<P> The practice of industrial animal agriculture is a relatively recent development in the history of agriculture, and the result of scientific discoveries and technological advances . Innovations in agriculture beginning in the late 19th century generally parallel developments in mass production in other industries that characterized the latter part of the Industrial Revolution . The discovery of vitamins and their role in animal nutrition, in the first two decades of the 20th century, led to vitamin supplements, which allowed chickens to be raised indoors . The discovery of antibiotics and vaccines facilitated raising livestock in larger numbers by reducing disease . Chemicals developed for use in World War II gave rise to synthetic pesticides . Developments in shipping networks and technology have made long - distance distribution of agricultural produce feasible . </P> <P> Agricultural production across the world doubled four times between 1820 and 1975 (1820 to 1920; 1920 to 1950; 1950 to 1965; and 1965 to 1975) to feed a global population of one billion human beings in 1800 and 6.5 billion in 2002 . During the same period, the number of people involved in farming dropped as the process became more automated . In the 1930s, 24 percent of the American population worked in agriculture compared to 1.5 percent in 2002; in 1940, each farm worker supplied 11 consumers, whereas in 2002, each worker supplied 90 consumers . </P> <P> According to the BBC, the era factory farming per se in Britain began in 1947 when a new Agriculture Act granted subsidies to farmers to encourage greater output by introducing new technology, in order to reduce Britain's reliance on imported meat . The United Nations writes that "intensification of animal production was seen as a way of providing food security ." In 1966, the United States, United Kingdom and other industrialized nations, commenced factory farming of beef and dairy cattle and domestic pigs . From its American and West European heartland factory farming became globalised in the later years of the 20th century and is still expanding and replacing traditional practices of stock rearing in an increasing number of countries . In 1990 factory farming accounted for 30% of world meat production and by 2005 this had risen to 40% . </P> <P> Factory farms hold large numbers of animals, typically cows, pigs, turkeys, or chickens, often indoors, typically at high densities . The aim of the operation is to produce large quantities of meat, eggs, or milk at the lowest possible cost . Food is supplied in place . Methods employed to maintain health and improve production may include some combination of disinfectants, antimicrobial agents, anthelmintics, hormones and vaccines; protein, mineral and vitamin supplements; frequent health inspections; biosecurity; climate - controlled facilities and other measures . Physical restraints, e.g. fences or creeps, are used to control movement or actions regarded as undesirable . Breeding programs are used to produce animals more suited to the confined conditions and able to provide a consistent food product . </P>

When did factory farming start in the us