Thermal stress of pigs can be shortterm or long term and may occur inconstant or fluctuating conditions.The first visible sign of how pigsreact to increasing ambient temper-atures is a change in behaviour.Wallowing was observed as the firstof all behavioural changes. It occursat relatively low ambient tempera-ture, 16 to 17°C. Lying on the slattedfloor and excretion on the solid floorstarted above 19 to 20°C.

The first physiological indicatorthat the pigs are reacting to highambient temperatures is an increasein respiration rate; on average thisoccurs at 22°C. The second step is

for rectal temperature to rise; thishappens when ambient temperatureis on average above 26°C. Thisimplies that pigs can avoid increas-ing their rectal temperature for atemperature range of about 4°C(Figure 1). Figure 1 also shows thatpanting allows pigs to maintain asimilar voluntary feed intake at atemperature of about 3 to 4°C higherthan the temperature at which res-piration rate increases. Within therange of ambient temperatures atwhich total heat production isdecreased, because of less activity ofthe pigs, to the point that voluntaryfeed intake decreases, the pigsmight use the saved energy forgrowth. However, this point shouldbe further investigated. Important inthis respect is the considerationbetween animal welfare and produc-tivity.

Effect of humidityThe influence of relative humidity

on the reactions of the pigs is lowcompared to the effect of tempera-ture. However, relative humidityinfluences the performance of theanimal at high ambient tempera-tures. At high relative humidity, theinflection point temperature forincreased respiration rate was lowerthan it was at lower relative humidi-ty (21.3°C at 80% RH, 22.6°C at 65%RH, and 23.4°C at 50% RH). The dailyweight gain per pig was also lowerat high relative humidity (at thesame ambient temperature) than atlow relative humidity.

Economical losses It should be noted that performanceis the culmination of all effects overa long period. In the study, for eachdegree Celsius ambient temperatureincrease above the inflection pointtemperature, the voluntary feedintake declined steadily by an aver-age of 95.5g (Figure 2). In general,under diurnal tropical condition pigs

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Heat stress in pigsFarmed pigs may experience heatstress throughout the world: in temperate areas during summermonths and in tropical areas duringthe whole year. Heat stressed pigshave low performance, poor welfare,and pollute the environment.Therefore, researchers have studiedthe reactions of pigs to heat in a highgrowing commercial breed to betterassess the indicators of heat stress.

By Thuy Huynh1,2 and André Aarnink1

1Livestock Environment Group, Wageningen UR, the

Netherlands 2Ministry of Agriculture and RuralDevelopment, Vietnam

Simple water baths for pigs were

studied at a small-scale farm in

Vietnam.

Pigs under high ambient temperature wallowed themselves in urine and

faeces to cool down. In hot conditions, this behaviour can be interpreted as a

decline in welfare.

responded by maintaining a highrespiration rate, especially duringthe hot part of the day. However, inthe field study, the maximum respi-ration rate in the afternoon was stillmuch lower than the maximumvalue in the controlled climatestudy: at similar ambient tempera-ture (approximately 32°C), the aver-age respiration rate of pigs in thefield study was 65/min, comparedwith 170/min for the pigs in theexperimental condition. The fluctu-ating temperatures between day andnight might explain the lower respi-ration rate in the field study. Thisgives pigs the opportunity to shiftfeed intake to the lower temperatureperiods of the day. Lower feed intakein the afternoon, the hot part of theday, reduces heat production andthe necessary heat loss. Thereforerespiration rate can be lower than inthe situation of a constant high tem-perature as in the controlled climateexperiment.

Impaired pig welfareWhen temperature rose, the pigs westudied displayed signs of discom-fort: they became inactive, extendedtheir body contact with the floorwhile lying, and avoided physicalcontact with other pigs. So, whenthey are hot, pigs show less spacesharing and lie more on their sidescompared to low temperatures. Thismeans that with increasing temper-ature, the need for physical surfacespace increases.

The inflection point temperaturefor change in lying behaviour wasrelatively low (19°C). We considerthis low inflection point temperatureas an indicator that welfare isimpaired. Lying on the slatted flooror excreting area is not the naturalmotivation of the pigs. Thus, pigshad to lie on slatted floor or in theirown urine and faeces to reduce heatstress; to do the latter, they had toabandon the natural desire to avoidcontact with excrements. In hot con-ditions this behaviour can be inter-preted as a decline in welfare.

Environmental problemWhen the animals wallowed in their

own manure and urine to increaseevaporative heat loss, the emissionof environmentally damaging gasessuch as NH3 increased. Previousstudies on ammonia emissionshowed that urine-fouled floor areaand urine-fouled animal body relat-ed positively with ammonia emis-sion. On average 40% of total emis-sion came from the pen floor (25% ofthe pen floor was slatted). It wasalso found that pen fouling washigher in summer than in winter,and that pen fouling increasedtowards the end of growing period. Itis clear that on exposure to heatstress the animals immediatelychanged their lying and excretionbehaviour. Those changes couldcause a large increase in ammoniaand odour emissions. The implica-tion of this finding is that it isimportant to offer the animals cool-ing systems like floor cooling, waterbath or sprinklers. During hot peri-ods, this could prevent the animalsfrom fouling their pen with manureand urine and thus from increasingemission of environmental pollu-tants.

Prevention of heat stress Heat stressed pigs responded posi-tively to cooling systems. Cooling asolid floor during hot periods in thesummer improves the feed intake

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Start todefecateon solidfloor

Decreaseheat production

Decreasefeedintake

Increaserectaltemperature

Increaseresprirationrate

Increaseevaporativewater

Increaselying onslattedfloor

Start towallow,reducehuddling

26°C25°C24°C23°C22°C20°C19°C18°C16°C

Figure 1. Adaptation of ad libitum fed 60kg pigs to increasing temperatures(the temperature scale should be read from left to right).

1000

1500

2000

2500

14 16 18 20 22 24 26 28 30 32 34

Room temperature, °C

Feed

inta

ke, g

/ d

50% 65% 80%

Fitted line 50 %RH Fitted line 65 %RH Fitted line 80 %RH

Figure 2. Responses of ad libitum fed 60kg pigs with respect to feed intake atdifferent humidity levels.

and daily gain. The cool floor alsoimproves the lying behaviour: morepigs chose to lie on the cool solidfloor instead of the slatted floor. Inthe experiments in practical condi-tions, cooling systems like a waterbath and sprinklers, in combinationwith an extra yard had positiveeffects on the physiological respons-es and behaviour. Growth rate andfeed intake were significantlyimproved in pigs provided with floorand spray cooling. Compared to therelative expensive and complex sys-tem of floor cooling, for developingcountries spray cooling is one of thecheapest and simplest ways ofreducing the negative effects of hotweather.

Conclusions• The best indicators for assessing

heat stress of finishing pigs are:increased respiration rate andwater to feed ratio, followed byreduced feed intake, and, finally,increased rectal temperature.Decreased feed intake andincreased rectal temperature aregood indicators of reduced perfor-mance of heat-stressed pigs.

• Observations of lying and excretingbehavioural changes of the pigs (adlibitum fed 60kg pigs) may be use-ful to assess their very first reac-tions in thermal regulation. We

found that huddling alreadydecreased above 16°C; wallowingoccurred above 16 to 17°C; lying onthe slatted floor graduallyincreased above 19°C, and as aresult of the changed lying behav-iour the excretions on the solidfloor increased when temperaturerose above 20°C. The number ofpigs lying on slatted floor could bethe first indicator for assessing thewelfare of pigs exposed to highambient temperatures.

• The indicators of heat stress foundin this study could be used as setpoints for cooling systems, in orderto improve animal performanceand welfare in hot conditions. Theavailability of cooling systems e.g.floor cooling, water bath or sprin-klers has beneficial effects on pigperformance compared to controlgroups without cooling. In terms ofcost, a water bath and sprinklersystem are more suitable for ani-mal production in a developingcountry like Vietnam. In hot humidtropical conditions these two sys-tems contribute only little to extrahumidity in the air.

• Generally, the researchers foundthat humidity had minor effects onphysiological parameters. However,a combination of high humidityand high temperature clearly has adetrimental effect on daily gain. PP

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A floor cooling system, studied in the Netherlands; cool ground water was

used to cool the solid floor of the lying area.