J. Anim Sci. 2007. 85:779-790. doi:10.2527/jas.2006-430
© 2007 American Society of Animal Science
Acclimation to high ambient temperature in Large White and Caribbean Creole growing pigs1
D. Renaudeau*,2,
E. Huc* and
J. Noblet
* Unité de Recherches Zootechniques, INRA, 97170 Petit Bourg, Guadeloupe, French West Indies, France; and
and
UMR Systèmes d’Elevage, Nutrition Animale et Humaine, INRA, 35590 St-Gilles, France
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Abstract
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The effect of breed [Creole (CR) vs. Large White (LW)] on performance and physiological responses during acclimation to high ambient temperature was studied in 2 experiments involving 24 (12/breed) growing pigs each. Pigs were exposed to 24°C for 10 d (d –10 to –1) and thereafter to a constant temperature of 31°C for 16 d (d 1 to d 16) in Exp. 1 and for 20 d (d 1 to d 20) in Exp. 2. For both experiments, the temperature change was achieved over 4 h on d 0. The first experiment began at 105 d of age, and the average BW of CR and LW pigs was 36.6 ± 2.5 kg and 51.7 ± 3.0 kg, respectively. The second experiment was designed to compare both breeds at a similar BW (about 52 kg on d 0). Pigs were individually housed and given ad libitum access to feed. At 24°C, ADG was lower (P < 0.01) in CR than in LW (602 vs. 913 g/d and 605 vs. 862 g/d in Exp. 1 and 2, respectively), but the ADFI was not affected by breed (190 and 221 g · d–1·kg–0.60 in Exp. 1 and 2, respectively). Short-term thermoregulatory responses during the 4-h transition from 24 to 31°C (d 0) were analyzed according to a linear plateau model to determine the break point temperature, above which rectal temperature (RT), cutaneous temperature (CT), and respiratory rate (RR) began to change. The CT increased linearly with temperature increase (0.22°C/°C) and was less (P < 0.05) in CR than in LW (by –0.3°C on average). In both experiments, the break point temperature for RT was not affected by breed (27.6°C on average), whereas for RR it was greater (P < 0.05) in CR than in LW (27.5 vs. 25.5°C, P < 0.01). On average, ADFI declined by about 50 g · d–1 · kg–0.60 from d –1 to d 1 (P < 0.01), and thereafter at 31°C, it gradually increased (23 g · d–1 · kg–0.60; P < 0.05), suggesting an acclimation to high exposure. This response was not influenced by breed. After the day that marked the beginning of the acclimation response (i.e., the threshold day), RR, CT, and RT declined over the duration of exposure to 31°C (P < 0.05) in both experiments. During this period, RT and CT were less in CR than in LW pigs (39.6 vs. 39.9°C and 37.9 vs. 38.2°C, respectively; P < 0.05), whereas RR was not affected by breed. The threshold day at which RT began to decline was less in CR than in LW pigs (0.18 vs. 1.17 d and 0.39 vs. 0.93 d in Exp. 1 and 2, respectively; P < 0.05). In conclusion, this study suggests that short- and long-term physiological reactions during heat acclimation differed when CR and LW pigs were compared at the same age or BW.
Key Words: acclimation • breed • heat stress • pig • thermoregulation
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INTRODUCTION
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The effect of elevated ambient temperature on pig performance is well documented. Increasing ambient temperature above the upper limit of the thermoneutral zone (approximately 25°C for growing pigs) reduces ADFI, with negative consequences on growth rate (Nienaber et al., 1987
; Le Dividich et al., 1998). In addition, recent studies have reported a specific effect of elevated temperature on energy utilization, i.e., on the partition between protein and lipid depositions (Brown-Brandl et al., 2000; Le Bellego et al., 2002).
However, most studies dealing with the effect of high ambient temperature on pig performance were performed on heat-acclimated pigs. In general, ADFI significantly decreases within the first 24 h of exposure to elevated temperature and thereafter remains constant or slightly increases over the period of thermal acclimation (Morrison and Mount, 1971; Giles, 1992). In addition, over the same period, rectal temperature (RT) and respiratory rate (RR) increase within 24 h and decline thereafter over successive days of exposure.
Variations in the short-term (a few hours) response to heat stress among breeds are described in halothane-positive and halothane-negative boars (D’Allaire and DeRoth, 1986; Tauson et al., 1998) or in high- and low-producing genotypes (Nienaber et al., 1997; Renaudeau, 2005), but little is known about the effect of breed on acclimation to elevated ambient temperature. In the French West Indies, a local Caribbean breed (Creole pig; CR) is known for its good adaptation to the harsh tropical environment (Renaudeau, 2005). For this purpose, it was introduced in our experimental facilities to study the genetic variability of heat tolerance in pigs.
The objectives of this work were to study thermal acclimation in growing pigs exposed to elevated ambient temperature and to determine if this acclimation response is influenced by breed.
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MATERIALS AND METHODS
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Care and use of animals were performed according to the Certificate of Authorization to Experiment on Living Animals, number 04739 (issued by the Ministry of Agriculture to J. Noblet).
Experimental Design and Pig Management
A total of 48 barrows [24 Large White (LW) and 24 CR] were used in 2 experiments, each conducted on 2 replicates of 6 pigs per breed at the experimental facilities of INRA in Guadeloupe, French West Indies. Within a replicate, 6 LW and 6 CR pigs originating from different litters were randomly selected and moved to a climatically controlled room for 36 or 40 d, including 10 d for adaptation at 24°C and 26 d (Exp. 1) or 30 d (Exp. 2) for the experiment. During the experimental period, pigs were kept at 24°C (thermoneutral zone) for 10 d (from d –10 to –1) and thereafter at a constant temperature of 31°C for 16 d (from d 1 to 16) and 20 d (from d 1 to d 20) in Exp. 1 and 2, respectively. Between the 2 experimental periods, on d 0, the temperature was changed gradually from 24 to 31°C within 4 h beginning at 0800 h. The relative humidity (RH) was kept at 80% over the total experiment.
Experiment 1 compared both breeds at the same age; it began when pigs were 105 d of age; the average BW of LW and CR pigs were 51.7 ± 3.0 and 36.6 ± 2.5 kg, respectively. The second experiment was initially integrated to another program, which explained its different length (30 vs. 26 d). Finally, results of this second experiment were presented with those of the first experiment in the present paper to allow a comparison of both breeds at a similar BW. Practically, the average BW was similar for both breeds on d 0 (52.6 ± 3.2 and 52.4 ± 3.6 kg for LW and CR pigs, respectively). Such a design was used to take into account the marked differences in ADG between CR and LW pigs (Renaudeau et al., 2006). Pigs were offered ad libitum a diet formulated with corn, wheat middlings, and soybean meal (Table 1).
Housing
In this study, an 800-m3 climatic room equipped with 12 individual metal-slatted pens (0.85 x 1.50 m) was used. Each pen was equipped with a feed dispenser and a nipple drinker designed to avoid water spillage. In the climatic room, ambient temperature and RH were controlled within ± 0.2°C and ±3%, respectively. The photoperiod was fixed to 12 h and 30 min of artificial light (0600 to 1830h) daily, and the ventilation rate was set at 50 m3 · h–1 · pig–1. Air speed was not controlled, but periodic punctual measurements indicated that it did not exceed 0.15 m/s.
Measurements
All pigs were weighed before and after a 24-h fasting period at the beginning and at the end of the experiment. An additional BW was determined without prior fasting on the morning of d 0, before the temperature increase. For each pig, the corresponding fasting BW was estimated from the fasting BW:full BW calculated at the beginning and at the end of the experiment. Every morning, feed refusals were manually collected from 0700 to 0800 h, weighed, and sampled for DM determination. A sample of the feed was also taken weekly for DM measurement, and samples were pooled at the end of each replicate for further chemical analysis. These samples of feed were measured for DM, ash, CP (N x 6.25), and lipid contents according to the AOAC (1990). The NDF and ADF contents were determined according to van Soest and Wine (1967).
Rectal temperature, cutaneous body temperatures (CT), and RR were measured 3 times daily (0700, 1200, and 1800 h) at d –10, –8, –6, –4, –2, 0, 1, 2, 3, 5, 7, 9, 11, 14, and 16 of experiment in Exp. 1 and at d –10, –7, –5, –3, –1, 0, 1, 2, 4, 7, 9, 11, 14, 16, 18, and 20 of experiment in Exp. 2. Additional body temperatures and RR measurements were carried out hourly during the transition from 24 to 31°C (d 0), so temperatures and RR were theoretically measured at 24, 25.75, 27.50, 29.25, and 31.0°C ambient temperatures. Corresponding actual temperatures (average ambient temperature during the period of RT, CT, and RR measurements) were 24.0, 25.6, 27.3, 29.1, and 31.2°C, respectively, in Exp. 1 and 24.4, 26.1, 28.1, 29.5, and 30.8°C in Exp. 2.
For each recording period, the following protocol was applied: first, RR was visually determined by counting flank movements over a period of 1 min, but only for resting pigs. Because the pig does not sweat, RR variation is considered a good indicator of the latent heat loss (Kamada and Notsuki, 1987). After RR measurements in all pigs were completed, RT was measured using a digital thermometer (Microlife Corp., Paris, France). Then, CT was measured on the back and belly (flank) of each pig using a digital thermometer (HH-21 model, Omega Engineering Inc., Stamford, CT) with a K probe. A variation in CT value under heat stress conditions is an indicator of an increase of blood volume in skin blood vessels to promote sensible heat loss (Mount, 1975).
Calculations and Statistical Analyses
For each pig, the daily feed intakes (ADFI in g of DM or in g of DM/kg0.60; Noblet et al., 1999) and ADG (g/d) and G:F (DM intake/gain) were calculated for both temperature levels (24 and 31°C). According to our experimental design, the effect of temperature (24 vs. 31°C) could be confounded with the increase of BW, age between the 2 periods, or both. For this reason, performance data were analyzed for each experiment and each temperature level using the GLM procedure (SAS Inst. Inc., Cary, NC), including effects of breed, replicate, and their interaction.
The CT was calculated as the average of CT measured on the back and flank locations. For each experiment, the effect of the ambient temperature rise from 24 to 31°C on d 0 on RT (°C), CT (°C), and RR [breaths per min (bpm)] responses was analyzed with a linear mixed procedure, with breed, actual temperature, and replicate as main effects. The repeated measurements option of the MIXED procedure of SAS was used with an autoregressive [AR(1)] covariance structure. The previous model was used to analyze the effects of day of exposure to 31°C, from d –10 to d 16 (Exp. 1) or from d –10 to d 20 (Exp. 2) on ADFI (g of DM · kg–0.60 · d–1), RT, CT, and, RR responses. The model of the covariance structure of error was chosen according to the REML estimation and Akaike and Bayesian information criteria.
The RT, CT, and, RR short-term responses to temperature change from 24 to 31°C on d 0 were submitted to a broken-line model (Huynh et al., 2005) to validate whether a break point could be determined (an inflection point temperature above which the response variables changed). This model can be described as:
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When the model failed to converge, the following linear regression model was used:
 | [model 2] |
where Y = the response variable (°C or bpm); y0 = a constant over a range of T (°C or bpm); v1 = the rate of increase of Y when T 
bpT (°C/°C or bpm/°C); and bpT = the break point temperature (°C).
According to Morrison and Mount (1971) and Giles (1992), the pig thermoregulatory response has a biphasic profile, consisting of an initial hyperthermia within the first 24 h of exposure to heat stress and a subsequent recovery period characterized by a gradual decrease of body temperature. To make a clear distinction between the periods during which RT, CT, or RR increased after the temperature increased from 24 to 31°C and declined over time of exposure at 31°C, we looked for a "threshold day" (the day which marked the beginning of the acclimation response) using a bilinear model described by Collin et al. (2002) and adapted from Koops and Grossman (1991):
 | [model 3] |
where Y = the response variable (°C or bpm), y0 = the value of Y at d = 0 (°C or bpm); Td = the threshold day (d of exposure); and v1 and v2 = the linear variations of Y before and after the threshold day (°C/d or bpm/d), respectively. In the approach of Koops and Grossman (1991), r determines the smoothness of the transition around the threshold day. In the current study, r was fixed at 0.25, assuming that it was not influenced by breed, the variables considered (RT, CT, RR), or both. The parameters of the model were estimated using the NLIN procedure of SAS. A preliminary analysis was conducted to evaluate the effects of data set size (data selected from d –2 to d 16 and d –1 to d 20 vs. from d 0 to d 16 and d 0 to d 20 in Exp. 1 and 2, respectively) on the accuracy of the nonlinear regression model, especially for the threshold day estimate. Using the asymptotic SE and the confidence interval as critera, the accuracy of the threshold day estimation was improved when the data from d –2 and d –1 in Exp. 1 and 2, respectively, were considered. As a consequence, the latter data set was used for the parameter estimation procedure.
A hypothesis concerning the effect of genotype on the fit of the models (model 1, 2, and 3) was tested using extra sum of squares principles (Van Milgen et al., 1992; Ratkowsky, 1993). The effect of genotype in parameter estimates was compared using a t-test (Van Milgen et al., 1992).
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RESULTS
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Actual temperature and relative humidity and their diurnal variations in the experimental room agreed with objectives of the experiments (Figure 1). Because of health problems (rectal prolapse and leg weakness), 2 pigs in Exp. 1 (1 CR in the first replicate and 1 LW in the second replicate) and 1 pig in Exp. 2 (1 CR in the first replicate) were removed.
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Figure 1. Variations in daily mean ambient temperature and relative humidity in Exp. 1 and 2 (means + SE).
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Growth Performance
According to the experimental design and breed difference in growth potential, the average BW was lower in CR than in LW pigs in Exp. 1 (Table 2), and the average age was greater in CR than in LW pigs in Exp. 2 at comparable BW (Table 3). In Exp. 1, ADFI was not affected by breed when expressed as g · d–1 · kg of BW–0.60 (190 and 143 g · d–1 · kg–0.60 on average at 24 and 31°C, respectively). In Exp. 2, ADFI was not influenced by breed at 24°C (221 g · d–1 · kg–0.60 on average), whereas it was lower in CR than in LW pigs at 31°C (143 vs. 168 g · d–1 · kg–0.60; P = 0.008). Whatever the experiment, the G:F was numerically greater in CR than in LW pigs, but the effect of breed was significant only for the second experiment. Figure 2 indicates a significant decline from d –1 to d 1 after the change from 24 to 31°C (–42 and –58 g · d–1 · kg–0.60 in Exp. 1 and 2, respectively; P < 0.001). This decline was not influenced by breed. Irrespective of breed, a gradual increase in ADFI was noticed over successive days of exposure to 31°C. This increase was significant (P < 0.05) only after d 8 in both experiments. Significant increases of ADFI were calculated from d 1 to 16 (25 g · d–1 · kg–0.60) and from d 1 to 20 (22 g · d–1 · kg–0.60) in Exp. 1 and 2, respectively. Finally, the ADFI was lower at 31°C than at 24°C (–44 and –52 g · d–1 · kg–0.60 in Exp. 1 and 2, respectively; P < 0.01).
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Figure 2. Effects of breed on ADFI [g/(d · kg of BW0.60)] profiles over the acclimation period at 31°C in growing pigs (dotted line = ambient temperature). Each point is the least squares mean of 11 pigs in each breed or 11 and 12 pigs in Creole and Large White pigs in Exp. 1 or 2, respectively. x = mean values are significantly affected by breed (P < 0.05).
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Thermoregulatory Responses
For the short-term effects of temperature change from 24 to 31°C on d 0, the broken line model fits for RT and RR (Table 4). In contrast, CT response was adjusted with a linear regression model. Effects of breed are presented in Figure 3. For both experiments, mean CT was lower in CR than in LW pigs (P < 0.05; Figure 2
). Whatever the experiment, the increase in CT with temperature was not influenced by breed (0.22°C/°C on average). With increasing temperature, RR remained constant at about 33 bpm in Exp. 1 and 2 until the break point temperature. In both experiments, the break point temperature for RR was greater in CR than in LW pigs (27.8 vs. 25.0°C and 27.2 vs. 25.9°C in Exp. 1 and 2, respectively). Above this break point temperature, the increase of RR was not affected by breed in Exp. 1 (11.8 bpm/°C on average), but it was greater in CR than in LW during the second experiment (16.0 vs. 11.8 bpm/°C; P < 0.05). Until the ambient temperature reached the break point temperature, RT was constant at an average of 39.3°C for both experiments. The break point temperature that marked the beginning of the rise of RT was similar for both breeds (27.3 and 28.0°C in Exp. 1 and 2). Above this break point temperature, RT increased by approximately 0.10 and 0.25°C/°C in Exp. 1 and 2, respectively; the effect of breed was significant (P < 0.05) only in the second experiment with a greater value in CR pigs (0.30 vs. 0.15°C/°C in LW pigs; Table 4).
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Figure 3. Effect of breed on rectal temperature (RT), cutaneous temperature (CT), and respiratory rate (RR) profiles during the transition from 24 to 31°C on d 0. Each point is the least squares mean of 11 pigs in each breed in Exp. 1 or 11 and 12 pigs in Creole and Large White pigs, respectively in Exp. 2. x = mean values are significantly affected by breed (P < 0.05). A broken line relationship was used to predict the RT or RR responses to temperature change. A linear relationship was used to predict the CT responses to temperature change.
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The long-term effects of exposure to 31°C on RT, CT, and RR responses in Exp. 1 and 2 are presented in Figure 4. For both experiments, CT at 24°C was lower in CR than in LW pigs (36.8 vs. 37.2°C and 36.5 vs. 37.0°C in Exp. 1 and 2, respectively; P < 0.05), but the breed effect was significant only on d –6 and –2 in Exp. 1 and on d –10, –5, and –3 in Exp. 2. The threshold day at which CT began to decrease when pigs were kept at 31°C was not affected by breed and averaged 0.97 and 0.65 d in Exp. 1 and 2, respectively (Table 5). Breed affected the linear CT rise before threshold day (v1), with a greater value in LW than in CR pigs (0.54 vs. 0.42°C/d and 0.99 vs. 0.70°C/d in Exp. 1 and 2, respectively; P < 0.05). The rate of fall after threshold day (v2) was similar for both breeds during the first Exp. (i.e., –0.022°C/d on average), whereas it was greater (P < 0.05) in LW than in CR pigs during the second experiment (Table 5). Whatever the stage of exposure to 31°C, CT was greater in LW than in CR pigs (0.4 and 0.3°C in Exp. 1 and 2, respectively). In both experiments, v1 and v2 values for RR were not affected by breed (Table 4). The threshold day for RR measurements was significantly greater in CR than in LW pigs (1.05 vs. 0.50 d; P = 0.011) in Exp. 1, but it was not affected by breed in Exp. 2 (0.90 d on average). At thermoneutrality, RT was not affected by breed in Exp. 1 (39.3°C on average), but it was lower in CR than in LW pigs in Exp. 2 (39.2 vs. 39.4°C; P < 0.05). For both experiments, threshold day value for RT was 24 and 12 h lower in CR than in LW pigs during Exp. 1 (0.18 vs. 1.17 d; P < 0.001) and Exp. 2 (0.39 vs. 0.93 d; P < 0.05), respectively. However, irrespective of the experiment, the increase of RT before threshold day and the rate of fall in RT between threshold day and d 16 or 20 were not affected by breed (Table 4). Except at d 14 and 16 in Exp. 1 and at d 4 in Exp. 2, RT was 0.3°C less in CR than in LW pigs during the whole acclimation period at 31°C (P < 0.05; Figure 4).
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Figure 4. Effects of breed on rectal temperature (RT), cutaneous temperature (CT), and respiratory rate (RR) profiles over the acclimation period at 31°C (dotted line = ambient temperature). Each point is the least squares mean of 11 pigs in each breed in Exp. 1 or 11 and 12 pigs in Creole and Large White pigs, respectively, in Exp. 2. x = mean values are significantly affected by breed (P < 0.05). The RT, CT, and RR responses were predicted using a nonlinear model from d –2 and d –1 in Exp. 1 and 2, respectively.
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DISCUSSION
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Pig Performance
The effect of increased ambient temperature on ADFI in growing pigs has been extensively described. In a review, Le Dividich et al. (1998) reported that the associated reduction of ADFI ranged from a minimum of 40 g · d–1 · °C–1 to a maximum of 80 g · d–1 · °C–1. According to these authors, this large variability can be explained by many factors including breed, BW range, diet composition, and to the greatest extent, temperature range. From our results, each degree increase in ambient temperature from 24 to 31°C resulted in a reduction of ADFI equivalent to about 8 g · d–1 · kg–0.60 in LW pigs when both experiments were considered. This value is similar to the value estimated from the equation proposed by Quiniou et al. (2000; 7 g · d–1 · kg–0.60)
where T = ambient temperature (°C). This reduced ADFI under heat stress corresponds to a short-term acclimation to hot conditions via a decrease in metabolic heat production.
The current study shows an effect of breed on performance traits at thermoneutrality. In particular, ADG was lower in CR than in LW pigs at thermoneutrality, whereas ADFI (expressed as g · d–1 · BW–0.60) was not affected by breed, which is consistent with previous results obtained at our research station (Renaudeau et al., 2006). In this latter study, fatter carcass in CR pigs was associated with a reduced G:F. Similar conclusions were reported when Meishan pigs were compared with conventional lean pigs (Noblet et al., 1994b).
Short-Term Acclimation Responses to Elevated Ambient Temperature
Little information is available in growing pigs on the change of CT with ambient temperature. In the current study, CT increased linearly during the 24 to 31°C increase of ambient temperature on d 0 (0.22°C per extra degree of ambient temperature), and the extent of the increase was not affected by breed. Our results agree with those obtained in individual housed pigs by Renaudeau (2005) (0.23°C/°C from 24 to 34°C) and Giles and Black (1991; 0.23°C/°C from 23 to 31°C) and Huynh et al. (2005) in group-housed pigs (0.25°C/°C from 16 to 32°C). The elevation in CT is explained by an increase of blood volume in blood vessels to promote nonevaporative heat dissipation. The gradient beween CT and air temperature became smaller with the increase of temperature leading to a reduction in sensible heat loss, which made the pig more dependent on water evaporation from their lungs to prevent a rise in body temperature. In the current study, the break point temperature for RR in LW pigs averaged 25.5°C, which is similar to the value reported by Brown-Brandl et al. (2001; i.e., from 24 to 28°C in individually housed synthetic line pigs). According to Kamada and Notsuki (1987), water loss from panting is closely related with RR. Because the pig did not sweat, it can be assumed that the break point temperature for RR was equivalent to the point at which there was an increase in evaporative heat loss. This assumption was confirmed by Huynh et al. (2005) in group-housed pigs kept at 80% RH who found that RR and evaporative water loss began to increase at a similar temperature value (21.3 and 21.2°C, respectively). To simplify, this point was called evaporative critical temperature. The lower evaporative critical temperature value reported by Huynh et al. (2005) may be the result of a reduced ability of group-housed pigs to lose heat by radiation or convection because they were surrounded by other warm bodies. However, average RR value before evaporative critical temperature and rise of RR after evaporative critical temperature seemed to be unaffected by pig group size, because the values reported by Huynh et al. (2005; 29 bpm and 12 bpm/°C) were similar to ours (33 bpm and 11 bpm/°C). As shown in the present work, the break temperature for RT in LW pigs averaged 27.5°C, which is in agreement with the value reported by Giles and Black (1991; from 26 to 29°C). However, Huynh et al. (2005) showed a lower break point temperature for RT (24.6°C at 80% RH), which could be related to the difference in housing conditions (individually vs. group-housed). Above this point, mechanisms implicated in body temperature regulation are saturated or not efficient enough to prevent hyperthermia. Consequently, RT rose by about 0.10°C for each degree increase in ambient temperature. This value for RT increment above the break point temperature for RT is similar with previous values measured by Brown-Brandl et al. (2001) and Huynh et al. (2005; 0.10 and 0.11°C/°C, respectively).
In contrast to other farm species (ruminants or poultry), the effect of breed on acclimation responses to elevated high ambient temperature is poorly documented in pigs. The existence of a different short-term heat stress responsiveness between breeds has been previously reported between halothane-positive and halothane-negative boars (Aberle et al., 1975; D’Allaire and DeRoth, 1986; Tauson et al., 1998). In the current study, the slope of CT responses to short-term heat stress was similar in CR and LW, suggesting that breed did not apparently influence the ability to lose nonevaporative heat. In the current study, evaporative critical temperature was higher in CR pigs (2.8°C) when both breeds were compared on a constant age basis (Exp. 1). When the effect of breed was studied at a constant BW (Exp. 2), the difference became smaller (1.3°C) but remained significant. Moreover, above evaporative critical temperature, the rise in RR with increasing temperature was higher in CR pigs.
According to our results, the RT short-term response was not affected by breed, whereas evaporative critical temperature was significantly higher in CR pigs. From this, one can conclude that the mechanisms involved in maintaining the thermal balance under heat stress would differ between CR and LW breeds. According to Van Milgen et al. (1998), fasting heat production is reduced when fat-type pigs are compared with conventional line pigs. This effect is related to breed variations in weights of metabolically active body compartments such as viscera or lean mass. From these results, it can be hypothesized that a lower metabolic heat production in CR pigs would explain its lower use of evaporation capacity for the short-term body temperature regulation.
Long-Term Acclimation Responses to Elevated Ambient Temperature
Compared with the average ADFI at 24°C (202 g · d–1 · kg–0.60 on average), feed consumption in LW pigs declined to about 80 g · d · kg–0.60 within the first 24 h at 31°C (on d 1), and, thereafter, it increased slightly over the remaining 15 or 19 d of exposure to 31°C (25 and 22 g · d–1 · kg–0.60 in Exp. 1 and 2, respectively). The initial decrease in ADFI was lower than previous results reported by Morrison and Mount (1971) and by Giles and Black (1991) within 24 h of exposure to 33 or 31°C in 50- and 90-kg growing pigs previously maintained at 22°C, respectively (–138 g · d–1 · kg–0.60 on average). Apart from difference in pig characteristics (BW and genotype), this effect could be explained by the greater range of temperature between the thermoneutral and the hot treatments in previous studies (11 or 9°C) than in ours (+7°C). According to Collin et al. (2001), the decrease of ADFI in hot conditions tends to reduce the associated heat production due to metabolic processes of nutrient intake. Thereafter, the gradual increase of ADFI with successive days of exposure to 31°C supports the find-ings of Morrison and Mount (1971) in 60-kg growing pigs kept at 33°C for 28 d. In contrast, in heavier pigs (90 kg on average), Giles and Black (1991) showed that ADFI remained unchanged throughout the 11-d exposure to 31°C. In this latter study, the lack of variation in ADFI over the period of exposure may be related to greater pig BW, the shorter period of thermal acclimation, or both. From our results obtained in both experiments, hypotheses could explain the gradual increase of ADFI on successive days of exposure. First, this increased feed consumption can be related to the increase in BW or maintenance requirements. However, ADFI was expressed per kilogram of metabolic BW (BW0.60) and then it indirectly took into account energy requirements for maintenance. In other words, the gradual increase in ADFI at 31°C was not related to the increase of BW. A long-term acclimation to heat stress may be an alternative explanation. According to the results of Morrison and Mount (1971) and Giles and Black (1991), the steady decline in RT over time of exposure to 31°C supports this hypothesis. As observed in the current study, the decline in the RR was also measured by Morrison and Mount (1971) and Giles and Black (1991). In the current study, the decline in RR and in CT with the period of thermal acclimation tended to suggest that the demand for body cooling was reduced. In consequence, these declines are considered as a consequence rather than a cause of a long-term acclimation to heat exposure (Bianca, 1959). Moreover, it can be hypothesized that this acclimation is mainly related to a decrease in heat production. This hypothesis is consistent with the decrease of O2 consumption reported by Giles and Black (1991). Such an explanation would be consistent with numerous reports in the literature demonstrating reductions in metabolic rate during acclimatization to heat in man (Chaffee and Roberts, 1971) and in pig (Derno et al., 1995). Assuming that total heat production decreased at 31°C, the gradual recovery trend measured for ADFI suggests that pigs would become metabolically more efficient.
Concerning RT measurements, more than 20 d were required for a steady value to be reached after the change to the high temperature, suggesting that the time of adaptation is greater than 20 d. Verhagen et al. (1988) showed that 5 d were required for 20-kg growing pigs to acclimate to a change in ambient temperature from 20 to 25°C. From these results, it can be suggested that the rate of acclimation would vary with BW, the temperature ranges, or both. In 60-kg pigs kept at 33°C, Morrison and Mount (1971) estimated the adaptation time to be 10 d. The difference in the RH (i.e., from 50 to 60% vs. 80% in our study) could also explain the discrepancy between the 2 studies. In other words, in high RH, the evaporative heat loss efficiency from the respiratory tract would decrease and slow down the rate of acclimation.
Concerning RT measurements, the threshold day for the onset of acclimation response was 24 and 13 h lower in CR than in LW pigs when both breeds were compared at a constant age basis (Exp. 1) and at a constant BW basis (Exp. 2), respectively. Our results showed that the effect of breed on the threshold day is partly explained by BW difference when pigs were compared on a same-age basis. Moreover, the slope of the thermoregulatory response was not affected by breed. In fact, when the threshold duration of exposure to high temperature was reached, mechanisms involved in thermo-regulatory response seemed to be identical for both breeds. Singh and Newton (1978) showed that the time required to respond to a rapid increase in temperature from 18.3 to 40.5°C was lower in heat-acclimated than in heat-nonacclimated calves. In addition, they also measured a greater rate of fall in RT with the period of acclimation to 40.5°C in heat-adapted pigs. From these authors, the observed breed change in thermoregulatory capacities was mainly explained by the difference in sweating rate. Our results show that the lower RT in CR pigs was related to changes in the onset of time threshold rather than to changes in the slope of thermoregulatory response.
The present experiment confirms the negative effect of high ambient temperature on growth performance. Whatever the breed, our results show an improved tolerance to heat stress with duration of exposure, indicating an acclimation to heat. It is suggested that decrease in heat production might play a part in the observed acclimation. Finally, this study also demonstrates that physiological reactions during heat acclimation are affected by breed. However, further studies are needed to understand the mechanisms underlying the effect of breed on the onset of acclimation responses following an increase in ambient temperature.
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Footnotes
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1 We thank C. Anaïs, K. Benony, B. Bocage, M. Bructer, M. Giorgi, J. L. Gourdine, A. Racon, F. Silou, and J. L. Weisbecker for their technical assistance and J. Van Milgen for critical evaluation of the manuscript. 
2 Corresponding author: David.Renaudeau{at}antilles.inra.fr
Received for publication July 4, 2006.
Accepted for publication October 23, 2006.
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Abstract
INTRODUCTION
