Metabolic and Metataxonomic Changes in Lactating Holstein Dairy Cows During the Transition from Heat Stress to the Recovery Period

This study investigated the changes in rumen fermentation characteristics, blood parameters, and rumen microbial communities of Holstein dairy cows in the early lactation stage during heat stress conditions and subsequent recovery. This study aimed to fill the significant knowledge gaps regarding the recovery of dairy cattle from heat stress during the early stages of lactation. Metataxonomic analysis was used to identify potential biomarkers and metabolites associated with metabolic disease prediction. The temperature-humidity index was recorded on a dairy farm to define the heat stress and recovery periods.  Using the Bray-Curtis dissimilarity index, principal coordinate analysis revealed that both the heat stress and recovery periods affected the overall composition of the rumen bacterial community. The first three principal coordinates explained 33.00%, 16.60%, and 11.60% of the total variation, indicating the significant (p &lt; 0.01) influence of temperature changes on the dominance of rumen microbes and the rumen environment. However, alpha diversity measurements were unaffected in either period. Metataxonomic analysis (average relative abundance 2%) of cows in both periods revealed ten predominant genera: <italic>Prevotella</italic>, <italic>Ruminococcus</italic>, <italic>Selenomonas</italic>, <italic>Gilliamella</italic>, <italic>Duncaniella</italic>, <italic>Succiniclasticum</italic>, <italic>Paraprevotella</italic>, <italic>Bacteriodes</italic>, <italic>Lentimicrobium</italic>, and <italic>Treponema</italic>. During heat stress, significant alterations were observed in the levels of three organic acids, six fatty acids, and thirteen amino acids. Furthermore, heat stress caused a significant increase in blood serum HSP27 and HSP70 levels (both p &lt; 0.01), whereas blood serum glucose (p = 0.001) and blood urea nitrogen (p &lt; 0.001) decreased. Heat stress increased blood serum ketone concentrations, decreased cholesterol and blood urea nitrogen concentrations, and altered total protein, aspartate aminotransferase, and total bilirubin concentrations. The levels of blood serum minerals, such as calcium, phosphorus, and magnesium, as well as ruminal pH, ammonia-N, acetate, and butyrate, were not affected during either period. Heat stress influenced propionate (p=0.006) and total volatile fatty acids (p=0.030). Overall, heat stress during early lactation resulted in significant shifts within the rumen bacterial community structure, accompanied by corresponding changes in blood metabolite profiles.