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Introduction
The domestication of Sus scrofa may have occurred as early as 10,000 B.C. in Southeast Asia (Lekagul and McNeely, 1977). Pig production was common in early agricultural societies; perhaps because pigs are highly resilient, mature quickly, have large litters and are able to sustain themselves on low-quality feedstuffs. Pigs were introduced into North America by many of the early explorers and settlers, but pork production began to flourish in the USA after the practice of finishing pigs on corn became established in the late 1600s (McGlone and Pond, 2003). This practice was so successful that, by 1847, there were nearly twice as many pigs as people in the USA (35 million pigs versus 20 million people) and Cincinnati was the largest pork market in the world (USDA, 1981). For most of the period after 1850, but particularly between 1890 and the present, the total number of pigs in the USA has remained between 50 and 60 million animals (USDA, 2008). But beginning about 1900 in the USA, and more recently in other parts of the world, the total number of swine farms has been declining, resulting in fewer farms with more pigs per farm (USDA, 2008).
Initial provisions for housing swine were rudimentary, as reflected in Spencer's (1919) comment that: 'The ordinary sty with a yard attached is unhealthy for a growing or matured pig, but in the colder weather it is simply cruel for newly born pigs'. An evolution in swine housing occurred concurrently with the decline in the number of producers in the 20th century. Experiments in the early 1900s led Danish agriculturalists to conclude that housing swine indoors ('intensive' production) provided for more efficient use of available land, protected animals from weather, eliminated fighting and improved feed efficiency (Shaw, 1938). Spencer (1919) marveled at the two-story barn he observed near Aarhus, Denmark in which the pigs were fed downstairs and slept upstairs, which they reached by walking up a ramp. Shaw (1938) made the crucial comment that removal of manure 'at least daily' was required to maintain sanitation under confinement conditions - a reflection of the abundant farm labor available at the time! The trend toward the production of swine in intensive systems...