Grain storage – updates for fumigation in large silos and grain protectants

Take home message

  • Successful grain storage is achieved by combining regular grain monitoring, good hygiene, aeration cooling, correct fumigation practices and use of grain protectants when appropriate
  • In larger silos (150 – 2000t) recirculation of fumigation gases within the sealed silo, using a small fan, ensures rapid and uniform distribution of phosphine gas
  • Without recirculation, it can take 2-5 days before the fumigant gas reaches all areas in a large silo, resulting in significant volumes of grain and insect pests being exposed to lower amounts of gas
  • A significant R&D gap is the inability to effectively ‘monitor grain’ in the now common, large flat bottom silos. A simple, robust system for monitoring grain temperature, relative humidity, gas concentrations during fumigation and detection of the start of insect infestations is needed
  • For grain protectant treatments, prior to applying, always read the label, check with potential grain buyers and seek advice if in doubt. Set up grain protectant spray application equipment to achieve good coverage and correct dose rate

Successful on-farm storage results

Fumigations and strategic use of grain protectant insecticides are only two of the five key tools used to maintain grain quality and achieve reliable pest control.  These five practices (outlined below), when combined, form the foundation for successful grain storage. With a clear focus on these, a producer builds a reputation with grain buyers and end-users as a reliable supplier of quality grain.

Top five practices for successful grain storage:

  1. Aeration:  Correctly designed and managed, it provides cool grain temperatures and uniform grain moisture conditions. Aeration reduces storage problems with moulds and insect pests, plus maintains a range of grain quality attributes including germination, pulse seed colour, oil quality and flour quality.
  2. Hygiene:  A high standard of storage facility hygiene is crucial in keeping background pest numbers to a minimum and reducing the risk of grain infestation.
  3. Monitoring:  To prevent nasty surprises undertake monthly checking of grain in storage for insect pests (sieving / trapping) as well as checking grain quality and temperature. Keep monthly storage records, including any grain treatments applied.
  4. Fumigation:  In Australia, only fumigant gases (e.g. phosphine) are registered to deal with insect pest infestations in stored grain. To achieve effective fumigations the storage/silo must be sealable – gas-tight (AS2628) to hold the gas concentration for the required time.
  5. Grain protectants: Used on specific parcels of grain like planting seed held on farm, or bulk grain where potential grain buyers have agreed to its use, grain protectant sprays provide another line of defence against storage pests.

Fumigation of large silos (150 t or larger)

The first step – ensure “gas-tightness of storage”

Gases are the only registered products in Australia we can now use to control live insect pests when detected in grain. The most commonly used fumigant is a range of phosphine products (aluminium phosphide formulations) such as tablets or blankets. Other gases for grain pest control include sulfuryl fluoride (ProFume®), ethyl formate (Vapormate®) and methyl bromide.

The controlled atmosphere method is also effective, making use of either carbon dioxide or nitrogen gas. These are most commonly used for pest control in organic grains.

For any fumigation to be effective at controlling storage pests, the insects need to be exposed to a sufficient gas concentration (C), for a specified length of time (T).  If this “C x T” exposure requirement is not achieved during the fumigation, insect survival is likely, especially of tolerant stages such as eggs and pupae. With fumigation failures, insects reappear in the grain within days or weeks.

Therefore, it is critical for Australian grain producers who store grain for more than a month, to have at least two sealable storages that meet the Australian silo sealing standard (AS2628).

A storage that is not gas-tight does not allow the fumigation “C x T” exposure level to be reached in all parts of a silo, whether large or small.  Achieving reliable pest control results is not possible with gas leakage and air dilution.  As well as not killing the pests, poor fumigation attempts also select for resistant insect populations.

To achieve effective fumigations, silos must be pressure tested to check they are sealed and gas-tight. This ensures they can hold high gas concentrations for the required time to kill pests.

Checking a large silo is ready for fumigation – useful equipment for pressure testing

  • Portable leaf blower or small aeration fan -used to add air to silo for pressure tests. High volume, low pressure air is required. Standard air compressors are generally not suited to this task (see Figure 1).
  • 50 mm poly fitting including a 50 mm shut-off value - fitted into the external section of silo aeration ducting. This is used to blow air into the silo.
  • The silo’s pressure relief valve, or a clear U tube manometer, or better still, a digital manometer (e.g. Extech HD 755 Differential pressure manometer 0 – 0.5 psi) -  Units seek to measure pressure changes within the 0 - 4 inches water gauge (w.g.) (0 - 1000 Pa) range.  (see Figure 2)
  • Spray bottle containing water & detergent -  used to check for leaks.  Often you can hear or feel air leaks from large silos during the pressure test.

This photo shows a leaf blower and 50 mm gate valve fitted to aeration ducting used to pressurise a 1400 tonne silo for testing. Figure 1. Leaf blower and 50 mm gate valve fitted to aeration ducting used to pressurise a 1400 tonne silo for testing.

This photo shows the use of a U tube manometer to pressure test a silo. A digital manometer is also shown. Figure 2. Using a U tube manometer to pressure test a silo. A digital manometer is also shown.

Pressure test – methods

New silos should be pressure tested by the silo supplier or manufacturer when completed on site. They should pass the Australian standard test (AS2628) to show they are sealable to a standard to allow for effective fumigation.

Sealable silos should then be pressure tested at least once a year to check for suitability for fumigations. Ideally, conduct a pressure test when a silo is full of grain. This places grain pressure on all silo surfaces and outlets, which is the condition the silo is in when you are fumigating.

Pressure tests should not be conducted when the sun is heating the silo’s external steel surfaces and warming / expanding the air inside the silo. Testing late in the afternoon when hot air in the silo is cooling is also a problem. A strong windy day is difficult, as silo surfaces are pushed around. Pressure test results under these conditions are meaningless.

Ideally test in the early morning before sunrise, or on a completely overcast day. In this way the air inside the silo is not heating or cooling (expanding or contracting) due to external conditions.

For small silos the pressure tests can be carried out by using a short burst (5 – 15 seconds) from the small aeration fan fitted to the silo.  For larger silos a portable leaf blower to push air into the silo via a 50 mm fitting can be used to initially pressurise the silo for a test. (see Figure 1).

The pressure decay time (250 Pa down to 125 Pa) can be checked using one of three options: the silo’s relief valves, a length of 15 mm diameter clear plastic tube in a “U” shape with water in it (manometer), or a digital manometer connected to the silo. See Figure 2. Also see GRDC Fact Sheet: “Pressure testing sealable silos

Common leakage points for large sealable silos

  • Silo roof vents not sealing – maintenance or design problems
  • Silo grain fill point at top of silo not sealing – damaged rubber seals on lid or sealing plate
  • Grain outload auger at base of silo – leaking seal plate
  • Bottom silo access manhole into silo - damaged seals, or poor design
  • Sealing plate covers for the aeration fan’s intake, often poor design
  • External aeration fan ducting, or the aeration fan itself not well sealed
  • For some cone-based silos, weight of grain in the silo can break the seal of the bottom outlet – poor design

Fumigation recirculation – why is it important for fumigation of larger silos > 150 t

During fumigation, phosphine gas is typically liberated over 4 - 6 days from tablets or blankets that have been placed in the silo. This gas however, only moves slowly through the grain.

If fumigating a medium to large silo (150 – 2000 t), the gas may take 2 - 5 days to reach all parts of the silo. In large silo fumigations, this may result in some grain at the furthest distance from tablets, only getting 6 days of phosphine gas, instead of the required 10 days or longer exposure period.  Six days is not enough time to kill all pest life cycle stages (especially tolerant eggs or pupae).

Figures 3 and 4 below show the difference in phosphine distribution speed in a silo with and without a fan. Phosphine concentrations required to kill all pests is a minimum of 200 ppm phosphine gas concentration for at least 10 days (horizontal blue line in Figures 3 and 4 below).

This line graph shows phosphine gas concentrations at 7 points in a silo during fumigation of 1420 t of wheat. Phosphine blankets were placed in the silo headspace with no recirculation. It took up to 5 days for all grain at the silo base to reach at least 200 ppm gas concentration.
Figure 3. Phosphine gas concentrations at 7 points in a silo during fumigation of 1420 t of wheat. Phosphine blankets were placed in the silo headspace with no recirculation. It took up to 5 days for all grain at the silo base to reach at least 200 ppm gas concentration.

This line graph shows phosphine gas concentations in a silo (1420 t wheat) where a small fan was used to draw gas from blankets in the silo headspace and pump it into the silo base via aeration ducts for the first 5 days of fumigation. Gas concentration in all areas of the silo reached over 800 ppm within the first 24 hrs.
Figure 4. Phosphine gas concentations in a silo (1420 t wheat) where a small fan was used to draw gas from blankets in the silo headspace and pump it into the silo base via aeration ducts for the first 5 days of fumigation. Gas concentration in all areas of the silo reached over 800 ppm within the first 24 hrs.

This photo shows a small fan (F370 – 0.37 kW) used during the first 5 days of fumigation to recirculate phosphine to give rapid uniform gas distribution in 1423 t wheat. See Figure 4. Figure 5. A small fan (F370 – 0.37 kW) used during the first 5 days of fumigation to recirculate phosphine
to give rapid uniform gas distribution in 1423 t wheat. See Figure 4.

Application options for fumigation recirculation

  • For all fumigation recirculation systems, the sealable silo needs to be gas- tight so there is no gas leakage during fumigation. In Figure 4, “Base wall north” shows the impact of a leak at the silo manhole, causing large daily fluctuations in gas concentrations
  • Phosphine blankets or tablets can be placed in the ‘silo headspace’. A small fan, sitting at the silo base, is connected to the headspace via 90 mm pipe plumbing coming down the silo wall from the roof. Phosphine gas is drawn from the headspace and pumped into the base of the silo via both aeration ducts (see Figure 5)
  • For ground level application of tablets or blankets, a sealable ‘phosphine box’ can be plumbed into this system, either a moveable box, or mounted permanently on each silo
  • Using a fan to force the phosphine gas movement around in silos during fumigation is generally recommended, rather than relying on a passive ‘thermosiphon’ approach and especially for medium and large silo fumigations (150 t or greater) or silos storing smaller grain sizes (e.g. millets, canola or lentils) that reduces air movement.  Fan forced recirculation provides rapid gas distribution that is helpful where the grain type (e.g. oilseeds) typically absorbs higher amounts of phosphine during fumigation

Equipment for fumigation recirculation

  • Sealable silo, gas tight, that passes a pressure test
  • Plumbing pipes (90 – 100 mm) from silo roof to ground level.  Use quality pipe, fittings and seals that will ensure many years of safe, gas- tight fumigations
  • Small fan (e.g. Downfield F370 - 0.37 kW) to recirculate air. In most case this fan size will be suitable for both small & large silos. In trials (Figure 4 & 5) this fan size provided a complete silo air change every 12 hours for a full silo holding 1420 t of wheat
  • Fittings for fan intake and outlet.  Flexible hoses (50 – 100mm) couplings and gate valves

Fumigation recirculation - operations

  • Pressure test the silo to check for leaks
  • Follow all label directions and place tablets / blankets in the ‘headspace’ or ‘phosphine box’
  • Run small recirculation fan for first 5 days of fumigation. Leave silo sealed for remaining days of fumigation exposure period as label requires (e.g. 7, 10, 20 days)

Fumigation notes

There are benefits to using the silo ‘headspace’ to locate the phosphine blankets or tablets. The large surface area of grain in the headspace provides safe, easy access for liberated gas to penetrate and diffuse into the grain.

Licenced fumigators may choose to use ‘bottled gas’ formulations of phosphine to undertake fumigations in large silos and other storage types, rather than using the solid phosphine formulations of blankets or tablets. An example is Cytec’s ECO2FUME® containing 20 g/kg phosphine in carbon dioxide handled in 31 kg liquefied gas cylinders. While applying the full dose of phosphine gas on day one into a storage has benefits, in most cases the use of a recirculation systems is still recommended to provide rapid, uniform gas distribution throughout the storage.

Warning: Always seek advice from a suitably qualified professional before fitting fumigation recirculation systems to silos / storages.  Some systems that are currently sold are not recommended because of unsafe design features. Phosphine is not only a toxic gas but can be flammable and explosive if restricted in a small area or used in a manner that causes gas concentrations to rise quickly to high levels.  Follow label directions and seek advice.

R&D gap:  One of the significant R&D gaps is the inability to effectively ‘monitor grain’ in the now very common, large flat bottom silos. We require simple but robust equipment to monitor grain temperatures, relative humidity, gas levels during fumigation and also clever ways of detecting the start of insect infestations in storages.

Products such as OPI grain cables are a good starting point, however in previous research trials in Australia we found OPI moisture cables fitted to silos failed after a standard phosphine fumigation. We assume the corrosive phosphine gas had entered via the relative humidity orifice on cable sensors.  We need regular, reliable monitoring of grain in these large storages.

Profume fumigation use in Australia

ProFume® (sulfuryl fluoride gas) has only been available for use in Australia for a relatively short time, 10 years. Phosphine fumigation products have been used to control grain pests for well over 50 years.

ProFume is manufactured and supplied by Douglas Products™ based in America. A-Gas Rural® based in South Australia has the importing and distribution rights for ProFume.  They also provide specialist product and safety training to licenced fumigators, allowing them to purchase and undertake ProFume fumigations.

ProFume can only be applied by a licenced fumigator to control storage pests in cereal grains. It has a valuable role in the grain industry as an alternative fumigation product to rotate with phosphine to help manage insect pest resistance in cereal grains.

Ten years ago, a very high level of phosphine resistance was first identified at a number of bulk handling sites in Eastern Australia. It occurred in a species of one of the flat grain beetles, also known as rusty grain beetle (Cryptolestes ferrugineus) (see Figure 6). Since then it has become more common, although to a lesser extent on farms.

If growers identify flat grain beetles surviving what should have been a successful phosphine fumigation, we would recommend sending in a sample of these insects to DAF’s postharvest laboratories in Brisbane for testing. If found to be this resistant strain, then a ProFume fumigation may be recommended.

For further details on ProFume use, read the label and search ‘ProFume’ on GRDC’s web site.

This photo shows flat grain beetles, Cryptolestes spp.

Figure 6. Flat grain beetles, Cryptolestes spp.

Grain protectant sprays update

Warning

Grain protectant notes below do not apply to the grains industry in Western Australia where their use is restricted.  In all cases, product labels are to be used to determine correct use patterns.

When to use grain protectants

  • Grain protectant sprays are not to be used to disinfest grain. When live insects are detected, fumigation in a sealed silo is required for effective control
  • Typically, protectant sprays are applied to cereal grain at harvest time as grain is augered into storages, providing storage pest protection for 3 - 9 months. Protectants are effective at controlling insects as they invade the grain during storage, or the immatures (eggs, larvae or pupae) produced by such insects
  • With many domestic and export markets seeking grain supplies which are “pesticide residue free” (PRF), always talk to potential grain buyers / traders prior to applying grain protectant sprays
  • Except for some chlorpyrifos-methyl products in lupins in Victoria only, NOprotectant sprays can be applied to pulses and oilseeds

Common ‘on-farm’ uses for grain protectants

  • Planting seed held on-farm – wheat, barley, oats
  • Grain held for an extended time in non-sealable storages (not suited for fumigation) and when the grain buyer has agreed to grain protectant use that is in line with directions for use on the registered product label
  • Grain held on-farm as feed for livestock with agreement from livestock agent or buyer and is in line with directions for use on the registered product label

Grain protectant choices

Examples of two products, which include a partner product, to control the main storage pest species:

1. Conserve Plus™ Grain Protector – a.i. 100 g/L spinosad, 100 g/L s-methoprene.  Used in combination with a compatible organophosphate (OP) product such as chlorpyrifos-methyl (ReldanTM), or fenitrothion.

Key recommendations

  • Always add the OP partner to Conserve Plus so the rice weevil (Sitophilus oryzae) is controlled
  • Spray equipment calibration and application care are critical to achieve correct dose and uniform coverage on grain
  • If treated grain is exposed to light, for example a semi open grain shed, cover the grain surface with a tarp or 80 - 90% shade cloth. Sunlight breaks down Conserve Plus over time
  • Take care to read notes on the web site (above) and seek advice when purchasing Conserve Plus

For label and details on product use, see Conserve Plus™ Grain Protector

2. K-Obiol® EC Combi, synergised grain protectant – a.i. 50 g/L deltamethrin, 400 g/L piperonyl butoxide.  Used in combination with an organophosate (OP) partner e.g. chlorpyrifos-methyl or fenitrothion.

For label and details on product use, see K-Obiol® EC Combi

Key recommendations

  • To control rice, maize and granary weevils (Sitophilus spp.) add a recommended OP partner (e.g. chlorpyrifos-methyl or fenitrothion) to the tank mix
  • To ensure effective pest control and that MRL’s are not exceeded, calibrate spray equipment to achieve correct dose rate & uniform coverage on grain
  • Grower users are required to complete a brief (approx. 60 minutes) online training course to be an ‘approved user’ prior to purchase of K-Obiol® EC Combi.  See above website

Insect resistant management

If possible, aim to rotate chemical active ingredients for storage pest control at your storage facility. Example, for two years use Conserve Plus™ product combination, followed by one or two years of K-Obiol® EC Combi.

Please read and follow all label recommendations and ensure that the product is registered for use in your state prior to application of any product.

Application for grain protectants

Grain protectant application requires care to achieve the correct dose and uniform grain coverage. This leads to effective pest control results and ensures MRL’s are not exceeded. See Figure 7 below.

  • Auger’s grain transfer rate.  Ensure you have good understanding of the grain flow rate, tonnes per hour, for the height the auger will be operating at
  • Calibrate your spray application unit with water and check appropriate nozzles and spray pressure are used to achieve the required application of 1 litre of spray mixture per tonne of grain

These photos show spray application equipment designed for good coverage by applying treatment at two points in the auger

Figure 7. Spray application equipment designed for good coverage by applying treatment at two points in the auger

Further information

GRDC booklet – Fumigating with Phosphine other fumigants and controlled atmospheres

GRDC Fact sheet – Pressure testing sealable silos

GRDC video – Fumigation recirculation

Dow™ AgroSciences - Conserve Plus™ Grain Protector

BAYER CropScience - K-Obiol® EC Combi

Acknowledgements

The authors acknowledge the research support by the past Plant Biosecurity Cooperative Research Centre, of which GRDC was a partner, specifically projects PBCRC3036 and PBCRC3150 under which the fumigant research was conducted. The authors would also like to thank DAF’s Postharvest research team members, GRDC’s national grain storage extension team, along with valued support from growers and other industry collaborators.

Contact details

Philip Burrill
Department of Agriculture & Fisheries, AgriScience Qld.
Hermitage research facility, 604 Yangan road, Warwick Qld. 4370
Mb: 0427 696 500
Email: philip.burrill@daf.qld.gov.au

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GRDC Code: PRB00001, PBCRC3036, PBCRC3150

GRDC Project code: PRB00001, PBCRC3036, PBCRC3150