Mice management and control

Mice are known to carry diseases that can pose a risk to livestock, most likely transmitted through contaminated hay and other feed sources.

Leptospirosis

Leptospirosis, commonly known as ‘Lepto’, is a disease transmitted through mice urine via direct contact or contamination of water and food sources. Strains of the disease can cause abortions and reproductive losses in cattle but not normally in sheep.

Cattle can be vaccinated against Leptospirosis, but this requires two injections four to six weeks apart. There is no vaccine available for sheep, but Leptospirosis is not thought to cause clinical disease in sheep.

Leptospirosis is also a zoonotic disease that can cause serious illness in people. For more information on this disease, visit NSW Health.

Botulism

Botulism is a disease seen in animals that eat dead decaying carcasses, including mice carcasses, or contaminated food sources. Symptoms include paralysis and sudden death, with very few animals recovering.

There is a vaccine available for both cattle and sheep, but it also requires two injections, so if you are planning on vaccinating, thinking ahead is key.

If landholders have concerns over the potential impact of mouse numbers on their livestock health, please contact your local district veterinarian or private veterinarian.

When mice levels are high, producers should be mindful that stored hay, or the hay you are buying in, could have some level of contamination from mice.

What's the risk of mice contaminated hay?

Mice not only do damage, they carry disease that can pose a risk to livestock.

It is important producers consider their options before using or purchasing potentially contaminated hay, as it can carry risks, such as:

• Livestock avoiding hay due to smell
• Leptospirosis in cattle
• Botulism in livestock.

A major risk is that mice urine can spread Leptospirosis, commonly known as “Lepto”. There are more than 250 different serovars (or “strains”) of Lepto. Two of these strains (L.hardjo and L.pomona) are known to cause abortion in cattle, and there is a vaccine available to prevent this. Hence, I would strongly recommend if you are planning on feeding out hay that has been contaminated by mice, that you fully vaccinate your cattle for Lepto well in advance of feeding (this means giving them two shots, 4-6 weeks apart). There is no vaccine available for sheep, but Lepto is not thought to cause clinical disease in sheep.

Mice carry many more strains of Lepto than just two though, and these other strains may infect humans that are handling the hay. Lepto is a debilitating zoonotic disease and there is no vaccine for people, therefore the best way to reduce the risk is to limit contact with mice urine. You may choose to do this by electing not to use the hay, using mechanical methods to move the hay so that people aren’t handling it, or wear appropriate PPE (long clothing, gloves, eye protection and a mask) when touching the hay.

Another real risk that can come from mice contamination of hay is Botulism. Botulism comes from a bacterial toxin that lives in the dead mice carcasses. When livestock eat the hay and pieces of the carcasses, they may become infected. It causes flaccid paralysis which results in animals that go down, cannot eat and drink and eventually die. Large numbers of stock may be affected and there is no treatment. Vaccination is available for cattle and sheep for botulism, but two shots are needed for most products available on the market, so again, plan ahead.

It may be prudent to vaccinate for botulism, regardless of whether you are planning to feed hay, due to the extreme contamination of paddocks with mice carcasses. Stock that are phosphorus or protein deficient will actively select to chew on carcasses and bones and are at a higher risk.

Assessing damage to hay bales

Monitoring is key when identifying the impact of mice on hay bales. Where possible, open them up and examine from various positions or sections of the haystack or shed. An assessment of the damage can be made by identifying the number and depth of mice holes and the amount of carcasses, urine of feces found.

For low levels of damage, it may be possible to remove damaged sections, and destroy them separately. However, this would only reduce your risk, not fully remove the possibility of the fodder source continuing to be contaminated.

When inspecting bales, it is important to wear personal protective equipment (PPE) and implement good hygiene practices. Mice damaged also pose a hazard because they can collapse.

What to do with mice damaged hay

Mice contaminated hay can be managed through burning, burying or spreading. When deciding which option is most appropriate, you should consider the quantity needing to be disposed, cost and local weather conditions.

Burning

Burning contaminated hay bales, in an isolated and open area, can be a cost-effective means of disposal. This practice requires minimal input but can be quite time-consuming if dealing with large quantities of hay and conditions are not conducive.

If employing this practice, be sure to comply with all NSW Rural Fire Service (RFS) requirements and implement appropriate strategies to mitigate the risk, such as firebreaks and access to water carts.

Burying

Burying contaminated hay in carefully chosen sites can be a suitable option for large amounts of hay, but cost and availability of machinery should be considered.

Pit locations should avoid sites prone to accumulating water or near water ways. Pits need to be deep enough to apply a minimum of a foot of soil between the additive height of the bales and the original soil surface. Excess soil must then be placed on top to compensate for future sinking of the pit.

Unlike other pits, these will not require lining, but will still require livestock to be restricted access from the site.

Spreading

Hay may be spread back onto paddocks. Spreading contaminated hay across a paddock with rakes can put nutrients, bound in the organic matter, back into the soil through decomposition. This will take a significant time to occur but may provide some benefit out of hay that can carry risks to livestock.

If you’ve been poisoning mice using either anti-coagulants or zinc phosphide, the actual bait itself is a poisoning risk to livestock if present in the hay when it’s fed out, however secondary poisonings of livestock from eating the carcasses is low.

More about mice in hay

For more tips and advice, you can read ‘What to do with mice damaged hay ’ by Tim Bartimote, Local Land Services Cropping Adviser.

For more information on managing mice impacted hay, contact your local vet or agricultural advisor.

Detecting mice early through monitoring is critical for managing numbers effectively, and reducing potential impacts on crops, feed and fodder and stored grain.

Monitoring can be carried out year-round, but generally the most important times to undertake monitoring are prior to sowing (March– April) and in early spring (September– October).

Observing and monitoring changes in populations so you can be familiar with what ‘normal’ levels of mouse activity are, is essential before any effective and targeted mouse control plan can be rolled out.

To help report and see what activity is like in your region, landholders can report mouse activity on the Mouse Alert website.

Monitoring techniques

Mouse chew cards and active burrow counts are simple and effective techniques that can help show if mouse numbers are increasing in your paddocks.

Mouse chew cards

Mouse chew cards are most reliable when alternative food is scarce. They should be soaked in canola or linseed oil and set out in-paddock overnight. The proportion of the card that has been chewed by mice is recorded when it is collected the following day.

The GRDC has produced a chew card template and instructions that can be downloaded on their website.

Active burrow counts

To look for active burrows, walk a transect of 100 metres across the crop and count active holes in a 1-metre-wide strip. This gives an area assessed of 100 square metres, but should be repeated 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.

The number of active burrows you find can then be used to estimate how many mice you may be dealing with. GRDC advises that if there are more than 2-3 active burrows per 100 metres, then you may have a problem with mice.

More information

All the information above has been adapted from the GRDC’s Tips and Tactics: Better Mouse Management and Mouse Control website.

If you would like to find out more, you can find a copy of Tips and Tactics from GRDC, or find a range of practical materials on the GRDC Mouse Control website.

You can also discuss your situation with your local agronomist, who will be able to provide advice on mouse management.

Livestock can be used as one of many practical tools to help control mouse populations on your property. Reducing food sources and habitat through effective grazing management in cropping and pasture paddocks can make it much harder for mice populations to rapidly increase.

Grazing stubbles

Grazing stubble not only provides valuable nutrition for livestock, but by reducing stubble and removing spilt grain from paddocks, you are also removing food sources for mice and making environmental conditions less favourable for breeding.

Grazing pastures

While mice are generally seen in cropping paddocks, they can still be found in pasture, particularly over winter, where they have ample feed and habitat. As temperatures warm up, they can then move back into cropping paddocks.

Food and habitat can be managed in pasture to some extent, but it is important to make sure there is no negative effects on the land and your livestock.

Which paddocks to graze?

Firstly, you need to decide which paddocks are the highest priority. Paddocks bordering cropping paddocks (or hay and grain storage) should be considered a high priority. Have a look at the amount of cover from dry grass and the amount of annual pasture that is coming through. In some cases, the dry grass may be inhibiting annuals from germinating. Also look for seed and seed heads both on the ground and on standing grass.

Reducing food source

Feed source can be existing seed from summer species or potential seed from annual winter grasses. If your paddock has existing seed and seed heads you may look at grazing the paddock to reduce this seed.

Sheep will do a much better job of picking seed off the ground, but they will generally only pick up seed heads or larger seeds. Cattle may eat standing seed heads but will not graze seeds from the ground.

Reducing habitat

When looking to reduce mouse habitat we are looking to remove tall dry grass, which will reduce the ability of mice to shelter and forage for food safely. Tall dry grass will be poor feed quality and, in most cases, unsuitable for sheep. Cattle may be able to utilise the grass, but feed quality will not be high enough for growing stock.

If feed quality is not high enough, you should consider supplement feeding your livestock.

More about grazing management

For more tips and advice, you can read ‘Managing mice in pastures to protect your crops in spring’ by Callen Thompson, Local Land Services Senior Land Services Officer – Mixed Farming.

For more information on grazing management, contact your local ag advisor.

Baiting is not a total solution for crop protection, but can assist in minimising potential damage from mice.

An integrated approach—employing a number of control mechanisms across the farm—is the best way to minimise damage, especially in seasons conducive to high mouse populations.

Zinc phosphide

Zinc phosphide is the currently registered chemical for controlling mice in paddocks, but it cannot be used around buildings, in towns or residential areas.

In NSW, zinc phosphide bait products are not restricted, and can be purchased directly from rural resellers.

Pesticides must be used in accordance with the label or Australian Pesticides and Veterinary Medicines Authority (APVMA) permit instructions on an approved pesticide.

How should I bait?

Aerial or ground application can be used to spread zinc phosphide bait according to the label instructions. To achieve the best baiting results, CSIRO and Grains Research and Development Corporation (GRDC) recommend you:

• Reduce alternate food sources for mice. Baiting is less effective when alternative feed sources are available and so, where possible, growers should endeavour to clean up grain spills, control weeds and reduce available food (e.g., graze sheep on stubbles)
• Ideally, mouse bait should be used in dry conditions to achieve maximum ingestion
• Lay bait when food source in the paddocks is at the lowest level, this will give mice the best chance to find the bait
• To protect recently sown crops, apply bait at the time of sowing, or within 24 hours. Damage is most severe for about two to three weeks after crop emergence and again around seed-set
• Coordinate baiting with other farmers to avoid reinvasion. Bait on as broad a scale as possible.

There are a number of different approaches landholders can take when it comes to spreading mouse bait, depending on the equipment they have, including conventional spreaders, 12-volt broadcast and air commodity cart.

The GRDC has produced a five-part video series that provides detailed information on these methods, as well as other practical baiting tips. You can view these videos here.

More about baiting mice

The baiting advice above has been adapted from the GRDC’s Tips and Tactics: Better Mouse Management and Mouse Control website.

If you would like to find out more, you see Tips and Tactics or find a range of practical materials on the GRDC Mouse Control website.

You can also discuss your situation with your local agronomist, who will be able to provide advice on mouse management.