Grain growers who have monitored mouse activity through summer and early autumn and applied baits early should now be considering baiting at seeding, or within 24 hours of seeding to reduce the impact of mice on winter crops.

While ongoing monitoring is an important part of all mouse control strategies, those growers who still have six weeks until they start seeding are now advised to check mouse numbers in the paddock and apply bait if needed.

Grains Research and Development Corporation (GRDC) Manager Pests Leigh Nelson says the organisation is currently working to help growers maximise bait effectiveness and paddock application this season amidst reports of increased mouse activity in the grain growing regions of NSW and Queensland.

Five quick tips for mouse control at seeding:

• Apply broad scale zinc phosphide bait: According to the label, at the prescribed rate of 1kg/ha.
• Calibrate spreader: Whether it is a 12 volt broadcast spreader or an adapted trailing or linkage spreader, ensure the spreader is delivering the bait rate accurately with an initial calibration (and checks of rates over known areas).
• Apply bait at seeding or within 24 hours: While seed is still covered by soil increasing the likelihood of mice taking the bait, prior to finding the seed. Rebait through the season as needed.
• Timing is critical: Delays of 4-5 days in baiting after seeding can give mice time to find crop seed.
• Minimise sources of food and shelter: Control weeds and volunteer crops along fence lines, clean up residual grain by grazing or rolling stubbles
• Monitor paddocks: Check paddocks regularly and update local data using the MouseAlert website

Dr Nelson says the GRDC understands the challenges facing growers when it comes to effectively controlling mouse numbers and is committed to working with them to overcome the issues.

“Broad scale application of zinc phosphide bait at the prescribed rate of one kilogram per hectare is currently the only method available to growers to control mice in their paddocks,” she says.

“However the GRDC and our research partners are working to try to address gaps in knowledge and other challenges through new approaches – to improve the efficacy of zinc phosphide baiting, to better understand mouse ecology and no-till systems and develop innovative alternative control tactics.”

Dr Nelson says effective mouse control hinges on keeping informed of paddock populations, so regular monitoring is critical with growers encouraged to bait when mouse activity is significant (1 burrow per square metre) at the recommended rate of 1kg/ha.

“Ongoing monitoring is critical to guide decision making for growers with bait able to be re-applied after 14 days as needed,” she says.

Know mouse numbers in the paddock

Growers should monitor paddocks to get an accurate idea of their problem as numbers can vary significantly within a district.

Monitor by walking a transect of 100 metres across the crop and counting active holes in a 1 metre wide strip (gives an area assessed of 100 square metres). Pull back stubble to check closely for mouse activity. Repeat a number of times across the paddock.

Active holes can be identified by sprinkling talcum powder around holes and inspecting the level of disturbance the following morning. One burrow per 100 square metres equals 100 burrows per hectare, or 200 mice per hectare. Burrows usually contain 1 to 4 mice but can have up to 40 mice.

Checks should be made across a paddock as populations can be patchy. Hole counts vary by soil type. In cracking soils, holes may be difficult to identify. In sandy soils, mice may dig many holes in search of seed, which can look similar to nesting burrows. In hard-setting soils, there may be few holes but each can contain many mice – up to 40 per hole during plagues.

Mouse chew cards or ‘Canola’ squares: this method is most reliable in late autumn/winter when food is scarce. It is less reliable as crops mature, because the crop provides a more attractive food source than canola-soaked card.

• Each sampling area requires 10 pieces of strong paper or light card (10cm by 10cm), marked with a 1cm grid and soaked in canola or linseed oil.
• Place the cards randomly across a paddock and peg them to the ground.
• If more than 10 squares per card are eaten overnight, significant mouse populations are emerging.
• If more than 20 squares per card are consumed in immature crops, there is a significant mouse problem.

Economic damage is likely when there are 200 to 300 mice per hectare at sowing (1 to 2 active burrows per 100 m transect) and baiting is critical. Two hundred mice per hectare can eat 1 per cent of the crop sown each night, or 14 per cent in two weeks.

When and how should I bait?

Aerial or ground application can be used to spread zinc phosphide bait. A rate of 1kg/ha provides 20,000 lethal doses per hectare. Note that 1kg/ha is a very low application rate for most spreading equipment, so an initial calibration - as well as regular calculated checks of volumes applied across known areas - should be carried out.

Adjustment of the spreading equipment or application travel speed can then be made accordingly. Growers should also check for any residue build-up when refilling the spreader and remove if needed.

Bait can be spread in stubble, pasture and crop, or a vegetative fallow, but not on bare ground. Baiting is less effective when alternative feed sources are available, so growers should endeavor where possible to clean up grain spills, control weeds and reduce food sources (eg graze sheep on stubbles).

A wide range of baits are registered for use in bait stations in and around buildings and farm storages (within 2m) or enclosed spaces, e.g. drains. Consult the APVMA for a complete listing of currently registered products.

Ideally, mouse bait should be used in dry conditions to achieve maximum ingestion.

Baiting at the time of sowing, or within 24 hours, is most effective for protecting recently sown crops. Damage is most severe for about two to three weeks after crop emergence and again around seed-set.

How many applications of bait are required?

Monitoring is the only method of establishing if further baiting is required. After baiting, mouse activity should continue to be monitored and rebaiting should not occur for at least 14 days. After 14 days reassess mouse activity and if numbers remain high then reapply.

Proactive mouse baiting program looks to control numbers up to 6 weeks prior to planting. If numbers persist then baits can be reapplied before the crop is planted and further reduce any potential damage to the crop emergence.

Zinc phosphide bait will tolerate some rain, but rain action erodes the bait quality, so rebaiting may be required if rainfall occurs within 1-3 days of baiting.

Modifying machinery for baiting

Based on current research, GRDC is advising growers to bait strategically rather than frequently, and a number of growers have modified farm machinery to distribute bait quickly and widely.

Growers have modified a variety of farm machinery to apply the 1 kg/ha of zinc phosphide required to control mouse numbers in the paddock. Some examples are as follows:

In Victoria, Brad Plant at Manangatang has built a mouse bait spreader that is ground-driven via his air commodity cart, with a keyless drill chuck and drill bits used to modify the application rate. Product is metered into the air-stream and blown out to a swath width of around 16m.

Griffith, NSW farmer Michael Pfitzner has adapted a ‘whale-tail’ spreader with bait product metered from the air commodity cart and the product drawn into a fabricated plenum mount for the whale-tail, which draws product in a venturi style.

Useful Resources

GRDC Mouse Control hub on the GRDC website.

GRDC 'Tips and Tactics: Better Mouse Management' on the GRDC website.

GRDC Update Paper 'Monitoring Mice in Australia' on the GRDC website.

MouseAlert website.

DPIRD MyCrop 'Diagnosing Mouse Damage in Crops' on the DPIRD website.

Ag Excellence Alliance ‘Controlling Mice – Baiting Strategies’ and YouTube video ‘Mice control – a challenge for conservation farming’.