Acclimation in Body Temperature and Haematological Indices of Large White and Crossbred Boars Exposed to Direct Solar Heat Stress

The study was aimed at evaluating acclimation to high ambient temperatures (AT) in Large White (LW) and crossbred (Large White x local, LW x LC F1) boars exposed to direct solar radiation. Twenty-four LW and crossbred boars (12 boars each, 6 months of age, 42.65 to 50.14 kg body weight) were used for the study. There were three treatments (T) (8 boars/treatment) namely T1 or control (housed), T2 (45 minutes exposure, 13:30 to 14:15 h) and T3 (60 minutes exposure, 13:30 to 14:30 h). Parameters evaluated were ambient temperature (AT), body temperature (BT), leukocyte count (LC), erythrocyte count (EC), haemoglobin concentration (Hb) and packed cell volume (PCV). Mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC) were determined. Boars in the control had normal values for the parameters measured and these were relatively stable across the experimental period. BT increased significantly (P ? 0.05) with duration of exposure in both genotypes but tended towards the values for the control from the 3rd week of exposure in crossbred boars whereas in LW boars exposed for 60 min. it maintained an increasing trend until the end of the study. Threshold day (d) was 14 d for boars exposed for 45 min., but 7 d for crossbred boars exposed for 60 min. whereas no threshold d was observed for LW boars exposed for 60 min. The haematological indices of exposed LW boars were significantly (P ? 0.05) different from those of the control to the end of the experiment while those of crossbred boars were predominantly similar to those of the control by the end of the study. It was concluded that more than 28 d would be required to attain complete acclimation in BTs and haematological indices of LW and LW x LC F1 crossbred boars exposed for 45 and 60 min. to direct solar radiation during 13:30 to 14:30 h. Considering the trends in the parameters measured, complete acclimation would be attained in BT earlier than in the haematological indices, in boars exposed for 45 min. earlier than those exposed for 60 min. and in the crossbred boars earlier than in their LW counterparts. Therefore the time period for complete physiological acclimation to heat stress would depend on the parameter considered, duration of exposure, degree of thermal stress and breed of animals.