Zinc phosphide mouse bait, used in the only widely registered mouse baiting product MouseOff can lose some of its potency after seven or more days in wet soils.

Linton Staples, Animal Control Technologies’ Managing Director, said MouseOff, the product made by his company, breaks down slowly in rainwater which can acidify by absorption of carbon dioxide from air.

The process is very slow over a week or two, so bait can be used effectively in moist conditions.

However, it is a different story when water gets more acidic.

Slightly acidic water leads to a chemical reaction with zinc phosphide which breaks down the active ingredient.

“The bait is fine on damp soil or after light rain, heavy dews or mist,” Dr Staples said.

“We just advise to avoid baiting before heavy rain to minimise the risk of it being washed into cracks, or onto one end of the paddock or becoming bogged into the mud.”

The active ingredient is coated onto wheat grains in an oil-based slurry, which helps repel moisture and make the bait highly attractive to mice, even when there is crop food available.

South Australian Mallee Grain grower Trevor Hancock said he has baited four paddocks up to three times this year to curb the worrisome mouse population on his Parilla property.

“There were a lot of mice in paddocks I had not cultivated, so I baited before slashing to remove cover. I had to bait a second time due to the number of mice and a third time after sowing,” he said.

Mr Hancock is aware that rain affects the bait and continues to closely monitor his new crops.

Dr Staples said recent trials have revealed that mice can move up to 300 metres a night to recolonise baited areas . Buffer zones should be considered if surrounding lands also have high mouse density, he said.