Productive adaptability of Karan Fries cattle evolved under subtropical environmental conditions in changing climatic scenario.

Climate change poses significant challenges to dairy cattle production systems, with heat stress causing substantial economic losses through reduced milk yield. Understanding the relationship between heat stress and milk production is crucial for developing adaptive strategies in tropical and sub-tropical dairy farming. This retrospective longitudinal study aimed to model lactation curves, develop resilience indicator traits, and quantify heat stress-induced milk losses in Karan Fries cattle under tropical conditions. Daily milk yield records (n = 417,642) from 1475 Karan Fries cows collected over 35 years (1990-2024) were analysed alongside meteorological data from NDRI, Karnal, India. The Temperature-Humidity Index (THI) was calculated and classified into four categories. Multiple lactation curve models (Wilmink, Cobby, and Monophasic) were evaluated, and the best-fitting model was used to estimate expected milk yield. Four resilience indicator traits were developed: number of days in heat stress (NDHS), total milk loss (TML), average milk loss per day (AML), and number of heat stress incidences (NOI). Segmented regression analysis was performed to identify THI breakpoints, and variations of heat stress indicators across months, seasons, lactation stages, and yield categories were evaluated. The Wilmink model demonstrated excellent fit (R²=0.896, RMSE = 0.361 kg). Segmented regression revealed distinct THI breakpoints across yield categories: low yielders (53 and 55), medium yielders (55 and 81), high yielders (60 and 73), and overall population (76 and 81). Animals experienced heat stress for an average of 63.556 days, with total milk loss of 106.232 kg and average daily loss of 1.671 kg/day during the study period. The least squares analysis of variance was performed to study the monthly, seasonal and lactation stage wise variations in the climate resilience parameters. Monthly analysis identified July (NDHS = 13.714 days; TML = 24.329 kg, P < 0.001) and August (NDHS = 13.856 days; TML = 21.577 kg, P < 0.001) as the most critical months. The rainy season showed the highest heat stress impact (NDHS = 37.574 days, TML = 61.001 kg, P < 0.001). Mid-lactation cows were most vulnerable (NDHS = 25.283 days, TML = 40.452 kg, P < 0.001). Regression analysis revealed milk yield decline by 0.061 kg per unit increase in THI (P < 0.001) in overall population, with medium yielders showing the steepest decline (-0.023 kg per unit increase in THI, P < 0.001). These findings indicate that heat stress significantly impacts milk production in Karan Fries cattle, with variation across production levels, months, seasons and lactation stages. The results provide a basis for further investigation into targeted management and breeding strategies for improving climate resilience in tropical dairy systems.