Conception rates and temperature-humidity index during 2024 of Korean Holstein cows in the Jeolla-do region

Background: The rise in annual mean temperatures caused by global warming has increased the risk of heat stress in livestock.Dairy cows are particularly susceptible, with both productivity and reproductive performance adversely affected.Previous studies have reported that THI 72 is generally considered the onset of heat stress.Against this background, the present study examines conception rate variations of dairy cows in relation to the THI in Jeolla-do region through 2024.Methods: The THI for the Jeolla-do region was collected from the Korea Meteorological Administration Data Portal, and artificial insemination conception records were compiled for 35,053 cows from 39 farms across Gwangju Metropolitan City and eight counties in Jeolla-do region (Gochang, Gwangju, Yeonggwang, Naju, Hampyeong, Muan, Jangseong, and Yeongam).Seasonal artificial insemination conception rates (spring: March-May, summer: June-August, autumn: September-November, and winter: December-February) were compared and analyzed in relation to the regional THI values (n = 35,053).Results: The THI threshold for heat stress in dairy cows (THI 72) persisted from May through September, and all locations, except Yeongam and Naju, reached their annual maximum THI in August.In Jangseong, THI remained 72 for the longest period, lasting 175 days.In most locations, the highest monthly conception rate occurred in July.By season, conception rates were highest in summer (June-August) at 58.5% and lowest in autumn (September-November) at 43.5%.Conclusions: The pronounced reduction in conception rates observed in autumn appears to be a consequence of the cumulative heat stress of the summer, which continues to impair reproductive performance even in autumn.Although farms commonly deploy cooling systems such as ventilation fans and sprinklers during the summer, management of heat stress may be neglected in autumn despite animals continuing to experience thermal stress, thus contributing to the observed decline in conception rates.These findings underscore the need for vigilant herd management and heat-stress mitigation not only in summer but also in autumn.The results can serve as baseline data for future research on the effects of heat stress on dairy cattle and on strategies for its mitigation.