It is now well-established that the climatic environment has a major influence on the growth and development of an animal. It influences the rate and efficiency with which dietary nutrients are utilised for the many metabolic processes within the body. The objective should be to keep animals within their zone of thermal neutrality, since within this range of temperatures, heat production is minimal and the energy available for production is maximal. The lower and upper limits of this zone are called the Lower (LCT) and Upper (UTC) critical temperatures, respectively. Outside this zone, heat production increases and the rate and efficiency of nutrient utilisation is reduced.

Under free-living conditions, animals may compensate for variations in their climatic environment by altering their rate and pattern of feed intake, by changes in behaviour and physical activity and by seeking protection. This means changing their rate of metabolism. However, the animals' ability to alter their environment has been limited through intensification of production where they are kept in housing situations at greater stocking densities than in the past. This restricts their freedom to choose their own living conditions. Since both the nutrient intake and climatic environment are thus carefully controlled, there must be a high degree of understanding of the effects of the environment on productivity. In addition, it is important to know the consequences for long-term growth, development and reproduction if optimal climatic conditions are not provided and an understanding of the climatic needs of the pig at the different stages of production is essential.

Defining the optimum environment

In pig production, interest lies in the determination of the range of environmental conditions within which heat loss or production is minimal, that is the thermal neutral zone. Under these conditions optimal utilisation of feed for carcass growth and development is attained. Both the lower and upper critical temperatures are often referred to as precisely defined temperatures, but no such well-defined temperatures can be obtained in practice, so that it becomes necessary to make an estimate of the 'effective critical temperatures'.

The position of the thermally-neutral zone and hence 'effective critical temperatures' on the ambient temperature scale is influenced by several animal, nutritional and environmental factors and it is important to know the extent to which temperatures below and above the critical levels increase heat loss or heat production because this indicates what reduction in performance and feed efficiency may be expected.

Animal factors
It is now well established that as body weight and insulation increase, the critical temperature decreases. Thus the new-born pig which is metabolically immature, with sparse pelage and limited body fat, has a lower critical temperature around 34^oC (Mount, 1968). However, as the animal increases in size, the lower critical temperature decreases from approximately 21^oC at 20kg body weight to 20 and 18^oC at 60 and 100kg body weight, respectively (Holmes and Close, 1977). The critical temperature of the weaned animal, although related to body weight, is also dependent upon the age of weaning, the extent of body fat loss in the immediate post- weaning period and the level of feeding it is able to sustain after weaning. Thus for piglets weaned at two weeks of age, Close and Stanier (1984) have suggested a critical temperature of 28^oC at weaning, decreasing by approximately 2^oC each subsequent week. Other factors such as breed, condition of the animal and group size can also influence critical temperature and these have been reviewed by Close (1981) and Curtis (1983).

Nutritional factors
Within and above the zone of thermal neutrality, plane of nutrition influences the rate of heat production and hence critical temperature. Thus the higher the level of feeding, the higher the rate of heat production and the lower the critical temperature. For practical purposes it is therefore necessary to have an indication of the relationship between ME intake and critical temperature for different classes of animals and Table 1 has been compiled for this purpose. The results show that both the LCT and UCT values are influenced by both body weight and feeding level. These values are appropriate for individual animals and will be reduced by some 2 - 3 ⁰ C for animals living in groups.

If the temperature falls below the LCT values presented in Table 2, then heat loss increases, and if energy intake is fixed, both the rate of energy retention and growth are reduced. However, several studies reviewed by ARC (1981) and NRC (1987), provided data from which the increase in heat loss and hence extra energy requirements may be calculated. If the energy content of the diet is known, then the extra feed needed to compensate for the colder environment and to ensure similar rates of retention and growth may be calculated (Table 2).

Compared with LCT, there is less information on those factors which influence the values of UCT. Christianson, Hahn and Meader (1982) estimated the UCT values of pigs as:

UCT (⁰ C) = 34.7 - 0.33 Body weight (kg) ^0.72

However, the feed intake of the animals also needs to be considered, as indicated in Table 1.

Environmental effects
Thermal components of the environment other than air temperature influence heat loss and heat production and must be considered in practical pig production if deviations in the housing environment are to be assessed in terms of a change in the environmental or effective critical temperatures.

Air movement
An increase in air movement disrupts the thermal insulation provided by the boundary layer of air around the animal, causing an increase in convective heat loss. However, the extent to which air movement causes an increase in heat loss is dependent upon such factors as body weight, group size, and the temperature and duration of exposure (Sällvik and Walberg, 1984). Younger animals are more susceptible to changes in air movement than older animals and low draughts of air are proportionately more effective in increasing heat loss than similar changes at high wind speed (Mount and Ingram, 1965; Holmes and Mount, 1967). Mount and Ingram (1965) calculated that each 0.05 m/sec increase in air movement was equivalent in its thermal effects to a 1^oC decrease in air temperature for an individual 20kg pig; for a 60kg pig, the equivalent effect was 0.10 m/sec whereas for groups of pigs it was 0.30 m/sec (Verstegen and van der Hel, 1974). Close et al (1981) also calculated that a 0.05 m/sec increase in wind speed was equivalent to a 1^oC increase in critical temperature of single pigs. The effect of increasing air movement to 0.60 m/sec was to increase the critical temperature of the animals to 30^oC, thereby indicating the beneficial effects of higher wind speeds under hot conditions. These values relate to draughts of air provided at the same temperature as that in the pen. In practice they may often be lower so that heat loss is increased, resulting in the further lowering of the equivalent air temperature.

Radiant environment
The difference between air and structure temperature determines the rate at which heat is lost by radiation and convection. Mount (1968) varied air and wall temperature independently of each other and found that when wall temperatures was below that of air, total heat loss increased as a result of increased radiation. A 1-2^oC change in the temperature of the surroundings was equivalent to a 1^oC change in air temperature, in agreement with that of Holmes and McLean (1977).

Relative humidity
The humidity of the air can have an important influence on the well-being of the animal at high environmental temperatures, particularly as the animal relies to a large extent on heat loss through evaporation of water. Morrison et al. (1967) have shown that as relative humidity increased from 30 to 90%, at an environmental temperature of 30^oC, the animal became more dependent on cutaneous water loss, although respiration rate had almost doubled. Evaporation from the skin, as a percentage of the total, increased from 35 to 67%. The importance of cutaneous water loss, in particular that associated with wallowing, has been discussed by Ingram (1965a, b). Heitman and Hughes (1949) found that at 35^oC the animal's respiration rate and body temperature were reduced and rate of gain increased on a wetted floor compared with that on a dry floor. Similar effects were experienced by increasing the rate of air velocity at these abnormally high temperatures. In terms of thermal equivalent, Holmes and Close (1977) have calculated that at 30^oC an 18% increase in relative humidity was equivalent to a 1^oC increase in air temperature.

Bedding and floor type
The nature of the floor determines the extent of conductive heat loss, and as approximately 20% of the animals' body can be in contact with the floor, there may be considerable heat loss through it. On a cold, uninsulated floor, the conductive heat loss may represent 20 to 25% of the animals' heat loss. Under hot conditions, however, this may be a valuable avenue of heat dissipation.

Comparisons between different floor types show the significant changes in both heat loss and critical temperature associated with bedding (Mount, 1967; Stephens, 1971; Verstegen and van der Hel, 1974). The latter calculated the effective critical temperature of groups of 40kg pigs to be 11-13^oC on straw, 14-15^oC on asphalt and 19-20^oC on concrete slats. The degree of wetness of the floor will also have a considerable effect upon the animals' heat loss. If pigs have to lie on a wet floor, conduction to the floor and evaporation from the animals' surface will increase. This will have a beneficial effect in hot, humid conditions whereas under cold conditions the effect will be detrimental to the animal. Thus Mount (1975) has calculated that a cold, wet floor may have a thermal effect equivalent to a 7-10^oC decrease in air temperature.

From the results of these various investigations it is possible to arrive at estimates of equivalent temperatures, that is the air temperatures at which similar levels of heat loss can be maintained under different housing and environmental temperatures. These equivalent temperatures are presented in Table 3 and from these the cumulative effects of the varying animal, nutritional and environmental factors on the critical temperatures have been calculated. These are presented for the growing pig in Table 4 and show how various management and housing situations may be manipulated to supply the correct environmental conditions under both cold and hot conditions (Table 4). If the air temperature for any particular situation is less than that indicated, then energy from the feed will be used to compensate for the temperature deficiency. This leads to a reduction in growth and a deterioration in feed conversion efficiency.

Fluctuating temperatures
The extent to which animals can tolerate fluctuating temperatures are primarily dependent upon body weight and food intake. Le Dividich (1981) varied the air temperature by 3⁰C each hour throughout the 24-h period and clearly demonstrated that the post-weaning performance of young pigs was less than that when the temperature was maintained constant. The incidence of post-weaning scours was also increased. On the other hand, Morrison, Heitman and Givens (1975) found that neither growth rate nor food conversion efficiency of 60-kg pigs was affected by a 10 or 20⁰C diurnal temperature, when compared with a constant temperature equal to the mean of the cycle if the mean temperature was within the thermal neutral zone. However, when a constant or diurnal temperature pattern was maintained at 6⁰C above the optimum, then performance declined with the 20 ⁰C cycle range. It is likely that the performance was reduced due to heat stress during the periods of extreme temperature which rose to 40⁰C at its highest value. Since the pigs were fed ad libitum, one possibility is that the effect was mediated through a change in food intake, rather than a direct effect of temperature per se. Thus, the animals on restricted food intakes may be less likely to tolerate larger fluctuation in temperature than those fed ad libitum. Lopez et al (1991), showed that finishing pigs consumed less feed and grew more slowly in a hot, diurnal temperature than in a constant thermo-neutral environment. In a cold, diurnal temperature, they consumed more feed, but grew more slowly. These results suggest that for growing pigs, temperature fluctuations of more than 3-5⁰C around the mean should be avoided. Evidence also suggests that since pigs have a diurnal pattern of heat production, with the lowest heat production occurring at night, they can tolerate colder temperatures at night, especially when housed in groups and given the opportunity to huddle (McCracken and Caldwell, 1989; Brumm, Shelton and Johnson, 1985; Brumm and Shelton, 1991).

Environmental effects upon production characteristics

Energy metabolism (including protein and fat deposition)
The environmental conditions within which animals are housed influences the extent to which ME intake is utilised for maintenance, on the one hand, and energy retention or growth on the other. At any given energy intake, exposure to temperatures outside the zone of thermal neutrality increases the maintenance energy requirements (ME_m), with a concomitant reduction in the energy available for production. From the results of various investigations, ARC (1981) calculated that the mean increase in ME_m at temperatures outside the zone of thermal neutrality was 20 kJ/kg ^0.75 per day. However environmental temperature is not the sole climatic variable influencing ME_m. Close et al (1981) have shown that an increase in air movement from 0.03 to 0.56 cm/sec increased ME_m from 706 to 881 kJ/kg ^0.75 per day at 10^oC, from 490 to 715 at 20^oC and 517 to 625 at 30^oC. This considerable increase in ME_m indicates the extent to which energy in the feed is used for thermoregulatory purposes.

If ME_m increases then at any given energy intake there must be a change in the energy available for production and hence energy deposition as protein and fat. The results of a number of experiments suggests that protein deposition is remarkably independent of environmental temperature (Fuller and Boyne, 1971; Close et al, 1978; Phillips et al. 1982) indicating that at similar levels of intake the increase in ME_m is achieved at the expense of fat deposition (Table 5). However, fat deposition is very dependent upon the environmental conditions within which the animals are maintained, so that at any given intake, the highest depositions occurred between 23 and 25^oC. Above 25^o, fat retention was reduced in association with the hyperthermal rise in heat production. The temperature-dependent change in fat deposition was greater than that of protein and was equivalent to a 28 g/day reduction in feed intake, compared with an equivalent reduction of only 4 g/day for protein.

Growth rate
The depression in growth rate associated with a decrease in environmental temperature results from an increase in the animals' maintenance energy requirement and a reduction in the rate at which protein and fat are deposited (Table 5). When the environmental temperature falls below the lower critical temperature the heat production rises by between 2 and 4% per 1^oC fall. These figures give an estimate of the demands made by the environment on the pig to produce heat, and this heat must be derived from dietary nutrient intake. The question which arises is, how significant is this rise in environmental demand for the growing pig's feed conversion and growth rate and there are many estimates indicating the extent to which growth rate is diminished when the environmental temperature is reduced(Close,1981; Curtis, 1983). From a review of the literature Verstegen et al. (1977) calculated over a wide range of breeds, feeding levels and housing conditions that growth rate was decreased by 8.1 g/day for each 1^oC decrease in temperature below the critical level. To compensate for this they calculated that feed intake under ad libitum feeding conditions increased by 19.5 g/day for each 1^oC below 15^oC.

In many situations it is not always possible to feed pigs ad libitum and for practice it is important to know how much extra feed is needed to maintain similar levels of production both below and within the zone of thermal neutrality. Table 6 has been compiled for this purpose and indicates that the additional feed requirements during the growing/fattening phase increase between 22 g/day per 1^oC at 20kg to 43 g/day per 1^oC at 100kg body weight. These requirements are useful in determining whether it is more economical to keep the environmental temperature for growing/finishing pigs at or above their critical temperature and hence spare feed, or to increase the animal's feed allocation in response to the cold stress and hence save the costs of providing additional heating in the building. The decision will depend upon the costs of feed energy relative to that of fuel energy. For sows Geuyen et al. (1984) have calculated an additional feed requirement of 75 and 40 g/day per 1^oC drop in temperature when kept either individually or in groups, respectively. This decrease in requirements reflects the beneficial effects associated with grouping.

Carcass effect
The variations in the protein and fat gains of the animal reflect changes in carcass composition. In agreement with the changes in protein and fat deposition, it may be concluded from both chemical analysis and carcass appraisal studies, that cold environments results in leaner carcasses (Holmes and Close, 1977; Close, 1981). Exposure to hot environments also influences carcass composition, but only through its effects on the animal's feed intake. Temperature per se influences the morphological characteristics of the animal. Pigs kept under cold conditions have shorter ears, denser coats, shorter limbs and fewer blood vessels in their skin (Fuller, 1965; Dauncey et al. 1983). Carcass length may also be reduced (Holme and Coey, 1967). In the fat depots of the animal, prolonged exposure to colder environments produces a high degree of unsaturated fatty acids which can influence the melting point and physical characteristics of the fat (McGrath et al. 1968; Fuller et al. 1974). Hot conditions, on the other hand, increase the water binding capacity of the muscle (Hacker et al, 1973) and this may influence muscle quality. Within breeds of animals, high environmental temperatures cause the phenomena known as malignant hyperthermia syndrome, which produces a pale, soft and exudative condition known as PSE in the muscle after slaughter.

Feed intake
Given access to an ad libitum supply of feed, the animals' voluntary food intake decreases with increase in air temperature. From a review of several experiments, Close (1989), suggested that the relationship between energy intake and environmental temperature was best described by the equation:

y = 9.6 + 0.075 x₁ + 0.52 x₂ - 0.012 x₁ x₂

Where y is the ME intake in MJ/day, x₁ is the temperature (⁰ C) and x₂ is the body weight (kg). ME intake therefore decreases by 0.12 and 1.12 MJ/day for each 1 ⁰ C reduction in temperature for pigs of 20 and 100 kg body weight, respectively.

More recently, Rinaldo and Le Dividich (1991), calculated that the relationship between feed intake and temperature to be:

y = 16.8 x Temperature - 0.8 x Temperature² + 1163.

y is the feed intake in g/day of a diet containing 12.4 MJ ME/kg and temperature is in ⁰C.

Temperature has a greater effect on the voluntary feed intake of the sow than on the growing or finishing pig. For example, Stansbury, Mc Glone and Tribble (1987) reported that at 30 ⁰ C, lactating sows consumed only 4.2 kg/day, compared with 6.46 at 18 ⁰ C. As a consequence, the animals lost considerable weight during lactation. However, they similarly showed the importance of evaporative or convective cooling in increasing the appetite of sows under hot conditions.

Temperature is not the sole component of the environment which influences appetite. Other components which are equally important are air movement and relative humidity. However, one component which is often over-looked and can significantly influence feed intake is stocking density or space allocation. This topic has been reviewed by Kornegay and Notter (1984) and NRC (1987). From these studies it may be calculated that each 0.1 m ² reduction in space allocation results in a 0.65 MJ/day reduction in energy intake. The space allowance for optimum feed utilisation was 0.40 m² for weaned piglets, 0.60 m² for growing pigs and 1.00 m² for fattening pigs. The extent to which the different components of the environment influence appetite in groups of 60 kg pigs is presented in Table 7.

Climatic effects upon reproduction

The environment influences sow productivity by influencing fertility, through its effects upon ovulation rate and embryonic mortality, and by altering the transfer of dietary nutrients within the maternal body and gravid uterus. The two main environmental components affecting fertility are ambient temperature and day length. Thus ovulation rate is significantly reduced at high environmental temperatures although it is possible that the observed reduction may be due to the reduced feed intake under these conditions rather than the direct effect of temperature per se (Wettemann and Bazer, 1985). However after the first three weeks of pregnancy, that is after the implantation period, the embryos appear to become resistant to heat (Steinbach, 1976). During the last three weeks of pregnancy, the sow becomes susceptible to heat and prolonged exposure at farrowing can lead to exhaustion and sometimes death (Omtvedt et al, 1981).

During pregnancy there is optimum utilisation of dietary energy for both net maternal and gravid uterus gain when the sow is maintained within thermal neutral conditions. It is generally assumed that the lower critical temperature of a normal sow at normal levels of feeding is within the range 18-20^oC (ARC, 1981). However if the environmental conditions are too cold and if feed intake is limited, then the dietary nutrients are unable to meet all metabolic demands. Mobilisation of body tissue may then occur and this limits both maternal body and gravid uterus gain. Repeated and prolonged exposure to these conditions will result in low litter sizes, piglets of low birth weights and a severely emaciated, infertile sow. Such a condition, the 'thin sow syndrome' has been reported in practice.

Although heat stress during the first three weeks of pregnancy can have a serious effect upon reproductive efficiency, during the final weeks it can cause thermal discomfort through an increase in rectal temperature and the respiration rate of the sow. Exposure to heat at this stage may also increase the number of still-born piglets (Omtvedt et al. 1976; Steinbach, 1971). This susceptibility to heat stress in late gestation may be associated with the increased metabolic rate of the sow which is alleged to occur at this time. At farrowing pigs are also susceptible to exhaustion and subsequent death after exposure to elevated temperatures (Omtvedt et al. 1971).

In terms of the thermal environment, the requirements of the lactating sow and those of her piglets are at variance with one another. Since the piglet nutrients needs can only be met through the milk supply of the mother, it follows that factors controlling maternal feed intake during lactation will determine the rate and efficiency of piglet growth. Thus exposure to high environmental temperatures reduces feed intake, limits milk supply and increases the loss of body fat from the sow so that the weight of the piglets at weaning may be reduced, as has been reported under tropical conditions by Steinbach (1976). On the other hand, the piglets require a warm environment for optimum growth and development. This conflict of requirements is apparent from the experiments of Lynch (1977) who showed that sows maintained at an environmental temperature of 27^oC decreased their feed intake by 12% relative to that at 21^oC. Even though the piglets at the higher temperature consumed larger quantities of creep feed to compensate for the reduced milk yield, the overall effect was a lower piglet body weight when weaned at 4 weeks of age, 6.76 kg at 27^oC compared with 7.75kg at 21^oC.

There is little evidence to suggest that cold conditions impair the reproductive function of the boar, provided sufficient feed is given to meet the nutrient requirements essential for normal growth and body development. However under hot conditions, the environment influences boar performance through its effect upon the retardation of sexual development, reduced libido, reduced sperm motility and a higher proportion of abnormal sperm cells (Christenson et al. 1972). These traits did not appear to become evident until some 16-30 days after exposure to heat and may persist for as long as 60-64 days post-treatment.

Practical implications for pig housing and conclusions

It is obvious that pigs will continue to be housed in buildings of different designs. Therefore, the general requirement for farming practice is to determine the range of environments and nutrition which allow maximum efficiency of feed utilisation. This necessitates the determination of the zone of thermal neutrality for all classes of livestock, and those factors which influence it. If the environmental conditions fall below or above this zone then growth rate is reduced, with concomitant effects upon feed conversion efficiency. Under cold conditions, where the animal may be below its LCT, actions such as structural improvements to the buildings, the provision of supplementary heating or increasing the animal's feed allocation may be taken to improve the environment. The decision will depend upon the cost of fuel energy relative to that of feed energy, and hence the prevailing economic circumstances.

In hot conditions, where the air temperature exceeds the higher critical temperature, the solution is not so simple since the requirement of the animal is to limit its heat production in order to avoid hyperthermia. It achieves this by reducing its feed intake and pattern of eating, by changes in posture and behaviour and by seeking shelter. By reducing feed intake and by eating 'on a little and often basis' the heat increment associated with feeding is reduced. Feeding behaviour and drinking also change so that the animal drinks more water. As the animal relies on its evaporative heat exchange for the maintenance of homeothermy it follows that ways to enhance water vapour renewal help adaptation and increase production. The use of wallows, hoses or automatic sprinklers and the control of relative humidity have been shown to improve productivity under tropical conditions. The provision of draughts of air around the animal may also be effective in alleviating heat stress.

Housing can therefore reduce the "thermal stress" of animals in both cold and hot conditions and improve their performance. It also allows better control of both the quantity and quality of feed, a reduction in the energy expenditure and improvement in animal comfort and welfare. However the costs of construction, maintenance and labour must be set against any improvement in performance. It is likely that the high cost and quality of the end-product, the improved use of feed and resources, the improvement in fertility and fecundity and the increase in survivability will far outweigh these structural costs. In this respect, studies on the climatic needs of the pig have played an integral role in permitting a rational approach to be used in the design of these buildings and in deciding the type of housing and management and husbandry practices most suited to the different climatic regions and types of production.

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