Objective: In Isfahan province, Iran, the health and productivity of dairy cows are frequently challenged by heat stress, especially in hot and arid regions, which can disrupt metabolic balance, transition period management indices, and postpartum health. The objective of this study was to assess the impact of season (summer vs. winter) and the intensity of heat stress on calving-related management indices, body condition, metabolic disorders, serum calcium status, and reproductive outcomes in Holstein dairy herds in Isfahan province.

Method: The study was conducted as a completely randomized design in four industrial Holstein dairy herds in Isfahan Province, Iran, during summer and winter seasons, with 855 dairy cows. Daily meteorological data were recorded to calculate the temperature–humidity index (THI). Body condition score (BCS) at calving, gestation length, dry period length, colostrum yield, incidence of metabolic disorders (retained placenta, metritis, hypocalcemia, and ketosis), culling rate up to 60 days postpartum, and conception rate at first AI were recorded. Serum calcium concentration was measured at calving and on day 3 postpartum, and serum beta-hydroxybutyrate concentration was measured on day 10 postpartum. The data were analyzed with appropriate statistical models, taking into account the effect of season, herd, parity, and their interactions. All statistical assumptions including normality of residuals and homogeneity of variances were tested and confirmed before final analysis.

Results: Results indicated that THI values ranged from 74 to 82 during the summer, suggesting moderate to severe heat stress. Gestation length was reduced in primiparous cows compared with multiparous cow within each herd (P<0.05). Body condition score at calving was influenced by herd management (P<0.05). Colostrum yield was lower in multiparous cows compared to primiparous cows (P<0.05), and tended to decrease during the summer season (P=0.07). Dry period length also decreased during the summer (P<0.05). Season, herd, and parity all influenced the incidence of retained placenta and metritis, with the highest incidence of these disorders in the winter season (P<0.05). Although dystocia increased during summer (P<0.05), serum calcium concentration was lower in summer than in winter (P<0.05). The prevalence of hypocalcemia and ketosis, especially in multiparous cows, was higher during summer (P<0.05). The culling rate up to 60 days postpartum varied among herds, with the highest rate observed in Herd C during both seasons.

Conclusion: Results of this study indicated that herd management and differences among lactation parities were important for the incidence of metabolic diseases and calving-related factors, whereas heat stress decreased the incidence of retained placenta and metritis, but was a limiting factor for dry period length and colostrum yield in dairy cows.

Keywords: Body condition score, Dairy co,; Heat stress, Metabolic disorders, Transition period.