Ruminal dCO2 and the pH scale

Carbon dioxide (CO2) is an essential component for life on Earth, playing a crucial role in various biological processes.

While CO2 is commonly known as a gas, it also exhibits high solubility in water. In the rumen, the first compartment of the stomach in ruminants, CO2 exists in two liquid forms: dissolved carbon dioxide (dCO2) and bicarbonate (HCO3-). These two molecules make up the ruminal buffer system, which is responsible for maintaining the proper balance of the rumen environment (Laporte-Uribe, 2016).

Traditionally, the ruminal pH scale has been suggested to control rumen function, with low pH leading to ruminal acidosis (Macmillan et al., 2017). However, it is dCO2 that drives optimal fermentation and pathological CO2 accumulation which in turn leads to ruminal acidosis, a conditions known as CO2 holdup (Laporte-Uribe, 2023). The ruminal pH scale however fail to recognize CO2 holdup, because it cannot measure individual components concentrations is a quotient.

Nevertheless, understanding dCO2 dynamics is essential for effectively managing ruminal health in livestock farming practices. Therefore, monitoring ruminal dCO2 is pivotal in driving optimal fermentation and preventing the onset of ruminal acidosis. By shifting the focus from pH to dCO2 monitoring, farmers can implement effective strategies to support the overall health and productivity of their livestock and reduce methane emissions.

The Interconnection between CO2 species equilibrium and Ruminal pH Scale

According to the Arrhenius theory (Arrhenius, 1887), water can undergo self-ionization, resulting in the formation of equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-) in a balanced state. This equilibrium is fundamental to determining the ruminal pH scale (Henderson-Hasselbalch, 1908-1917).

The Bjerrum plot is a graphical representation that aids in understanding the equilibrium between different CO2 species in aqueous solutions in relation to the pH scale. In the rumen, the Bjerrum plot illustrates the interrelationships among dissolved carbon dioxide (dCO2) and bicarbonate ions (HCO3-), the main acid and base. The plot represents the shifting balance (proportion) between these species which are reflecting on the change in pH, an spurious relationship.

As the concentrations of ruminal dCO2 increases the pH of the solution decline, because dCO2 drives water ionization. Similar effect can be seeing if a glass of pure water is left unattended in a bench, the pH of the water will go from a equilibrium at 7 to values closer to 6.4 corresponding to the CO2 gas concentration in the atmosphere (pKa 6.4, NTP). Moreover, within the ruminal pH scale, between 5.5 and 7, ruminal dCO2 is the main acid. The pKa or equilibrium for VFAs and Lactate is below 5, meaning that are mainly found as bases, their concentration is stable to around 100 to 150 mM, in fact the main changes that we see is in the proportion of each VFAs rather than their main concentration.

Ruminal dCO2 comes from fermentation, VFA absorption and exchange with the ruminal epithelium, saliva and the diet. Ruminal dCO2 concentrations are far from constant ranging from 0 to 130 mM, this range and variation makes ruminal dCO2 the main acid and the driver for water dissociation (ruminal pH fluctuations). Therefore, it is ruminal dCO2 accumulation that leads to ruminal acidosis and other nutritional related diseases (Laporte-Uribe, 2016) and not the concomitant pH decline, a spurious relationship.

The pH scale cannot measure concentrations

The Henderson-Hasselbalch equation (HH, 1917) suggests that:

pH = pKa’ + Log [Base/Acid]

Hence, the ruminal pH scale cannot measure concentrations, only the equilibrium between bases and acids (quotient).

For instance, two solutions of bicarbonate/dCO2, Solution A with 100/100 mM and Solution B with 10/10 mM will have similar pH. Solution A with 100mM of dCO2 can have potentially fatal effects for ruminants.

The only way to reduce the risk of CO2 holdup reducing the risk of many endemic diseases in ruminants is to monitor directly ruminal dCO2.

Our patented technology is the only way to achieve continuous ruminal dissolved carbon dioxide monitoring