Seasonal heat stress alters rumen fermentation, microbial community, and microbial amino acid composition in cattle

In this study, we investigated the effects of seasonal changes on ruminal temperature, pH, fermentation characteristics, microbial composition, and microbial amino acids (AAs) in cattle. The experiment was conducted using six fistulated Holstein steers in two separate 9-d trials conducted in spring and summer (3 d adaptation and 6 d sampling). The average ambient temperature and relative humidity were 10.7 °C and 58% in spring and 24.5 °C and 83.5% in summer, respectively. Ruminal temperature was approximately 0.9 °C higher in summer than in spring, whereas ruminal pH was 0.15 units lower in summer. Among rumen fermentation parameters, butyrate concentration increased during summer, while the concentrations of acetate, propionate, branched-chain fatty acids, total volatile fatty acids, and NH₃-N decreased (p < 0.05). Despite identical feed intake, ruminal degradation rates increased after 24 h in summer, with significant increases in the degradation of dry matter, crude protein, and neutral detergent fiber (p < 0.05). Microbial community analysis revealed that at the phylum level, Spirochaetota decreased, whereas Planctomycetota and Chloroflexi increased during summer (p < 0.05). At the genus level, Succiniclasticum and Treponema decreased, while Eubacterium coprostanoligenes and Oscillospiraceae increased compared with spring (p < 0.05). Changes in microbial composition were accompanied by alterations in microbial AA profiles, characterized by increases in alanine, glutamate, histidine, and non-essential AAs, and decreases in glycine, serine, methionine, threonine, and essential AAs during summer (p < 0.05). Notably, methionine content decreased by 13% in summer relative to spring. Overall, these findings demonstrate that seasonal thermal conditions directly alter rumen fermentation, microbial ecology, and microbial AA composition, even under identical feeding conditions. This study provides foundational evidence for developing nutritional and management strategies to optimize rumen function under heat stress conditions.