drying process will help keep the difference in grain moisture from the bottom to the top of the stack to a minimum, by ensuring the fans get adequate run time to push each drying front right through the grain stack.
Heat can be added to aeration drying in proportion to the airfl ow rate. Higher airfl ow rates allow more heat to be added as it will push each drying front through the storage quick enough to avoid over heating the grain close to the aeration ducting. As a general guide, inlet air shouldn’t exceed 35°C to avoid over heating grain closest to the aeration ducting.
Regardless of whether supplementary heat is added to the aeration drying process or not, the grain should be cooled immediately after it has been dried to the desired level.
Dedicated drying machines are the next step up from aeration drying because they rely far less on the ambient conditions. For growers and bulk handlers who have large volumes of grain at high moisture contents, (above 16 per cent) dedicated drying machines are a more reliable option to dry grain quickly.
Designed for drying high-moisture grain in moderate quantities, batch dryers can typically remove about 3 per cent moisture content from 8–10t/hr depending on the type of grain, size of dryer and the ambient conditions. A batch of grain is put into the dryer, usually with mesh walls, and high volumes of pre-heated air are forced through the grain to dry it quickly. After grain is dried to the desired level the heater is turned off and the fan is left running for a period of time to cool the grain before augering it back into storage.
At the higher end of the grain drying equipment scale, continuous flow dryers are the most efficient way to dry large quantities of high-moisture grain. Typical operating capacity removes 3 per cent moisture content from 10–37t/hr depending on the type of grain, size of dryer and ambient conditions. Continuous flow dryers blow pre-heated air through a stream of grain before another fan blows cool air through the grain just before it leaves the dryer. The efficiency of a continuous flow dryer is largely due to the fact that the heaters remain on for the whole time and grain never stops moving.
Caution: Research on unregistered pesticide use Any research with unregistered pesticides or of unregistered products reported in this document does not constitute a recommendation for that particular use by the authors or the authors’ organisations. All pesticide applications must accord with the currently registered label for that particular pesticide, crop, pest and region.
The combination of meticulous grain hygiene plus well-managed aeration cooling generally overcomes 85 per cent of storage pest problems. For grain storage, three key factors provide significant gains for both grain storage pest control and grain quality – hygiene, aeration cooling and correct fumigation.
HygieneThe first grain harvested is often at the greatest risk of early insect infestation due to contamination. One on-farm test found more than 1000 lesser grain borers in the first 40 litres of wheat passing through the harvester. Remove grain residues from empty storages and grain handling equipment, including harvesters, field bins, augers and silos to ensure an uncontaminated start for new-season grain. Clean equipment by blowing or hosing out residues and dust and then consider a structural treatment (see Table 2, page 3). Remove and discard any grain left in hoppers and bags from the grain storage site so it doesn’t provide a habitat for pests during the off season.
Freshly-harvested grain usually has a temperature around 30°C, which is an ideal breeding temperature for storage pests (see Table 1, page 2). Studies have shown that rust-red flour beetles stop breeding at 20°C, lesser grain borer at 18°C and below 15°C all storage pests stop breeding. Aim for grain temperatures of less than 23°C during summer and less than 15°C during winter. When placing grain into storage, run aeration fans continuously for the first 2-3 days to push the first cooling front through the grain and to create uniform moisture conditions. Then run the fans during the coolest 9-12 hours per day for the next 3-5 days. This will push a second cooling front through the grain bulk. Aeration cooling generally only requires air-flow rates of 2-4 litres per second per tonne. Finally the grain requires approximately 50 hours of appropriate quality air each fortnight during storage. Use an aeration controller that will perform the cooling process at the right time and continue to aerate the grain selecting the coolest air to run fans. An effective aeration controller will also ensure fans don’t operate when the relative humidity is higher than 85 per cent, which can re-wet and damage grain if operated for extended periods.
Fumigation with phosphine is a common component of many integrated pest control strategies. Taking fumigation shortcuts may kill enough adult insects in grain so it passes delivery standards, but the repercussions of such practices are detrimental to the grains industry. Poor fumigation techniques fail to kill pests at all life cycle stages, so while some adults may die, grain will soon be reinfested again as soon as larvae and eggs develop. What’s worse, every time a poor fumigation is carried out, insects with some resistance survive, and pass the resistance gene into their progeny making control more difficult in the future.
Effective fumigationUsing the right type of storage is the first and most important step towards an effective fumigation. Only use fumigants, like phosphine, in a pressure-tested, sealed silo. Research shows that fumigating in a storage that is anything less than pressure sealed doesn’t achieve a high enough concentration of fumigant for a long enough period to kill pests at all life cycle stages. For effective phosphine fumigation, a minimum of 300 parts per million (ppm) gas concentration for seven days or 200ppm for 10 days is required. Fumigation trials in silos with small leaks demonstrated that phosphine levels are as low as 3ppm close to the leaks. The rest of the silo also suffers from reduced gas levels. Achieve effective fumigation by placing the correct phosphine rates (as directed on the label) onto a tray and hanging it in the top of a pressuretested, sealed silo or into a ground level application system if the silo is fitted with recirculation. After fumigation, ventilate grain for a minimum of one day with aeration fans running, or five days if no fans are fitted. A minimum withholding period of two days is required after ventilation before grain can be used for human consumption or stock feed. The total time needed for fumigating is 10-17 days. As a general rule, only keep a silo sealed while carrying out the fumigation (for example, one to two weeks). If grain moisture content is low (8-12%) the silo can remain sealed after fumigating but regular monitoring is essential to check for insect infestation and moisture migration to the head space.
When grain is put into storage it needs monitoring just like it does when it’s in the paddock – regularly. Check stored grain at least monthly, taking samples from the bottom, and if safe, the top of the storage.
Things to monitor:
When buying a new silo, buy a quality, sealable silo fitted with aeration and check with the manufacturer that it meets the Australian Standard for sealable silos (AS2628). Experience has shown that at least two sealable, aerated silos on farm provide the option for an effective fumigation and delivery program. Many older silos are not designed to be sealed and cannot be used for fumigation, however retrofitting aeration can reduce insect multiplication through grain cooling.
Seed that is dry, cool and sound (not weather damaged) will remain viable for longer. In well-managed storage, germination percentages can be expected to reduce by only 5 per cent after six months. To achieve this, keep grain moisture content below 12%. Grain temperature also has a major impact on germination. Aim for grain temperatures of 20°C and below in seed storage by using aeration cooling (with auto control). Wheat at 12 per cent moisture content stored at 30-35°C (unaerated grain temperature) will reduce germination percentages and seedling vigour when stored over a long period. Position small seed silos in the shade or paint them reflective white to assist keeping grain cool. WA growers can treat seed with a grain protectant combined with a dyed grain fungicide in combination with aeration cooling to maximise insect control.
Insect control options are limited for stored pulses and oilseeds. Aeration and phosphine fumigation are the main methods and controlled atmosphere (inert gasses such as carbon dioxide or nitrogen) may be an option. The effectiveness of phosphine fumigation on oilseeds is often reduced due to phosphine sorption during treatment. Monitoring gas concentrations with a gas monitor is essential to ensure the correct concentration is achieved for the correct length of time. Use sound grain hygiene in combination with aeration cooling to reduce insect activity. Small seed-size grains, such as canola, may need larger-capacity aeration fans to combat the greater amount of back pressure in the storage. Always store these grains at their recommended grain moisture content level.
Aeration cooling creates cool, uniform conditions throughout the grain to preserve grain quality, restrict or prevent mould growth and slow or stop insects breeding. Mould and insect development is slowed at temperatures below 200C and most insects stop reproducing at temperatures below 150C. Unlike aeration drying, aeration cooling can be achieved with airflow of 2-3 litres of air per second per tonne of grain and is best managed with an automatic aeration controller. For more information on how to manage fans for cooling see ‘Aeration cooling for pest control’ or ‘Aerating stored grain, cooling or drying for quality control’
Aeration that provides more than 15 litres of air per second, per tonne of grain can be used to reduce grain moisture content by a few percent. The risk of using less air (smaller fans) is that moisture will be pushed partway up through the grain but not all the way out the top of the silo. This leaves a band of even higher moisture grain susceptible to mould and insect attack. For more information about aeration drying and how to manage fans for drying, see ‘Dealing with high-moisture grain’ or ‘Aerating stored grain, cooling or drying for quality control’
Intermediate studies have shown insects can generally be found in vegetation areas, barns, sheds, spilt grain and hay stacks. Some species can fly over 5km, other means of travel include on machinery. The best time to clean-up around storages to prevent insects being attracted is during the cooler months of the year before they become mobile when the weather warms up. For more information see ‘Storage pests beat around the bush.’
Grain could be transferred into a gas-tight, sealed storage for fumigation or in some cases it is possible to fumigate in the grain bag. Fumigating in a grain bag requires a precise application method in a completely sealed bag and a fan must be used to ventilate the gas following the fumigation. For more information on how to fumigate in a grain bag see the Grain bag fumigation video