Intensive pig farming, also known as pig factory farming, is a system of pig production in which pigs are raised indoors at high density under controlled environmental conditions. It has become the dominant method of commercial pork production in most industrialized countries, though traditional systems remain significant in many parts of the world. Typical facilities, sometimes called piggeries, house pigs raised for slaughter in group pens on slatted floors made of concrete, with a manure pit beneath the floor. Breeding sows are kept in gestation crates or in group housing during pregnancy, and moved to farrowing crates to give birth and nurse piglets. The waste falling through the slats is sometimes transferred to manure lagoons. The system developed over the second half of the 20th century. Features include purpose-built indoor facilities with mechanical ventilation and climate control; specialized genetic lines selected for fast growth, large litter sizes, and feed efficiency; the use of formulated diets heavier in grain than previous practice and with nutrients tailored for each life stage; and routine use of vaccines and antibiotics to manage disease. The new system replaced the mixed outdoor and barnyard operations that had previously dominated. These changes produced substantial gains in feed conversion ratio and growth rate, while requiring fewer human staff per pig. These gains have supported a large expansion in global pork production. Production also became more concentrated, with smallholder farms declining and a growing share of output coming from a smaller number of large operations. Piglets are usually separated from their mother around 3 weeks old, an earlier weaning than in traditional agriculture, and their diet switched from the sow's milk to solid feed. Other standard practices include tail docking, teeth clipping or grinding, and the surgical castration of male piglets, often performed without anesthesia. Producers cite injury prevention, disease control, and labor efficiency as rationales. These practices, along with close confinement of sows, have drawn sustained criticism from animal welfare researchers and advocacy groups. The European Union restricts the use of gestation crates to the first four weeks of pregnancy, and several U.S. states, including California and Massachusetts, have passed laws limiting confinement of breeding sows. Most other states of the United States permit broader use of crates, as do other major producers including China, Brazil, and Mexico. Because intensive operations concentrate large numbers of animals and their manure in small areas, they are associated with more acute environmental impacts from pig farming than traditional pig farming, including water pollution from nutrient runoff and air pollution from ammonia released by manure storage. The routine use of antibiotics in intensive systems has also been linked to the spread of antibiotic-resistant bacteria that can infect humans. The high density of genetically similar animals raises concerns about the potential amplification of zoonotic diseases such as swine influenza ("swine flu").
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