The three rat species most commonly causing problems in pig farms are the brown rat, the black rat, and the house mouse. Collectively referred to as "commensal rodents," these species readily infiltrate human-occupied environments and tend to cause significant trouble. Each has its own characteristics and distinct behavior patterns, so understanding the differences is essential when planning a control strategy.

The brown rat is a large species that tends not to fear humans. It favors areas near water, making it closely associated with manure and urine processing equipment, drainage channels, and wastewater systems.
In pig farms, brown rats often burrow into bare ground, grassy areas, or drainage embankments around the pig house, where they build up their numbers out of sight. Poor drainage management can quickly turn these areas into prime rat habitat.

The black rat is one of the most commonly encountered species in pig farms. It is highly cautious, fast-moving, and excels at navigating three-dimensional spaces.
It readily moves into elevated areas such as attic spaces, behind insulation, above ceiling panels, and inside ventilation ducts — places that are difficult for people to monitor. This makes it particularly hard to control, and it is common for damage to have already spread by the time it is discovered.

The house mouse is a small species capable of squeezing through extremely narrow gaps. Living in a semi-wild state, it readily finds its way into feed stores and materials warehouses.
Its small size makes it easy to overlook, but its high reproductive rate means numbers can increase rapidly — so it should never be underestimated.

Rats are said to consume roughly one-third of their own body weight in food per day. With the average brown rat weighing around 300g, a single rat can consume a considerable amount of feed on its own.
When scaled up to the size of a livestock operation, the extent of this damage becomes even clearer.
For example, in a poultry house with 10,000 laying hens, assuming a feed price of ¥40,000 per tonne, the following levels of feed damage may occur depending on rat population density:

• Population density below 10% (approx. 500 rats): approx. ¥2,000 in feed loss per day
• Population density up to 20% (approx. 1,000 rats): approx. ¥4,000 in feed loss per day
• Population density above 20% (approx. 2,000 rats): approx. ¥8,000 in feed loss per day

Even a few thousand yen per day adds up to a significant sum over a month or a year. On top of direct consumption, additional losses arise from feed being contaminated or bags being torn open — meaning the actual financial impact is often even greater.
Feed losses from rats are difficult to see directly, which means they may be quietly eroding profitability without the farm operator ever realizing it.

Rats gnaw on a wide variety of materials to wear down their constantly growing teeth. Among the most serious consequences is the gnawing of electrical wiring, which can cause short circuits and equipment failures.
Rat-related wiring damage has been implicated in the shutdown of windowless pig house ventilation systems on more than one occasion. Wiring failures not only halt equipment operation — they also carry a significant risk of fire, making this a particularly serious concern.

Rats can transmit pathogens such as Salmonella. In cases where stunted growth in pigs occurs frequently without an obvious cause, rat involvement may need to be considered as a contributing factor.
Rats also carry external parasites such as mites and fleas, contributing to a deteriorating sanitary environment. In pig farming, rats are not simply a nuisance — they represent a significant risk factor for overall hygiene management.

Pig farms provide a consistently available food source, giving rats a stable and reliable supply of nutrition. In particular, environments with high-protein feed are known to support especially rapid rat reproduction.
The straightforward dynamic of "rats congregate where food is available" applies directly to pig farms.

Because food is close at hand, rats have no need to travel far. As a result, their activity range remains narrow and population density tends to increase within a concentrated area.
This clustering behavior means that once conditions allow, damage can escalate rapidly — and delayed action can lead to a serious infestation.

Feed stores, materials warehouses, under equipment, behind insulation panels, inside attic spaces, and along drainage channels — pig farms offer numerous places for rats to conceal themselves.
The more areas that are out of human sight, the more comfortable rats feel settling in.

Rats give birth 5 to 6 times per year, producing an average of around 6 pups per litter. They mature quickly and begin reproducing themselves within just a few months.
As a result, even if 50% of a rat population is eliminated, numbers are said to recover to their original level within approximately three months. In feed-rich environments like pig farms, reproduction is especially rapid, making it very difficult for control measures to keep pace.

Rats are sensitive to sudden changes in climate. For this reason, winter and summer are periods when their reproductive activity is somewhat reduced — making these seasons the ideal time for thorough and intensive control measures.
Targeting these lower-activity periods and applying repeated treatments is key to preventing the spread of damage.

Coumarin-based rodenticides have long been the primary tool in rat control. However, in recent years, there have been increasing reports that they are becoming less effective.
Second-generation coumarin compounds have been developed that deliver a stronger effect even at low doses, but it is becoming clear that relying on chemicals alone has its limits.

Physical methods such as glue traps, capture traps, and ultrasonic devices are also available. However, their effectiveness in covering an entire large pig farm is limited.
While they may offer partial solutions, they rarely lead to a fundamental resolution of the problem.

Repellents are also used to protect wiring and materials from rat damage. For example, cycloheximide (CHI) works by exploiting the rat's tendency to avoid anything that previously caused an unpleasant sensation — in this case, an unpalatable taste.
When applied to materials or electrical cables, it can be effective in reducing damage. This approach of "keeping rats away" rather than simply "killing them" is an important element of a comprehensive pest control strategy.