Consequences of Heat Stress on Physiology, Microbiome Dynamics, and Multi-Omics in Dairy Cows: More than Meets the Eye.

Heat stress (HS) is at the top of the challenges facing modern dairy production, with annual losses according to global projections, under high-emission scenarios, reaching US$14.7-40.0 billion by the end of the century. This review emphasizes three interconnected topics that account for most of the proportion of the productive and reproductive losses during HS. First, the physiological consequences of HS are reviewed, with emphasis on the pair-fed thermal neutral (PFTN) paradigm, which established that reduced dry matter intake (DMI) accounts for only 35-50% of the observed milk yield decline, with the remainder arising from tissue-level effects of hyperthermia on mammary function, metabolism, and reproductive performance. Second, HS-induced microbiome disruption is examined as an active pathophysiological amplifier, whereby rumen dysbiosis compromises intestinal barrier integrity and drives systemic endotoxaemia, chronically amplifying the immune suppression already imposed by the thermal insult. Third, we focus on the integration of multi-omics platforms as a management approach, since single-omics analyses capture only a fraction of the biological complexity underlying the HS response. As the available datasets expand in coverage and scale, their integration through AI-driven analytical frameworks has the potential to substantially advance beyond the current fragmented picture, progressively building toward a systems-level model of thermal stress. Evidence-based mitigation strategies spanning environmental cooling, targeted nutritional supplementation, and genomic selection are critically evaluated within this framework, with emphasis on equity of access to evidence-based solutions across global dairy production systems.