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When compared to other species of farm animals, pigs are relatively sensitive to high environmental temperatures because the pig cannot sweat and is relatively poor at panting. Little information is available about the ambient temperatures above which group-housed pigs start to adapt their mechanisms of balancing heat loss and heat production. The temperature above which an adaptive response occurs (also called the critical temperature or inflection point temperature) may well differ depending on which physiological or behavioral parameter is studied. The objective of these studies was to determine the reactions of pigs to a hot environment (experiment 1) and to study the effects of different cooling systems (sprinklers and water bath) on behavioral, physiological and productive parameters (experiment 2). In experiment one 12 groups of 10 pigs of 60 kg were studied in respiration chambers. Each day, the temperature was increased by 20 C from low (160 C) to high (320 C). In experiment two 12 groups of 5 pigs were studied under the humid tropical climate of Viet Nam. The pigs first visible reaction to increasing ambient temperature was a change in behavior. Wallowing was the first of all behavioral changes. It occurs at relatively low ambient temperature: from 16 to 170 C. The first physiological reaction to high ambient temperatures was an increase in respiration rate; on average this occurred at 22.40 C. An increase in rectal temperature occurred above an average ambient temperature of 26.10 C. This implies that pigs can prevent an increase in their body temperature for an ambient temperature range of about 3.70 C. An increase in rectal temperature and an extra reduction in feed intake are indicators that room temperature is clearly above the upper limit of the thermal neutral zone. Pigs in pens with sprinklers or water bath had lower respiration rate and skin temperature than pigs in control pens, especially during the hot period of the day in pens with outside yard. No effect of cooling on the rectal temperature was found. Pigs in pens with outside yard without cooling were less active than pigs in cooling pens. Pigs in control pens were lying more in lateral position than pigs in the cooled pens. Pigs in pens with sprinklers had the highest feed intake and the highest daily gain.

It is concluded that the availability of cooling systems reduces heat stress in pigs. The indicators of heat stress found in this study could be used as set points for these cooling systems, in order to improve animal performance and welfare in hot conditions.