Thermal Resilience in Dairy Cattle: A Comparative Study of Heat Stress Impact on Milk Yield and Chemical Composition Across Three Breeds

This study examined the effects of seasonal climatic variations and parity on the milk yield and composition of Holstein (HST), Brown Swiss (BSW), and Montbéliard (MBM) dairy cows in North Tunisia from 2020 to 2024. Test day records (TD) from 199762 observations for different classes of parities (1, 2, 3 and 4+), with calculation of the Temperature-Humidity Index (THI) through Ambient Temperature (AT, ℃) and Relative Humidity (RH, %), were merged with meteorological data for analysis of the effect of seasonal heat stress and parity on the milk yield and composition. Meteorological data indicated a significant seasonal variation, with THI surpassing heat-stress thresholds during summer (mean = 75.29) and reaching its maximum in July and August. According to the results, Holstein cows had the highest peak milk yields (about 24.5 kg/d), but they were also the most sensitive to heat stress, as shown by their significant production decrease in summer. On the other hand, despite environmental challenges, BSW and MBM showed better thermotolerance and maintained consistent lactation. Analysis of milk composition revealed a strong seasonal concentration effect in all breeds, with the percentages of milk fat (MF) and milk proteins (MP) increasing in the summer and autumn months, with BSW producing the highest solid concentrations (up to 3.87% fat). Statistical analysis demonstrated significant interactions among breed, season, and parity (p < 0.05) for all traits except parity with milk fat. The results indicate that, whereas Holsteins maximize performance in temperate environments, incorporating Brown Swiss and Montbéliard genotypes provides a more resilient strategy for maintaining dairy productivity and milk quality in the increasingly hot Mediterranean climate of North Tunisia.