Studying Resistance of Some Dairy Cattle Breeds to Heat Stress in Relation to Milk Yield

The objective for our work was to estimate the milk yield and blood biochemical constituents of and weight daily gain of calves recently born from imported cows under hot summer conditions in Egypt. Thirty late- pregnant cows, aged 22-24 months, chosen randomly were used in this study. Experiment was carried out during the period from third stage of pregnancy and continued to 8 weeks postpartum. Imported late-pregnant cows divided into three groups, each group was 10 cows. 1st group was Brown Swiss (BS) cows, 2nd group was Simmental (S) cows and 3rd group was Holstein (H). Results showed that H cows have a lowest rectal temperature (RT) and respiration rate (RR) in compare with BS and S cows. Results showed that H cows have a highest daily milk yield while S cows have lowest values. BS and H cows have significantly higher total protein, albumin and urea concentrations than S cows. While, globulin and glucose concentrations were significantly higher in BS than its concentrations in S and H cows, respectively. Total cholesterol, phospholipids and creatinine concentrations showed the lowest values in BS cows as compare with S and H cows. Otherwise, BS cows have the highest triglycerides concentration as compare with H and S cows. About liver function showed the highest activity for AST in S cows and for ALT in H cows as compared of other groups. BS cows have significantly higher T4 and Estradiol17β levels than its levels in S and H cows, respectively. Birth weight of newborn both male and female calves as well as daily body weight gain of calves produced from the three imported cows were not differs Significantly. Two months weight of calves produced from S cows was significantly higher than BS and H cows, respectively. In conclusion, the three imported breed cows appears good resistance to heat stress effects under hot summer conditions in Egypt with extremely resistance for H cows.