In animal husbandry, a concentrated animal feeding operation (CAFO; popularly known as factory farm), as defined by the United States Department of Agriculture (USDA), is an intensive animal feeding operation (AFO) in which over 1,000 animal units are confined for over 45 days a year. An animal unit is the equivalent of 1,000 pounds of "live" animal weight. A thousand animal units equates to 700 dairy cows, 1,000 meat cows, 2,500 pigs weighing more than , 10,000 pigs weighing under 55 pounds, 10,000 sheep, 55,000 turkeys, 125,000 chickens, or 82,000 egg laying hens or pullets. CAFOs are governed by regulations that restrict how much waste can be distributed and the quality of the waste materials. As of 2012 there were around 212,000 AFOs in the United States, 19,496 of which were CAFOs. Livestock production has become increasingly dominated by CAFOs in the United States and other parts of the world. Most poultry was raised in CAFOs starting in the 1950s, and most cattle and pigs by the 1970s and 1980s. By the mid-2000s CAFOs dominated livestock and poultry production in the United States, and the scope of their market share is steadily increasing. In 1966, it took 1 million farms to house 57 million pigs; by 2001, it took only 80,000 farms to house the same number. Definition There are roughly 212,000 AFOs in the United States, of which 19,496 met the more narrow criteria for CAFOs in 2016. The Environmental Protection Agency (EPA) has delineated three categories of CAFOs, ordered in terms of capacity: large, medium and small. The relevant animal unit for each category varies depending on species and capacity. For instance, large CAFOs house 1,000 or more cattle, medium CAFOs can have 300–999 cattle, and small CAFOs harbor no more than 300 cattle. The table below provides some examples of the size thresholds for CAFOs: The categorization of CAFOs affects whether a facility is subject to regulation under the Clean Water Act (CWA). EPA's 2008 rule specifies that "large CAFOs are automatically subject to EPA regulation; medium CAFOs must also meet one of two 'method of discharge' criteria to be defined as a CAFO (or may be designated as such); and small CAFOs can only be made subject to EPA regulations on a case-by-case basis." A small CAFO will also be designated a CAFO for purposes of the CWA if it discharges pollutants into waterways of the