No pressure? Forget fumigation!
With the winter harvest mostly put away, the Grains Research and Development Corporation (GRDC) is encouraging farmers considering fumigating grain stored on farm to do it wisely and carefully.
The GRDC says there are several key points when you are looking to fumigate stored grain, including effectiveness and safety.
Agri-Science Queensland grain storage expert Philip Burrill says phosphine is the single most relied upon fumigant to control stored grain pests, and as such it is everyone’s responsibility to use it wisely.
“Resistance is a problem for the entire community, and misuse of phosphine is resulting in poor insect control and increasing resistance in certain pest species,” Mr Burrill said.
“The most common problem is not fumigating in a gas tight, sealed silo. That means if you have just recently purchased a new sealable silo it should meet the Australian standard; a five minute half-life pressure test. For older existing sealable silos, passing a three minute half-life pressure test shows the silo can hold the correct phosphine gas concentrations for the required time,” he said.
A pressure test to determine whether or not a silo is properly sealed involves blowing air into the silo or storage using the aeration fan, an air compressor, hand held blower or vacuum cleaner until a small positive pressure is created. The air flow is then switched off and sealed, and then the time it takes for the pressure to halve recorded.
“Farmers may think a silo is sealed because it keeps out rain, or because it was originally purchased as a “sealed silo” but in reality even new sealable silos can have construction or design faults resulting in leaks.
“It is a good idea to pressure test silos when they full or part full of grain, which puts weight on the bottom outlet slide, a common gas leakage point. Unless tested regularly you have no way of guaranteeing your silo is gas tight.
“This is problematic because if you do have leaks, when you fumigate the gas levels won’t remain high enough to kill all life stages of stored grain pests. You may kill some of the adults, but most eggs and pupae will remain, meaning your problem insect will quickly reappear,” Mr Burrill said.
In order to kill grain pests at all stages of their life cycle (egg, larvae, pupa, adult), including pests with strong resistance, phosphine gas concentration levels need to reach and be maintained at no less than 300 parts per million (ppm) for seven days (when grain is above 25°C) or above 200ppm for 10 days (grain 15–25°C).
Growers are advised to check with silo manufacturers prior to purchase that any new “sealable silo” meets the Australian standard AS2628 which is based on a new silo meeting a five minute half-life pressure test once erected on your property. It allows growers to refer to an industry benchmark when buying a gas-tight, sealable silo.
If testing an older existing sealable silo, preferably full of grain, a three minute half -life pressure test is a good result that will provide for effective fumigations.
Grain storage specialist Peter Botta says the message about pressure testing silos is one of the most important for farmers storing grain.
“Repeatedly using phosphine in unsealed storage leads to resistance, you simply do not control all the life stages of the insect, even a single fumigation in unsealed storage will select for resistance.
“The best way to prevent resistance is to use phosphine correctly,” Mr Botta said.
“Ensuring you have a gas tight sealable silo will enable effective fumigation. This combined with aeration gives you more proactive pest management and a shorter ventilation period after fumigating – one day instead of five.”
It is important to conduct a silo pressure test early in the morning or late in the afternoon to prevent the unwanted effect of the sun heating the silo walls and roof and causing the air inside the silo to expand. Conducting a pressure test in the middle of a hot sunny day will give a false pressure test result.
“Check oil levels in the relief valve on the silo before pressurising the silo. Fill with light oil, such as hydraulic oil to the middle indicator mark,” he said.
If no aeration fan is fitted to a sealable silo, install an air valve to pressurise the silo with an air compressor. Remember you need a fair ‘volume’ of air going into a silo, not high pressure, to conduct a pressure test. For larger silos install a male PVC fitting to connect to a venturi gun that fits on the end of the air line. Ensure aeration fans are not left blowing air into a sealed silo for more than 10–15 seconds as this can cause damage to a silo. When pressurising the silo with the fan take extra care placing fan seal covers over the intake.
“If oil levels are bubbling or further than 25mm apart, wait until the oil stops bubbling and is 25mm apart. The time taken to drop from 25mm to 12mm apart should be no less than five minutes on a new silo. For older silos, full of grain it should be three minutes.” Mr Botta said.
“If the half-life is less than it should be, you have a leak. Find them by pressurising again and spraying around the seals with soapy water. Fix the leaks and then test again.
“By making pressure testing part of your regular maintenance you can ensure that your grain is protected now, and phosphine is available for use in the future.”
GRDC has a range of useful material and guides for pressure testing sealable silos at www.grdc.com.au or at www.storedgrain.com.au. In particular two new publications, Fumigating with phosphine, other fumigants and controlled atmospheres, GRDC booklet January 2011 and Pressure testing sealable silos, GRDC Fact Sheet September 2010. Look in Insect Control, under the Phosphine heading.
For information about grain storage workshops, contact Philip Burrill – 0427 696 500, Peter Botta 0417 501 890, Chris Warrick 0427 247 476.
Photo 1 – grain storage expert Peter Botta
Photo 2 – grain storage expert Philip Burrill
Photo 3 – Test silos, regardless of age, regularly to ensure effective fumigation
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