An update on heat stress impacts on rumen microbiome composition, fermentation parameters, and mitigation approaches in ruminants.

Over the past decade, there has been an increasing amount of literature on heat stress (HS) effects on ruminant production systems, yet much uncertainty still exists about the relationship between HS and rumen microbial ecology across species. This narrative review synthesizes peer-reviewed evidence from 2020 to 2025, with the aim of providing a critical appraisal of HS effects on rumen microbiome composition and fermentation parameters in cattle, buffalo, sheep, and goats. A convergent pattern emerges from the available data: HS consistently reduces cellulolytic bacteria (Fibrobacter, Ruminococcus) while increasing lactate-producing and starch-fermenting taxa. Acetate proportions declined by 29-33% in buffalo and beef cattle, whereas total VFA in sheep increased during mild HS, reflecting species-dependent fermentation responses. A strong relationship between breed-level heat tolerance and rumen microbiome diversity has been reported in several recent studies, providing converging evidence that heat tolerance may be characterized as a holobiont phenotype. It has been demonstrated that HS extends beyond classical VFA changes to disrupt B-vitamin synthesis, amino acid metabolism, biogenic amine homeostasis, and bile acid biotransformation. Nutritional interventions, including probiotics, chromium, herbal supplements, and rumen-protected amino acids, have been shown to partially restore microbial balance, although responses vary with HS severity and host genotype. Notwithstanding these findings, the generalisability of much published research on this topic is limited by methodological heterogeneity across studies. Taken together, these findings highlight the need for standardized experimental protocols, multi-omics integration, and microbiome-targeted intervention strategies.