Acclimation to high ambient temperature in Large White and Caribbean Creole growing pigs

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 [degrees]C for 10 d (d -10 to -1) and thereafter to a constant temperature of 31 [degrees]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 [+ or -] 2.5 kg and 51.7 [+ or -] 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 [degrees]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.sup.-1] * [kg.sup.-0.60] in Exp. 1 and 2, respectively). Short-term thermoregulatory responses during the 4-h transition from 24 to 31 [degrees]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 [degrees]C/ [degrees]C) and was less (P < 0.05) in CR than in LW (by -0.3 [degrees]C on average). In both experiments, the break point temperature for RT was not affected by breed (27.6 [degrees]C on average), whereas for RR it was greater (P < 0.05) in CR than in LW (27.5 vs. 25.5 [degrees]C, P < 0.01). On average, ADFI declined by about 50 g*[d.sup.-1] * [kg.sup.-0.60] from d -1 to d 1 (P < 0.01), and thereafter at 31 [degrees]C, it gradually increased (23 g * [d.sup.-1] * [kg.sup.-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 [degrees]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 [degrees]C and 37.9 vs. 38.2 [degrees]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