Comparative study of hematological responses to thermal stress in Sahiwal and Hardhenu calves

The present study was conducted to compare the hematological responses of Sahiwal and Hardhenu calves to different levels of thermal stress, expressed as Temperature-Humidity Index (THI), during the first 90 days of life. Blood samples were collected monthly in the morning (09:00-10:00 h) from 102 female calves, comprising a crossbred strain (Hardhenu, n = 54) and an indigenous breed (Sahiwal, n = 48), maintained at the Cattle Breeding Farm, LUVAS, Hisar, India, from July 2020 to June 2021. Calves were categorized into neonatal (≤ 30 days) and post-neonatal (31-90 days) groups. THI was classified as < 55, 55-75 and ≥ 75. Data were analyzed using a linear mixed model, with breed and THI as fixed effects and animal as a random effect. Estimated marginal means (± SE) were used to evaluate differences among groups. The results indicated that both breed and THI influenced several hematological parameters, with variations observed across age groups. Indigenous Sahiwal calves showed relatively higher erythrocytic indices (RBC, Hct and MCV) and neutrophil (NEU) percentages, suggesting better adaptability to thermal stress. In contrast, crossbred Hardhenu calves exhibited higher leukocyte (WBC) and lymphocyte (LYM) responses, indicating greater sensitivity to environmental conditions.Moderate THI (55-75) was generally associated with more stable hematological profiles, whereas higher THI (≥ 75) was linked with increased variability and changes in several parameters, reflecting heat stress effects. The study revealed significant effects of breed and THI on several hematological traits in calves, indicating breed-related differences in hematological responses under varying thermal environments. These findings may help in developing management strategies for dairy calves under tropical conditions. However, further studies with larger datasets and direct indicators of heat stress are required to validate their application in selection and management programmes under changing climatic conditions.