Rumen fermentation shifts and microbial dynamics in mid-lactating holstein dairy cows experiencing heat stress and subsequent recovery periods.

Objective: In this study, we investigated the effects of heat stress (HS) on rumen fermentation, blood parameters, and ruminal microbial communities in mid-lactating Holstein dairy cows in Korea.

Methods: Our study involved 12 mid-lactation Holstein dairy cows aged 55.54 months with 2.5 ± 0.65 parities and 100 to 200 days in milking (DIM), fed a total mixed ratio (TMR) diet. Samples were collected during HS (temperature-humidity index (THI) = 81.69) and recovery (RC) period (THI 69.84). The samples were analyzed for rumen fermentation, blood parameters, heat shock proteins, and microbial communities in dairy cows.

Results: The milk yield, milk fat, milk protein, and milk urea nitrogen (MUN) levels differed significantly between two -time points (p < 0.05). Rumen pH and volatile fatty acid (VFA) concentrations, the pH was not significantly different (p=0.619) between HS and RC periods; however, the ammonia nitrogen (NH3-N) levels increased during HS period ), however, there was no significant difference (p>0.05). Blood total protein significantly increased during HS period compared with that during RC period (p < 0.05), while no significant differences were observed in other parameters between the two periods. HSP27, HSP70, and HSP90 increased in dairy cows under HS conditions compared with those during the RC period. Taxonomic classification revealed that Firmicutes and Bacteroidetes dominated the bacterial community. PERMANOVA and PERMDISP showed significant differences in rumen bacterial diversity between HS and RC periods, based on Unifrac metrics (p=0.044 and p=0.015, respectively), indicating taxonomic variations. Microbial networks with correlations of > 0.8 (p < 0.05) showed a complex structure with equal positive and negative connections, indicating Anaerohabdus furcosa and Ruminiclostridium cellobioparum as key species during the HS and RC periods respectively.

Conclusion: Heat stress significantly impacts Holstein dairy cows' physiological and metabolic processes, altering rumen fermentation, blood biochemistry, and gut microbiota during mid-lactation.