There are also concerns that animals on mega farms may be over-medicated with antibiotics, as if one gets sick the whole herd is generally required to be treated. The animals are often kept indoors throughout their lives, though on some farms they are allowed access to outdoor areas at least part of the time. Mega-farms and intensive farms are controversial because they require keeping tens of thousands of animals in a small space, which campaigners and independent experts say can hamper their ability to express natural behaviours, such as moving naturally and nesting.

There are now 1,099 mega-farms of this size in the UK.

There is no legal definition in the UK of a mega-farm. The Environment Agency, and its regional counterparts, classify livestock farms as 'intensive' if they have hold at least 40,000 poultry or 2,000 pigs or 750 breeding sows. In the US, larger concentrated animal feeding operations (CAFOs) are defined as those housing 125,000 broiler chickens, 82,000 laying hens, 2,500 pigs or 700 dairy or 1,000 beef cattle.

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