Martínez Díez, Elena NiceasCastillo Rodríguez, CristinaAvendaño-Reyes, LeonelHernández Bermúdez, JoaquínBenedito Castellote, José LuisRico, A.García Herradón, Pedro JoséMuiño Otero, Rodrigo2025-07-302025-07-302025-07-22Martínez, E.N., Castillo, C., Avendaño-Reyes, L., Hernández, J., Benedito, J.L., Rico, A., García, P., & Muiño, R. (2025). Impacts of heat stress under oceanic climate on fertility and reproductive physiology of dairy cows subjected to hormonal synchronization. "International Journal of Biometeorology". https://doi.org/10.1007/s00484-025-02969-6https://hdl.handle.net/10347/42637Regions with oceanic climates are experiencing unprecedented climatic changes. This study assesses the presence of heat stress in these regions and its impact on reproductive in Holstein cows using the Temperature-Humidity Index (THI), a measure of thermal stress. Data was collected from 159 Holstein cows across experiments. Cows underwent a modified double-ovsynch synchronization protocol, while 96 cows were inseminated based on observed estrous. Cows were examined for body temperature, follicle and uterus examination, and serum progesterone concentrations to evaluate the impact of heat stress. The results of cows pregnancy rate in double-ovsynch synchronized cows were 48.5% and 43.3% in summer and winter seasons, respectively. While the pregnancy rate in non-synchronized cows were 29.5% and 32.7% in summer and winter season, respectively. Elevated THI levels (THI ≥ 72) have adversely effects on reproductive physiology, reduced estrous expression, decreased uterine blood flow, and altered progesterone concentrations. Mild heat stress during summer months in oceanic climate negatively impacted reproductive efficiency in dairy cows, for adaptive management strategies. The double-ovsynch protocol effectively stabilized fertility across seasons, demonstrating its value in improving reproductive outcomes under varying thermal conditions.eng© The Author(s) 2025Attribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/Temperature-humidity indexDairy cattleOceanic climateHeat stressModified double-ovsynch2509 Metereologíajournal article10.1007/s00484-025-02969-61432-1254open access