Seasonal variation in fertility and milk production in dairy cattle under Mediterranean climate

Heat stress affects fertility and productivity in dairy cattle, yet field studies from the Mediterranean area remain limited. This retrospective, observational cohort study aimed to investigate seasonal variation in: (1) conception and pregnancy rates (at 30 and 60d post-AI, respectively), number of services per conception, submission rate, calving to conception interval, and gestation length; (2) milk production and composition in Holstein Friesian cattle in two Italian farms during three consecutive years. Reproductive data, milk production, and composition were analyzed both monthly and based on three categories: Minimum risk temperature-humidity index (THI) < 56, Low risk (56 < THI < 68), and High risk (THI> 68). High, Low, and Minimum risk conditions occurred for approximately 4 months/year. In heifers that were not exposed to cooling system, the monthly submission rate peaked in May (17.9%) and was the lowest in September (9.4% P < 0.001); also, a greater submission rate was recorded under Minimum and Low risk compared to High risk (P < 0.001). In lactating cows which were exposed to cooling systems when THI exceeded 68, conception rate at 30d post-AI, pregnancy rate at 60d post-AI, number of services per conception, monthly submission rate were influenced by month (P < 0.001). Under High risk conception rate at 30d post-AI (High risk = 37.2%; Minimum risk = 55.2%; P < 0.05), pregnancy rate at 60d post-AI (High risk = 29.4%, Minimum risk = 45.8%; P < 0.05) and submission rate (High risk = 13.1%, Minimum risk = 17.9%; P < 0.001) were reduced compared with Minimum risk. Milk production peaked in spring and decreased in summer and was lower under High (10.90 ton/day) than Minimum risk (11.91 ton/day; P < 0.01). When THI increased, milk fat, protein and lactose percentages decreased, whereas somatic cell count and milk urea nitrogen increased (P < 0.05). Evidence indicates that cattle in the Mediterranean region can be exposed to mean THI ≥ 68 for four consecutive months; high environmental temperatures may decrease conception rates by nearly 20% and impair milk production in cows.