Pigs are not solitary animals and will generally benefit from social contact with each other, even if only by sight or smell. At the same time, groups of pigs will always establish a social hierarchy and this starts at birth when the piglet struggles to commandeer a teat position.
If strange pigs are penned together later in life, fighting invariably ensues, and this can lead to considerable stress and physical damage to individual pigs. Once settled, however, pigs will huddle together in order to retain body heat in cold weather.
In common with the female of those species, sows are notably more docile during pregnancy than they have just produced young. Just prior to farrowing, the sow will prepare a nest from her bedding. She is often irritable during this period, and if she is confined without access to bedding material, this can lead to stress during the farrowing process.
Contrary to popular belief the pig is not a dirty animal and will normally defecate and urinate away from its resting or lying areas. However, this pattern tends to break down if pigs are overcrowded or stressed in other ways. Also, when temperatures are high they will often roll in their own faeces and urine in an attempt to increase evaporation and keep cool.
Recent studies have highlighted the importance of the interaction between pigs and humans in relation to productivity. If pigs live in fear of their stockman, both growth and reproductively performance are likely to be depressed.
In order to cope with forest conditions, the pig has better mechanisms for retaining heat, especially a well-developed subcutaneous fat cover, than for losing heat from the body. Because the pig possesses sweat glands only on the snout, it is unable to dissipate large amounts of heat by sweating.
Furthermore, the skin of certain breeds of pigs, e.g. Large White and Landrace, has no protection against the sun, and unless they have access to shade, or mud in which they can wallow, they can become badly sun burnt.
Like man, the pig is a homeotherm, and needs to maintain its deep body temperature constant. Nature has designed the metabolism of the pig to operate most effectively at 39oC.
For a certain range of environmental temperature, known as the zone of thermal neutrality the pig finds this easy and can maintain the correct body temperature by varying blood flow to and from the skin. The extent of this zone changes quite markedly according to the weight of the pig.
At the bottom end of this zone a lower critical temperature is reached when the pig required diverting feed energy to increase heat production in order to maintain body temperature. The lower critical temperature will vary between pigs according to a number of factors, for instance how fat (well-insulated) the pig is, how much feed it is eating and therefore how fast it is growing.
Whether it has bedding to help prevent heat loss, whether it is in huddle with pen mates, and whether it can make postural changes to minimize heat losses. Eventually, with decreasing ambient temperature the pig can no longer maintain its body temperature in spite of high heat production, and hypothermia and death ensues.
Of greater interest in the tropics is the effect on the pig as ambient temperature rises. When the environmental temperature approaches body temperature, the pig will attempt to increase evaporative heat loss by sweating (through its limited sweat glands), panting, postural and positional changes, and wallowing in water, mud or excrement. In addition, it will reduce its energy output by decreasing its feed intake.
However, as the means of dissipating heat in the pig are not very efficient, particularly in the absence of the opportunity to wallow, it will soon reach an upper critical temperature. This is associated with hyperthermia and heat stress and the pig will die if the situation cannot be reversed.
At the other end of the scale, the piglet at birth is particularly sensitive to low ambient temperatures. Pigs are born with virtually no subcutaneous fat cover and limited carbohydrate reserves and therefore at birth they will suffer an immediate drop in body temperature.
In the case of weaker piglets they may battle to obtain an adequate milk supply; if they require energy to keep warm they very quickly develop the condition of low blood sugar (hypoglycaemia), and die of cold.
From the foregoing it is clear that stress factors can take many forms, and in fact can involve fear, pain, temperature, direct sunlight, restraint, fatigue and interference with natural behaviour patterns.
Stress will quickly lead to reduced performance and productivity, and specifically to gastric ulcers (just as in humans), greater susceptibility to infectious diseases and higher mortality rates.
It is therefore paramount that we understand what constitutes the major stress factors in pigs in different circumstances so that production systems can be designed to minimize these effects.
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