Alterations in Blood Markers, Rumen Fermentation, and Microbiota Due to Heat Stress in Holstein Cows During Dry and Early Lactation

In this study, to evaluate physiological and microbial responses under heat stress conditions, blood biochemical parameters, rumen fermentation characteristics, and microbial composition in Holstein dairy cows during the dry period and early lactation stages under summer conditions in Korea. Fourteen cows were observed during the hot summer month (from the first to the third week of August), with the temperature-humidity index (THI) recorded in the barns during the dry and early lactation periods being 80.80 and 81.66, respectively. Blood and rumen fluid samples were collected to evaluate physiological responses and changes in blood parameters, rumen fermentation, and microbial composition. Blood analysis revealed significant variations between the two stages. Early-lactating cows exhibited lower glucose, blood urea nitrogen, and cholesterol levels but higher ketone and aspartate aminotransferase levels, indicating increased energy demands and protein metabolism. A complete blood count showed reduced red blood cell count, hematocrit, and hemoglobin levels during the early lactation period, whereas white blood cell counts increased. The levels of heat shock proteins (HSPs), such as HSP27, HSP70, and HSP90, also differed significantly. Rumen fermentation analysis revealed lower ammonia nitrogen concentrations but significantly higher propionate and total vol-atile fatty acid concentrations during the early lactation period, indicating adaptive changes in rumen function. Microbial analysis revealed significant differences in bacterial diversity and composition. Early-lactation cows exhibited relatively high abundances of Bacteroidota and Prevotella, whereas the dry period was dominated by Clostridia and Eubacteriales. Network analysis highlighted shifts in microbial interactions, with specific keystone species identified at each stage. These findings suggest distinct physiological and microbiological adaptations in response to HS, with early lactation characterized by heightened metabolic demands and significant shifts in rumen bacterial communities. Such insights could inform tailored management strategies to mitigate the impact of HS on dairy cows during their critical production stages.