Ruminal CO2 Holdup Monitoring, Acidosis Might Be Caused by CO2 Poisoning

Abstract

The ruminal buffering system is composed of bicarbonate (HCO3-) and dissolved CO2 (dCO2). While low pH indicates high dCO2 formation, the pH scale is a ratio between acids and bases in a solution, i.e. HCO3- and dCO2, and fail to provide individual component concentrations. For instance, modern feeding practices can reduce CO2 gas fugacity from the ruminal fluid or "CO2 holdup". Under those conditions, not only dCO2 can reach critical concentrations, but the buffering system might favour HCO3- formation resulting in normal ruminal pH values, the quotient, regardless of the harmful dCO2 accumulation. Consequently, subacute ruminal acidosis (SARA), traditionally associated with low or variable pH, might be triggered by CO2 holdup. This observational study aimed to continuously monitor ruminal dCO2 and characterised CO2 holdup within the ruminal fluid targeting the specific infrared signal of dCO2 with an attenuated total reflectance infrared (ATR-IR) spectrometer. Three lactating dairy cattle were longitudinally exposed to diets designed to elevate both ruminal dCO2 and SARA risk. Indwelling pH sensors and ruminal fluid samples served as references for dCO2 analysis, while a categorical analysis detected CO2 holdup from the output of the ATR-IR sensor. Milk yield, milk components, and feed intake supported the known positive role for high dCO2 in rumen function. However, SARA was associated with ruminal CO2 holdup, suggesting that prolonged exposure to critical dCO2 concentrations during extended postprandial periods might trigger SARA. Continuous dCO2 monitoring with the proposed methodology and analysis may offer a valuable tool for optimising rumen function and prevent SARA risk.