Altered rumen microbiome and correlations of the metabolome in heat-stressed dairy cows at different growth stages

Heat stress is one of the major stressors affecting dairy cow production, which causes great losses in animal husbandry production and economic development. This study combined 16S rDNA sequencing, metagenomic sequencing, and metabolomic analysis to investigate the effects of heat stress on the rumen microbiome and metabolism of cows. To achieve this, 10 cows each of growing heifers, heifers, and lactating cows were selected for sample collection in April and August (a total of 60 cows). Ruminal fluid was collected, filtered through gauze, and immediately transferred to liquid nitrogen prior to metagenomic, 16S rDNA sequencing, and metabolomic analyses. Heat stress increased the abundance of pathogenic bacteria, such as Treponema_2, Sphingobacterium, and Streptococcus, in cattle at all growth stages, with the structure and composition of rumen microbial communities being the most affected by heat stress in growing heifers and the least affected by heat stress in lactating cows. Heat stress led to a significant decrease in carbohydrate metabolic function in growing heifers and heifers, whereas microbial function was the most stable in lactating cows, suggesting that changes in microbial function are not only more conserved than microbial composition but are also affected by age. Heat stress led to significant enrichment of fatty acyl metabolites in growing heifers, heifers, and lactating cows, and the heat stress indicator species Bacillus, Lactococcus, Streptococcus, and Enterococcus were significantly associated with fatty acyl compounds in growing heifers and heifers. Therefore, heat stress alters the metabolic status of cows by modulating fatty acid biosynthesis, the PPAR signaling pathway, and arachidonic acid metabolism through the regulation of long-chain fatty acids and other biomarkers in cows at three growth stages.IMPORTANCEHeat stress is one of the main causes of economic losses in the dairy industry worldwide; however, the mechanisms associated with the metabolic and microbial changes in heat stress remain unclear. Here, we characterized both the changes in metabolites, rumen microbial communities, and their functional potential indices derived from rumen fluid and serum samples from cows at different growth stages and under different climates. This study highlights that the rumen microbe may be involved in the regulation of lipid metabolism by modulating the fatty acyl metabolites. Under heat stress, the changes in the metabolic status of growing heifers, heifers, and lactating cows were closely related to arachidonic acid metabolism, fatty acid biosynthesis, and energy metabolism. Moreover, this study provides new markers for further research to understand the effects of heat stress on the physiological metabolism of Holstein cows and the time-dependent changes associated with growth stages. Heat stress is one of the main causes of economic losses in the dairy industry worldwide; however, the mechanisms associated with the metabolic and microbial changes in heat stress remain unclear. Here, we characterized both the changes in metabolites, rumen microbial communities, and their functional potential indices derived from rumen fluid and serum samples from cows at different growth stages and under different climates. This study highlights that the rumen microbe may be involved in the regulation of lipid metabolism by modulating the fatty acyl metabolites. Under heat stress, the changes in the metabolic status of growing heifers, heifers, and lactating cows were closely related to arachidonic acid metabolism, fatty acid biosynthesis, and energy metabolism. Moreover, this study provides new markers for further research to understand the effects of heat stress on the physiological metabolism of Holstein cows and the time-dependent changes associated with growth stages.