Changes in ruminal lipopolysaccharide, fermentation characteristics, nutrient digestibility, blood variables, and microbiota in sheep with subacute ruminal acidosis and its regulation by polymyxin CP7.

The objectives of this study were to (1) determine the effects of induced subacute ruminal acidosis (SARA) on ruminal lipopolysaccharide (LPS), nutrient apparent digestibility, ruminal fermentation parameters, hematological indices, and ruminal microbiota; and (2) evaluate the responses associated with polymyxin CP7 supplementation during SARA in Hu sheep. Eight rumen-fistulated Hu sheep sequentially underwent three 29-day stages: a control stage (CON, basal diet), a SARA induction stage (SAR, high-barley diet), and a bacteriocin-supplemented stage (BAC, high-barley diet with 500 mg/kg polymyxin CP7). The sequential design was adopted because SARA is not immediately reversible, rendering crossover designs problematic due to potential carry-over effects. Rumen pH remained below 5.60 for 315 min/d during the SAR stage, fulfilling the diagnostic criteria for SARA. Ruminal LPS concentration in the SAR stage was higher than that in the CON and BAC stages (P < 0.05). The apparent digestibility of organic matter and neutral detergent fiber in the BAC stage was higher than that in the CON and SAR stages, whereas dry matter intake was lower than that in the CON stage (P < 0.05). The propionate proportion was higher in the BAC stage than in the CON and SAR stages (P = 0.005). The butyrate proportion and lactic acid concentration in the SAR stage were higher than that in the other two stages (P < 0.05). Red blood cell hemoglobin and mean red blood cell volume in the SAR stage were lower than those in the CON and BAC stages (P < 0.05), whereas mean red blood cell hemoglobin concentration, platelet, and platelet hematocrit were higher in the SAR stage than in the other two stages (P < 0.01). Additionally, the monocyte proportion in the BAC stage was higher than that in the CON and SAR stages (P < 0.01). The relative abundance of the genus Moryella in the SAR stage was higher than that in the CON and BAC stages (P < 0.05), whereas the relative abundance of the genus Treponema in the BAC stage was lower than that in the CON and SAR stages (P < 0.05). In conclusion, although ruminal acidosis persisted during the BAC stage, lower ruminal LPS concentration, higher nutrient digestibility, and alterations in ruminal fermentation and microbial composition were observed during this stage. These findings suggest that polymyxin CP7 may mitigate selected SARA-associated responses.