Why learning how to fumigate safely is important

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As the number of growers storing grain on-farm increases, learning how to fumigate is becoming more and more important

Fumigants containing phosphine are popular among grain growers for controlling insect infestation
Fumigants containing phosphine are popular among grain growers for controlling insect infestation

Staff at the Port of Brisbane were forced to reject more than 70 truckloads of grain for export in 2016 as their levels of toxic phosphine gas exceeded the workplace safety limit – a time weighted average of 0.3 parts per million.

According to researchers from the Queensland Department of Agriculture & Fisheries, while the majority of trucks rejected had gas concentrations of between 0.5–2.5 ppm, some had very high concentrations of 32, 82 and 440ppm.

Fumigants containing phosphine, often sold as phostoxin or fumitoxin, are popular among grain growers for controlling insect infestation – they are inexpensive, accepted by international markets and control almost every pest when used correctly.

But because phosphine is an indirect toxin that reacts with oxygen in the cells to kill pests, it means that a small amount can be very effective, if given time to work and if used in the correct way.

In an update paper published earlier this year, the Grain Research and Development Corporation (GRDC) recommends that farmers fumigate within three weeks of the completion of harvest – while the grain is still warm and any insects that may have come into storage are at their most active.

In order to be suitable for fumigation, a silo needed to be pressure-tested and gas-tight – with all seals meeting Australian Standard AS2628-2010.

"To pressure test, pressurise the silo with an air compressor to 25mm water gauge (250Pa or 0.036psi). The pressure in the silo should not drop below 12.5mm (125Pa) over a three-minute period," the GRDC says.

"Most silos have a marked semi-opaque or clear pressure relief valve to measure the 25mm and 12mm pressure lines, but if not, use a home-made u-tube manometer using a length of clear hose with some water in it."

The GRDC also recommends that you make sure the pressure relief valve has light hydraulic oil (ISO46) in it, though wetter can be used as an alternative.

Should a silo fail that test, the GRDC recommends that they only be used to store untreated grain, generally being used for seed – with any treatments only being used on outload, though this should be checked with potential buyers first.

"This also means you can sell this surplus pesticide residue free (PRF) grain if you do not use it at seeding," it says.

When they are used, phosphine tablets should be spread out no more than two-deep on a tray in the headspace or in the ground-level applicator.

Phosphine is usually applied at two tablets per tonne of capacity of wheat regardless how full the silo is, the GRDC says, but users should check the label on any products used to be sure.

"Do not mix tablets in with grain — there is nothing to be gained by doing this — phosphine is a very active gas," the GRDC says.

It also recommends against wetting phosphine tablets, saying that this makes the products more dangerous – greatly increasing the likelihood of it reaching a flash flammability point or explosive concentration levels.

The GRDC also recommends the use of personal protective equipment including elbow-length PVC gloves and a breathing respirator when introducing the phosphine and when ventilating it at the end of the fumigation cycle.

While phosphine is known to have the smell of decaying fish, WorkCover Queensland says farmers should not rely on this as a way to determine whether the atmosphere is safe.

"The odour threshold for phosphine is above the exposure standard. If the odour threshold for phosphine is detected, evacuate the area immediately," it says.

The ventilation cycle should take no less than five days, the GRDC says, with all tablet residue removed and with aeration fans opened and operating for at least one day in silos where they are attached.

Once fully ventilated, the grain should be held for a further two days before it is delivered or used for either animal feed or human consumption.

"The total time required for fumigation ranges from 10–17 days accounting for the minimum exposure period, ventilation and withholding period," the GRDC says.

For further information, call the GRDC grain storage extension team on 1800 WEEVIL (1800 933 845) and go to www.storedgrain.com.au to check on anything grain storage related.

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