Disinfestion of farm-stored grain immediately before delivery to customers has attracted a lot of interest from growers.
A frequent suggestion is rapid, last-minute fumigation. While this may sound attractive, research shows there are some good reasons why it's not practical or safe.
Phosphine, the only fumigant that is readily available for on-farm use throughout Australia, is most often supplied as a solid phosphide preparation. It comes under a number of trade names and in a variety of forms such as tablets, pellets, sachets, 'blankets' or 'plates'. These formulations react with moisture in the grain and surrounding air to gradually release phosphine gas.
Siroflo™ is another way of delivering phosphine to less well-sealed silos. Phosphine from cylinders is diluted with air and is continuously added to the silo at a controlled rate for a specified period.
The release of phosphine from solid preparations can take up to five days under Australian conditions. The delivery or transport of grain that has recently been fumigated could thus present a severe hazard to people or animals. The grain might contain residual fumigant gas and/or preparations that are still releasing gas. The strong message is that fumigation must not be rushed.
Some phosphide preparations leave a powdery residue after fumigation is completed which can release phosphine at a later stage. Research at SGRL has shown that a direct mixture of phosphide preparation with grain can result in unacceptable amounts of the material in the grain at receival, especially when the grain has been treated a number of times.
Time is of the essence
Time is an important factor in the effectiveness of phosphine in killing insects. Depending on the concentration maintained, and the species present, it can take between 5 and 28 days to kill all grain insects. The findings underline the fact that effective fumigation with solid phosphide preparations must be carried out in sealed silos.
Product labels typically advise 5-21 days under fumigation, followed by 2-5 days ventilation followed by a further two days withholding period. That's a total of 9-27 days, ensuring that the solid preparation (if used) has finished releasing gas and that all gas has dissipated from the grain.
More concentrated is not the answer
There is a perception that increasing the concentration will lead to a faster fumigation. Increasing the concentration above the label dosage is illegal and unacceptable. It increases the risk of excessive residues of phosphine, causing problems further down the grain stream. Moreover, it may not kill all the insects present because it disregards the time factor central to the effectiveness of phosphine.
Rule of Thumb
In a sealed silo the following steps will help ensure a safe and effective fumigation. For all steps: under no circumstance enter any silo bin when it has grain in it.
- Ensure there is enough time for all stages of the fumigation to be completed before the grain has to be moved.
- Seal the silo to pressure test standard.
- Read the preparation label.
- Use the correct amount of preparation (see the label).
- Do not contaminate grain with phosphine preparation residues.
- Do not heap phosphine-producing preparations (when heaped they can heat up and cause afire).
- Do not add water to phosphine preparations (there is a risk of fire during the reaction with free water).
- Reseal the structure after the preparation has been added.
- Install a prominent notice that the silo is under fumigation.
- Allow the full exposure period plus an airing time before removing the preparation residue or the grain.
- Be very careful when handling preparation residues: they are likely to have some unreleased phosphine gas; there may, under some circumstances, still be significant amounts of unreleased phosphine
- Correctly dispose of the residues according to label or local regulations (if not otherwise instructed bury to about 0.5 m on-site; do not transport residues in a closed vehicle).
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