Rumen CO(2) species equilibrium might influence performance and be a factor in the pathogenesis of subacute ruminal acidosis

This experiment was conducted to explore rumen carbon dioxide (CO(2)) species equilibrium. Three lactating, fistulated cattle were consecutively exposed to three dietary treatments tailored to produce low rumen pH and increase the risk of subacute ruminal acidosis (SARA) by reducing physically effective neutral detergent fiber (Low (pe)NDF), increasing rumen degradable starch (High RDS) or both (Combined). Under these conditions, high and varied rumen concentrations of the CO(2) associated to water or dissolved CO(2) (dCO(2)) were found. The results suggest that the activity of dCO(2) and bicarbonate (HCO(3)(−)) represents an important component of the rumen environment. Rumen CO(2) holdup was associated with high dCO(2) and HCO(3)(−) activity as well as changes in the viscosity and surface tension of the rumen fluid. All dietary treatments produced low rumen pH, <5.5 for >3 h/d, a condition associated with SARA, but clinical SARA was observed only during CO(2) holdup. This pilot study highlights the possible role of CO(2) holdup and rumen CO(2) species in cattle performance and nutritional diseases. In the future, better estimations of CO(2) species might help clarify these findings.