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Comparative assessment of commercial rodenticides

Table 16 (below) contains a comparative summary of the toxicity of all active compounds found in commercial rodenticides for use against house mice (M. musculus) and brown/Norway rats (R. norvegicus). While most scientific research on the effects of rodenticide compounds has been performed on brown/Norway rats, there is limited information on the toxicity of these compounds in black rats (R. rattus). Given their close biological relationship and the lack of published data, it is reasonable to conclude that the toxicity of various commercial rodenticides will be similar for both breeds.

Feed requirement for LD50 refers to the amount of commercial bait containing that active compound that the target rodent will have to eat to ingest a median lethal dose. It is calculated using the range of active concentrations at which active compounds are found in commercial rodenticides, animal bodyweight and species LD50 value (from literature). The results in this table assume 20 grams bodyweight and 25 grams daily feed requirement for mice, and 320 grams bodyweight and 20–30 grams daily feed requirement for rats.

Against mice, zinc phosphide has the smallest feed requirement, followed by second-generation anticoagulants (brodifacoum, bromadiolone, difenacoum, difethialone and flocoumafen) and cholecalciferol (vitamin D3). First-generation anticoagulants have a relatively high feed requirement. Repeat bait feeding over several days (difficult to achieve on-farm) is needed for effective control. Against rats, zinc phosphide and second-generation anticoagulants (except difenacoum) have the lowest feed requirement. The first-generation anticoagulants, cholecalciferol and difenacoum all require repeated feeding of bait for an LD50 and are not suitable for controlling rats.

Liver half-life is the time required for the concentration of a rodenticide to reduce by one half in the liver of a target rodent. It influences the level of secondary poisoning and residue risk associated with a particular rodenticide. Zinc phosphide does not accumulate in the tissues of baited rodents, which reduces the risk of secondary poisoning risk. However, because birds are highly sensitive to zinc phosphide bait too, care should be taken to separate birds from bait sources. Second-generation anticoagulants have a lower feed requirement, but they are highly persistent in the livers of baited rodents. Therefore, their use is associated with a higher risk of residue contamination. Producers using anticoagulant compounds should be aware of this risk, and should minimise contamination of production areas with the bait and baited rodents. Cholecalciferol persists in the liver and fat tissue of baited rats for up to 81 days (data not available for mice). However, birds are less susceptible to the metabolites of vitamin D3 than rodents, which reduces the risk of secondary poisoning.

In summary, there are no rodenticides that are free of risks or drawbacks. Producers should consider their production system, identify the rodent species they encounter, select an appropriate rodenticide, follow instructions for safe use, and devote resources to facility maintenance and hygiene. This will maximise the likelihood of effective rodent control and minimise the risk of contamination, secondary poisoning and the accumulation of residues. For an in-depth assessment of each commercially available rodenticide, refer to the relevant sections in this manual.

Table 16. Comparative toxicity summary of APVMA-registered rodenticides against house mice and Norway/brown rats

BaitMice (Mus musculus)Rats (Rattus norvegicus)
ActiveFeed requirement for LD50 (grams of bait)Time to deathLiver half-lifeFeed requirement for LD50 (grams of bait)Time to deathLiver half-life
Acute poisons
CholecalciferolSingle feed (1.1–3.6g)3–21 daysUnknown (birds less susceptible to metabolites)Repeated feed (18.7g)2–11 days81 days (birds less susceptible to metabolites)
Zinc phosphideSingle feed (0.02–0.05g)20 minutes: several days (dose dependent)No accumulationSingle feed (0.35–0.77g)20 minutes: several days (dose dependent)No accumulation
First-generation anticoagulants
CoumatetralylRepeated feed (2.5–54g)3–21 days16 daysRepeated feed (0.66–14.3g)**3–17 days55 days
DiphacinoneRepeated feed (56.4g)3–21 days2–4 daysRepeated feed (19.2g)3–14 days3 days
WarfarinRepeated feed (15–29.9g)6–8 days67 daysRepeated feed (37–74g)3–17 days10–26 days
Second-generation anticoagulants
BrodifacoumSingle feed (0.16g)3–18 days307 daysSingle feed (1.66g)3–14 days114–130 days
BromadioloneSingle feed (0.4–0.8g)3–19 days28 daysSingle feed (3.6–4.8g)2–16 days170 days
DifenacoumSingle feed (0.32g)4–22 days62 daysMultiple feed (11.6–16g)4–13 days120 days
DifethialoneSingle feed (0.38–1.03g)2–20 days29 daysSingle feed (3.7–6.5g)2–16 days108 days
FlocoumafenSingle feed (0.4–1g)4–19 days94 daysSingle feed (1.6–3.6g)3–11 days220 days
**Coumatetralyl bait concentration ranges from 0.37–8g/kg, 8g/kg products may be lethal for rats in a single feed, all other products require repeated feeding of bait for effective control.
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