High temperature-humidity index is negatively associated with milk performance and 8 quality in Korean dairy system: big data analysis

28 Objective: The aim of this study was to investigate the effects of heat stress on milk traits in South 29 Korea using comprehensive data (dairy production and climate). 30 Methods: The dataset for this study comprised 1,498,232 test-day records for milk yield, fat- and 31 protein-corrected milk (FPCM), fat yield, protein yield, milk urea nitrogen (MUN), and somatic cell 32 score (SCS) from 215,276 Holstein cows (primiparous: n=122,087; multiparous: n=93,189) in 2,419 33 South Korean dairy herds. Data were collected from July 2017 to April 2020 through the Dairy Cattle 34 Improvement Program, and merged with meteorological data from 600 automatic weather stations 35 through the Korea Meteorological Administration. The segmented regression model was used to 36 estimate the effects of the temperature-humidity index (THI) on milk traits and elucidate the break 37 point (BP) of the THI. To acquire the least-square mean of milk traits, the generalized linear model 38 was applied using fixed effects (region, calving year, calving month, parity, days in milk, and THI). 39 Results: For all parameters, the BP of THI was observed; in particular, milk production parameters 40 dramatically decreased after a specific BP of THI ( p < 0.05). In contrast, MUN and SCS drastically 41 increased when THI exceeded BP in all cows ( p < 0.05) and primiparous cows ( p < 0.05), respectively. 42 Conclusion: Dairy cows in South Korea exhibited negative effects on milk traits (decrease in milk 43 performance, increase in MUN, and SCS) when the THI exceeded 70; therefore, detailed feeding 44 management is required to prevent heat stress in dairy cows.