Effect of heat stress on milk production, rectal temperature, respiratory rate and blood chemistry in Holstein, Jersey and Australian milking Zebu cows

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 (p2 (kPa), bicarbonate (HCO ₃ ^-, 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 (p2SAT, %) in Jersey and AMZ cows but lowered it (p2 (kPa) in Holstein and Jersey cows but lowered it (p