LAUSR

Abstract This experiment was conducted to explore rumen carbon dioxide (CO2) 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 peNDF), increasing rumen degradable starch (High RDS) or both (Combined). Under these conditions, high and varied rumen concentrations of the CO2 associated to water or dissolved CO2 (dCO2) were found. The results suggest that the activity of dCO2 and bicarbonate (HCO3−) represents an important component of the rumen environment. Rumen CO2 holdup was associated with high dCO2 and HCO3− 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 CO2 holdup. This pilot study highlights the possible role of CO2 holdup and rumen CO2 species in cattle performance and nutritional diseases. In the future, better estimations of CO2 species might help clarify these findings.