Current pig production systems reflect management decisions taken over a period of many years. The range of constraints on pig producers has grown rapidly and priorities are in the process of change. On the basis of the available evidence, the following conclusions may be made.

Less than half the pig breeding enterprises in the United Kingdom use sow housing which involves individual confinement and more than half provide dry sows with bedding. At present, there is a strong current interest in group housing systems.

In the parturition and lactating stages, farrowing crates are used in the vast majority of herds and the main current practical interest is in improving crate and pen design and husbandry techniques to assist in minimizing piglet mortality.

Less than half of the herds use fully slatted accommodation for newly weaned pigs and nearly one-third provide bedding. In the follow-on stage, less than one-fifth of herds use fully slatted floors and ‘fully controlled environment’ type accommodation.

There is strong current interest in fully slatted finishing housing but so far less than one-tenth of herds use it. Nearly two-thirds of herds use finisher houses which are not in the ‘controlled environment’ category.

Although the environment comprises all aspects of the animals' surroundings, it is the climatic component which has the biggest effect upon productivity. The most convenient way to assess the environmental requirements of the pig is to determine its lower critical temperature and those animal, nutritional and environmental factors which influence it. This knowledge can then be used to assess the suitability of the various nutritional, husbandry and management systems for pig production including their manipulation to supply the correct environmental and housing conditions for optimal growth, development and reproduction. If conditions are not optimal then characteristics such as growth rate, food utilization, carcass composition, acquisition of immunity, sexual development and reproductive performance will be impaired. However, knowledge of the climatic requirements of the pig allows action to be taken to correct for any inadequacy of the environment and thus ensure optimum productivity. In addition, it allows greater precision in assessing food requirements and permits a rational approach to be used in the design of buildings and in deciding the type of housing, and management and husbandry practices most suited to the different types of pig production.

Within typical pig buildings highly specialized sub-units have been created, each design to cater for a different age group or function, or both. These systems of housing have been developed from a desire by the farmer to overcome welfare problems as they have arisen and also for sound economic reasons. Unfortunately, although new housing systems have solved some problems they have created others. Pig health may be influenced in a direct simple way by housing but more often by the outcome of complex interactions between organisms, the pig and its environment. Examples of typical problems which may arise because of the influence of housing systems on the pig are outlined and discussed.

One of the limiting factors in pig production is the ill health of the animals. Besides the effect of overt disease, subclinical disease and more subtle disturbances in health may be a continual problem affecting all production parameters. These disturbances range from reproductive problems such as infertility, neonatal health problems, gastrointestinal and respiratory problems during the growing-finishing period, and injuries at all production stages.

The causes of many of these disturbances and losses are multifactorial, resulting froman interaction between different environmental factors (stressors) and the animals. Many health problems are not initially due to infections but infections may appear as a secondary factor. Where bacteria, viruses and parasites are involved, the pig's immune status and ability for immune defence is often the determinant for disease. Unfavourable environmental factors do not result in a specific disease if the agent is missing.

When animals are housed, their life form is altered. Housing creates limits in their choices, freedom of movement, activities, and feeding. While pigs generally have a talent for adapting to the environment, modern production methods have made it difficult for some animals to cope with their situation. They become stressed, and especially if this stress is prolonged, negative effects appear, generally reflected in their state of health.

Studies in the USA, Sweden and the Netherlands have shown that workers in enclosed piggeries have high levels of respiratory injury, apparently caused or exacerbated by inhaled factors in their working environment. Three types of airborne hazard are known in piggeries — gases, dusts and infectious agents such as bacteria.

Of the gases that accumulate in enclosed piggeries, carbon dioxide, methane and carbon monoxide, are asphyxiating, while dihydrogen sulphide and ammonia are irritant and toxic. The long-term effects of exposure to subclinical concentrations of these gases and their rôle in producing chronic respiratory effects in workers are unknown.

Studies have shown that organic dusts occur in enclosed piggeries at undesirably high concentrations and that a relatively high proportion of this dust is respirable. The components of these dusts consist mostly of fractions of foodstuffs and pig faeces but with significant amounts of pigskin and pig gut epithelial cells. Also microbes, especially fungi and bacteria, have been found to be numerous. Different components of piggery dust could be irritant, toxic, allergenic, inflammatory or infectious, and a single component could have more than one of these effects. Studies suggest that bacterial endotoxins are present in the dust of enclosed piggeries at concentrations that could induce respiratory diseases in workers.

It is recommended that in designing piggeries more attention should be given to the health and comfort of piggery workers and that a survey be carried out immediately to assess the level of respiratory disease in British piggery workers. Also a greater effort should be put into the health education of piggery workers and managers and of those involved in piggery design. Practical and effective systems for the control of dust and gas levels in piggeries, and appropriate management techniques, need to be developed for new piggeries and for already constructed buildings, and the financial cost of poor control of dust and gas contaminants brought home to producers. Overall, a watching brief needs to be maintained on trends in the design and management of pig rearing systems for their likely impact on workers' health.

In the last decade average breeding herd size has increased. This increase has been associated with a reduction in average weaning age, with improved performance and with systems that have become more intensive, have been based increasingly on housing with slatted floors and slurry handling systems and which, in partial consequence, have had reduced labour inputs. For pregnant sows, stalls and tethers have become increasingly used but the introduction of electronic feeders may herald a change from this system in the future. For farrowing sows, reductions in piglet mortality have been obtained by effecting a closer control over the environment, by using better equipment, and by using more hygienic materials in construction. The farrowing crate and pen have come to dominate the scene and it appears unlikely that this will change much in the foreseeable future. Weaning ages are unlikely to drop below the 25-day national average and flat-deck housing is likely to continue to predominate for young weaners for the next 5 years. Fully slatted floored pens for finishing pigs have become more popular but straw-bedded systems, which are capable of being improved in the future, have been tried.

Choice of any one housing system depends on a number of factors relative to individual needs. Capital cost, likely performance responses, ease of management and of construction and maintenance costs all have to be considered. In the future, outside influences relative to animal welfare and environmental factors increasingly will have to be considered.

Innovations in pig housing which have been both successful and unsuccessful are discussed and the progress achieved is summarized. The possibilities for the future are contemplated where a more formal testing procedure to make information available for would-be users is considered as desirable.

Material inputs into pig production systems are considered as animals, food, water and bedding material. Consequential outputs are considered as animals and manure. The influence on these inputs and outputs of building design, of the design of equipment for handling, transporting and feeding animals and for the disposal of manure is discussed.

The design of a building affects control of climate in several ways. Its thermal insulation can be impaired by inadequate thickness of insulating material, faulty installation, settling, condensation and destruction by vermin and insects. Also its design interacts with the ventilation system which must take into account not only location, size and shape of the building, but also details such as air leakage, deflection of air jets by obstructions and dimensional changes such as occur in a timber structure. In contrast, a smooth or rough surface can respectively enhance or impede the throw of a jet as required. A permeable floor in a building may help to remove sedimented dust.

A ventilation system needs to be matched to the class of livestock. Systems can be classified and described, namely, glass-fibre ceiling, high-speed jet, polythene ducts, recirculation (both damper and twin fans), hybrid (two systems combined), down-jet, Udiflo tent, and automatically controlled natural ventilation. Cyclical timing of fans gives close control of very low ventilation rates for batch rearing. Kennelled accommodation needs improved control for stock and stockman.

It may be possible to reduce dust by optimizing the many factors listed that predispose towards lower dust levels. An alternative solution is to use an internal air cleaner. This can be an electrostatic precipitator, a wet scrubber or a dry filter. The latter appears to be the most appropriate and a design that discharges filtered air downwards at a speed of 0–4 m/s towards the stock has been developed and tested in a flat-deck piggery.

Conventionally heat consumption forms a small proportion (about 002) of the total cost of producing finishing pigs. Heating costs are incurred from farrowing to about 20 kg live weight and generally represent about 0–05 of production costs to this stage.

Apart from the incorporation of adequate insulation in the building structure, the main means of restricting heating costs is by good control of minimum ventilation rate. Efficient control systems are available but operators do not always fully understand how they are intended to work, so effective training is as important as clear operating instructions.

Techniques exist for the reduction of fossil fuel consumption but all involve the expenditure of additional capital and have running costs of their own. In relation to the current low costs of fossil fuels, oil and propane, even the simplest of these methods are difficult to justify in commercial practice. They include: heat recovery by static recuperator from ventilation exhaust air; and heat recovery by heat pump from low temperature sources such as aerobically treated slurry, ground water and exhaust air.

Alternative non-fossil fuels include biogas and straw. Biogas production by anaerobic digestion of slurry is expensive in capital and is only feasible where the slurry must be treated for other reasons, such as odour control. Heat production from straw may be economical where the straw is available at low cost and simple stoking aids (e.g. existing tractor fore-loaders) are used.

Fail-safe equipment to protect stock in the event of forced ventilation system breakdown must be carefully designed and installed to be reliable. Methods are available to suit the range of ventilation and housing systems. Failure of natural ventilation systems is less likely to cause problems, and indications to the stockman of abnormal temperature conditions should be sufficient to prevent loss of stock.

Electronic control systems offer a big potential for improved environmental control inpig buildings. Their use is however limited to those types of controllers that can replicate the features of older electromechanical controls. Current research and development work, which is not confined to pig building applications, is largely concerned with extending the use of microprocessor-based controllers to incorporate features such as: (a) control based on parameters other than simply air temperature; (b) control based on algorithmswhich predict the pigs’ needs; and (c) control which integrates alarm, fail safe and monitoring.

To accommodate these features more advanced controllers are used, however their use in practice is likely to be limited by a number of technical, economic and ergonomie constraints.

The recent history of animal welfare is discussed with particular reference to the ethical and scientific factors which have influenced and will continue to influence the development of pig husbandry systems.

The Farm Animal Welfare Advisory Committee report on the original codes of recommendations for the welfare of livestock, including the code for pigs, a review by the Farm Animal Welfare Council (FAWC) and the re-issue of the code for pigs in 1983, the 1980/81 report of the House of Commons Agriculture Committee and the recent FAWC proposals for legislation are examined in some detail.

Research work that has been undertaken in pig welfare is mentioned and finally, reference is made to the various protagonists in the debate, not forgetting the pig itself. There is no attempt to draw any definitive conclusions.

Although a relatively young science, farm animal ethology has a considerable contribution to make to the development of housing systems. Two areas are highlighted in this review. First, the evaluation of existing housing systems is exemplified by studies of the effects of close confinement on sows. From such studies it has been concluded that close confinement can cause distress in sows. Secondly, the development of new housing systems is exemplified by studies on how pen design can control aggression.

In the United Kingdom, environmental protection is based on a dual approach. Guidelines and voluntary action have generally proved effective but, increasingly, Community of Europe directives as well as public pressure are resulting in legislation.

Water pollution is largely regulated by Part II of the Control of Pollution Act 1974 and odour by the Public Heath Act 1936 and subsequent subsidiary legislation. Planning is controlled under the Town and Country Planning Acts and dependent General Development Orders. The Order relevant to agriculture is being amended to bring in a greater measure of planning requirement where piggeries are near human housing. Alongside these laws, the Ministry has produced a Code of Good Agricultural Practice and Guidelines on Housed Livestock to establish appropriate standards.

Other countries are also developing their legislative control, most notably Germany, the Netherlands and Denmark. It is notable that, although the ‘polluter pays principle’ is often referred to, all countries have found practical problems in any simple-minded attempt to apply the principle to the effects of animal farming.

The paper reviews these areas in detail and suggests that research is needed on the effects of small quantities of straw on slurry systems and to produce a definition of the minimum of straw needed for welfare purposes. Other research which is already in hand is also mentioned.

The management of pig manures and slurries has not responded to the changes that have occurred in the development of pig farms. Management systems have remained unchanged despite the fact that the majority of pigs (64%) are kept on 10–3% of all pig units. The pressures of heavy applications of manures on restricted areas of land and the large numbers of animals kept in buildings has given rise to environmental pollution by odours and by contamination of water and of the soil.

The cause of odour formation in buildings is discussed, together with the effects of dust on odour dispersion. Methods of reducing odours and the effect of straw on odour formation and slurry handling are discussed.

Methods of controlling odours from buildings, stores and from manure applied to the land are reviewed and areas which need further research and development are proposed.

Environmental pollution by manures and slurries of water and soils and measures to control pollution are reviewed.

In relation to desirable future developments in the provision of suitable climatic conditions and accommodation for the pig, consideration has been given to the established physiological and behavioural requirements and the extent to which current systems fail to cater for these needs. Suggestions are made for improving the provisions for pigs at various stages of the production process including breeding, pregnancy, parturition and early lactation, weaning and in the growing-finishing period. In formulating future perspectives for accommodating pigs at all stages of the production process, an attempt has been made to maintain an appropriate balance between the objectives of catering more effectively for the requirements of the pig, acceptability from the welfare aspect of both pigs and stockmen, ease of management and cost-effectiveness.