Effect of heat stress on respiratory rate, rectal temperature and blood chemistry in Omani and Australian Merino sheep

The effect of heat stress (HS) on changes in respiratory rate, rectal temperature and blood chemistry parameters were evaluated in mature black wooly Omani and white wooly Australian Merino wethers. These animals were subjected to cool environment when the mean temperature humidity index (THI) was 72 ± 1.4 (dry bulb temperature of 22.2-24.4°C and relative humidity of 100-60%) during the month of December. This experiment was repeated during the hotter month of July of the following year, when the mean THI was 93 ± 3.1 (dry bulb temperature of 35.6-43.9°C and relative humidity 95-35%). Blood pH, bicarbonate (HCO₃^-; mmol/l), creatinine (CR; μmol/l), respiratory rate and rectal temperature increased (P < 0.01), but blood pCO₂ (kPa), base excess (BE; mmol/l), plasma calcium (Ca; mmol/l), creatinine phosphokinase (CPK; IU/l) and aspartate aminotransferase (AST; IU/l) decreased (P < 0.01) when the animals were subjected to heat stress. In addition, heat stress increased (P < 0.01) the pO₂ (kPa), oxygen saturation (O₂SAT; %) and blood urea nitrogen (BUN; mmol/l) in Omani sheep, but decreased (P < 0.01) these estimates in Merino sheep. Also, heat stress increased (P < 0.01) plasma glucose (Glu; mmol/l) in Merino but decreased (P < 0.01) it in Omani sheep. There was no effect of heat stress on the total hemoglobin (THb; g/dl) in Omani sheep but heat stress decreased (P < 0.01) THb in the Merino sheep. Heat stress had no effect on plasma potassium (K; mmol/l) in Merino but decreased (P < 0.01) it in Omani sheep. Plasma chloride (Cl; mmol/l) increased (P < 0.01) with heat stress in both the breeds to the same magnitude. These results indicate that both breeds of sheep were able to maintain a normal acid-base balance when heat stress increased their rectal temperature and respiratory rate and that Omani sheep is more heat tolerant than Merino sheep.