Impact of Climate Change and Heat Stress on Milk Production in Korean Holstein Cows: A Large-Scale Data Analysis

SIMPLE SUMMARY: Climate change driven by global warming and greenhouse gas emissions is a major global concern, particularly in Republic of Korea. Heat stress is a prominent issue in livestock farming, particularly in Holstein dairy cows, due to its negative impact on productivity. However, a limited number of studies have evaluated its effects in Republic of Korea. This study aimed to evaluate climate change trends in Republic of Korea and assess the impact of heat stress on milk production in Holstein cows using comprehensive large-scale data. The results showed that both milk production and lactation persistency sharply declined after surpassing their respective temperature-humidity index (THI) break points. Understanding the relationship between climate change, heat stress, and dairy cow productivity allows stakeholders to make informed decisions to ensure the sustainability and resilience of the livestock industry amid changing environmental conditions. ABSTRACT: This study investigated the effects of heat stress on milk production in Korean Holstein cows using large-scale data. Heat stress was assessed using the temperature-humidity index (THI). Weather records (2016 to 2020) were collected from 70 regional weather stations using an installed automated surface observing system (ASOS). A dataset of 2,094,436 milk production records from 215,276 Holstein cows obtained from the Dairy Cattle Genetic Improvement Center was analyzed. Stepwise selection was used to select the input variables, including the daily maximum THI (THI_max). Least-squares means were calculated for milk yield, fat and protein corrected milk (FPCM), fat and protein yield, fat-to-protein ratio, solids not fat, and lactation persistency. Segmented linear regression analysis determined the break points (BPs) of the THI_max. Over the five years, heat stress exposure increased, particularly from May to September. This study identified BPs around THI_max of 80–82 for milk yield and FPCM. Similar patterns for other milk traits were observed, which significantly decreased beyond their respective BPs. These findings indicate that THI variations adversely affect milk yield and composition in dairy cows, highlighting the importance of appropriate feeding management strategies to ensure the optimal productivity of Holstein cows under varying climatic conditions.