Rumen microbiota of heat-stressed Holstein cows supplemented with potassium bicarbonate
Heat stress severely impairs the health of lactating dairy cows and degrades the nutritional density of milk, particularly milk protein content, posing a significant threat to the global supply of high-quality dairy products. While dietary potassium bicarbonate (KHCO3) supplementation is commonly employed to mitigate these negative effects and maintain osmotic balance, the production responses among individual cows exhibit considerable heterogeneity. The primary goal of this study was to elucidate the ruminal microbial mechanisms driving these divergent phenotypic responses. We conducted an intervention trial where mid-lactation Holstein cows under continuous environmental heat stress were supplemented with KHCO3. Based on the macroscopic recovery of milk protein content and apparent crude protein digestibility, the subjects were retrospectively categorized into positive responders (PR) and limited responders (LR). We utilized 16S rRNA gene amplicon sequencing (V4 region) of rumen fluid to characterize the structural shifts and functional predictions of the rumen microbiota. Our investigation reveals that the rescue of milk protein is fundamentally driven by microbial "potassium sensitivity". The responsive (PR) microbiome specifically enriches taxa (such as Succiniclasticum and Lachnobacterium) with an enhanced capacity for K+ uptake, enabling these active consortia to utilize supplemented K+ to alleviate cellular osmotic stress. This microbial adaptation drives a profound metabolic reprogramming, characterized by a shift toward highly efficient propionate-dominant fermentation, enhanced carbohydrate degradation, and upregulated de novo amino acid synthesis that significantly reduces ruminal nitrogen loss. The relevance of this research lies in bridging the gap between mineral nutrition, rumen microecology, and the systemic rumen-mammary axis under thermal load. By characterizing how distinct microbial architectures modulate post-ingestive carbon and nitrogen turnover, this study provides a mechanistic foundation for developing precision, microbiome-targeted interventions. Ultimately, these insights offer novel biotechnological approaches to improving animal heat resilience and securing the processing suitability and nutritional consistency of raw milk in the face of escalating global warming.