The effect of heat stress on changes in milk production, rectal temperature, respiratory rate and blood chemistry was evaluated in three groups of six mature Holstein, Jersey and Australian Milking Zebu (AMZ) dairy cows. These animals were subjected to a 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%). Holstein cows produced more (p<0.01) milk than AMZ and Jersey cows during the cooler months of the year and all the cows were dry during the hotter months from June until September. Heat stress increased (p<0.01) rectal temperature and respiratory rate in all three breeds. Heat stress had no effect on blood pH in Holstein and AMZ cows but lowered (p<0.01) blood pH from 7.42 to 7.34 in Jersey cows. In addition, heat stress lowered (p<0.01) blood pCO2 (kPa), bicarbonate (HCO3–, mmol/L), base excess (BE, mmol/L) and plasma chloride (Cl–, mmol/L) in all three breeds. The total haemoglobin (THb, g/dl) was elevated (p<0.01) in all three breeds when they were subjected to heat stress. Heat stress increased (p<0.01) oxygen saturation (O2SAT, %) in Jersey and AMZ cows but lowered it (p<0.01) in Holstein cows. On the other hand, heat stress increased (p<0.01) pO2 (kPa) in Holstein and Jersey cows but lowered it (p<0.01) in AMZ cows. Heat stress increased (p<0.01) plasma potassium (K, mmol/L) and calcium (Ca, mmol/L) only in Holstein and Jersey cows but lowered them (p<0.01) in AMZ cows. The plasma glucose (GLU, mmol/L) increased (p<0.01) with heat stress in Holstein and AMZ cows but decreased (p<0.01) in Jersey cows. Heat stress increased (p<0.01) plasma creatinine (CR, (mol/L) but lowered (p<0.01) plasma creatinine phosphokinase (CPK, IU/L), aspartate aminotransferase (AST, IU/L) and blood urea nitrogen (BUN, mmol/L) in all three breeds. These results indicate that heat-stressed Holstein and AMZ cows were able to maintain their acid–base balance with a marginal change in their pH of 0.02 when their rectal temperatures increased by 0.47 and 0.38°C, respectively. When heat stress increased the rectal temperature in Jersey cows by 0.70°C, the pH decreased (p<0.01) from 7.42 to 7.34. However, even with this decrease 0.08 the pH is still within the lower physiological limit of 7.31.
blood chemistry dairy cows heat stress milk production rectal temperature respiratory rate