Although there are as many systems of pig production as there are individual farms, these can be divided into two major types: indoor or outdoor pig production.
Indoor pigs farms feature herds of pigs kept in a relatively small, closely controlled area, usually with some form of climate control, often with liquid feeding systems, and (increasingly) 'high health. "These systems are often referred to as factory' or 'intensive' production.
Outdoor pigs feature breeding pigs (sows and litters) being kept on free-draining arable fields for one or two years per site, using 'arks' and electric fencing. More than a third of the UK herd are now being kept this way, with an increasing number of pigs being raised to slaughter weight outdoors too.
Both systems have their 'pros' and 'cons': let's start by examining the positive features of both.
Indoors you have the advantage of environmental control: piglets can be born and raised at the right temperature; adult animals can be kept cool in the summer and warmer in the winter - they also do not get the opportunity to get sunburnt; and airflow, especially the occurrence of draughts, so detrimental to pig health, can be controlled. You can also control the feed intake of rented pigs, and are better able to reduce waste (so important in these days of increasing feed costs) - it's also easy to install computer controlled feeding methods, such as automatic sow feeders and liquid feeding for fattening stock. Indoor farms tend to be more productive than outdoors given the ability to control feed and environment - it's possible to achieve a greater level of supervision and measurement and therefore control of the many variables in an indoor situation. It's also possible to establish and maintain a high health status for your herd, significantly reducing disease risks and challenges.
Outdoors though, you'd benefit from much reduced capital costs, lower running costs, a real marketing benefit in these days when 'freedom food', 'outdoor bred' and even 'organic' hold sway over consumers who might be persuaded to part with a premium price for such environmental friendship. There is a perception of higher welfare in operation for the outdoor pig (more natural, better able to express it's 'inner pig'. arable land in need of weeding and refreshing.
Great advantages, but what about the downsides?
Indoors, the set up costs are three times higher (on a per sow basis) than for an outdoor unit. Energy costs are high, and slurry disposal can be a problem (despite welcomed by the arable boys once spread and incorporated into the soil), and certainly a significant cost. The high population density of an intensive farm has it's own problems too: diseases spread like wild-fire should they gain access to the herd, and the smell can get offensive, especially on hot days. Welfare considerations are very important too - it's easier to fall foul of the law indoors than out (stocking densities and environmental enrichment come to mind).
Outdoors, the biggest problems are lower productivity and extremes of weather (on my outdoor unit I watched water free as it came out of a four inch valve on a bowser one winter). Getting quality staff is an increasing problem too - every day outside is not necessarily idyllic. Vermin control and the health status of the herd is a potential problem too, as is the management of the feeding hasd that should be kept outdoors (appetite control, feed conversion, growth rates and feed wastage will all be big challenges that will need a healthy premium price to justify).
There you have it. "Swings and roundabouts" as they say - "six of one and half a dozen of the other". Perhaps the best is a compromise - well-designed buildings and slurry management systems, with pipeline fed fat pigs (using dairy waste for example), loose housing and lots of straw. Throw in some high welfare features like Electronic Sow Feeders and plenty of environmental enrichment, and maintain a high health status, then maybe you'll have the best of both worlds? One thing I know for sure is that pigs get as miserable as we do on snowy, icy, wet and windy days, and, like us, they find draughts and high temperature equally uncomfortable.