Why Ruminal dCO₂, Not pH, Defines Acid–Base Balance

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

Although CO₂ is commonly recognized as a gas, it is also highly soluble in water. In the rumen — the first compartment of the ruminant stomach — CO₂ exists primarily in two liquid forms: dissolved carbon dioxide (dCO₂) and bicarbonate (HCO₃⁻). Together, these molecules constitute the ruminal buffer system, which is critical for maintaining environmental balance in the rumen (Laporte-Uribe, 2016).

Traditionally, ruminal function has been assessed using the pH scale, with low pH considered indicative of ruminal acidosis (Macmillan et al., 2017). However, it is dCO₂ that drives fermentation dynamics, and pathological accumulation of dCO₂ — a condition known as CO₂ holdup — that leads to ruminal acidosis (Laporte-Uribe, 2024). The pH scale, being a quotient of acid–base equilibrium, cannot detect CO₂ holdup because it does not measure the absolute concentrations of individual components.

Understanding dCO₂ dynamics is therefore essential for effective rumen health management. Direct monitoring of ruminal dCO₂ provides a more accurate picture of fermentation status, enabling proactive prevention of acidosis. By shifting the focus from pH to dCO₂, farmers can implement strategies that safeguard animal health, improve productivity, and reduce methane emissions.

The Interconnection between CO2 species equilibrium and Ruminal pH Scale

According to Arrhenius theory (Arrhenius, 1887), water undergoes self-ionization, producing equal concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻) at equilibrium. This balance underpins the pH scale as later formalized by Henderson and Hasselbalch (1908–1917).

The Bjerrum plot is a graphical tool that illustrates the equilibrium between different CO₂ species in aqueous solutions relative to pH. In the rumen, it shows the interrelationships between dissolved carbon dioxide (dCO₂) and bicarbonate ions (HCO₃⁻) — the primary acid and base pair. The plot depicts how the proportion of these species shifts with pH, but this relationship is spurious, as pH reflects equilibrium ratios rather than absolute concentrations.

As ruminal dCO₂ concentrations rise, pH declines because dCO₂ drives water ionization. A similar effect can be observed if a glass of pure water is left exposed: its pH drops from ~7.0 to ~6.4 as it equilibrates with atmospheric CO₂ (pKₐ ≈ 6.4, NTP). Within the ruminal pH range of 5.5–7.0, dCO₂ is the dominant acid. By contrast, volatile fatty acids (VFAs) and lactate, with pKₐ values below 5, exist predominantly in their dissociated (base) forms. Their total concentration remains relatively stable (≈100–150 mM); what fluctuates is the relative proportion of individual VFAs, not their overall abundance.

Ruminal dCO₂ derives from microbial fermentation, VFA absorption, epithelial exchange, saliva, and diet. Its concentration is highly variable, ranging from 0 to 130 mM. This dynamic variation makes dCO₂ the primary acid in the rumen and the main driver of water dissociation (pH fluctuations). Consequently, it is the accumulation of ruminal dCO₂ — not the associated pH decline — that leads to ruminal acidosis and other nutrition-related diseases (Laporte-Uribe, 2016).

The pH scale cannot measure concentrations

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

pH = pKa’ + Log [Base/Acid]

Hence, the ruminal pH scale does not measure absolute concentrations, but only the equilibrium ratio between bases and acids.

For example, two bicarbonate/dissolved CO₂ solutions — Solution A at 100/100 mM and Solution B at 10/10 mM — will exhibit a similar pH. However, Solution A, with 100 mM of dissolved CO₂, can have potentially fatal effects for ruminants.

The only way to reduce the risk of CO₂ holdup — and thereby lower the incidence of many endemic ruminant diseases — is to directly monitor ruminal dissolved CO₂.

Our patented technology is the first and only solution that enables continuous monitoring of dissolved CO₂ in the rumen.