VFAs? No — the primary ruminal acid is dCO₂.

Volatile fatty acids (VFAs) in aqueous environments exist in equilibrium between their acid and base forms, for example, acetic acid and acetate (Arrhenius theory). The Henderson–Hasselbalch equation allows us to describe the dissociation state and concentration of each conjugate pair, provided we know the total concentration, the dissociation constant (pKₐ), and the ruminal pH.

Literature examples from different diets under normal and SARA conditions show that VFAs in ruminal fluid are present predominantly as bases. By contrast, the higher pKₐ of CO₂ species means that dissolved CO₂ (dCO₂) is the predominant acid in the rumen (Laporte-Uribe, 2024).

Table 1. Standard equilibrium constant for various molecules of significant importance in relation to the proper functioning and overall efficiency of the rumen system.

The decline in ruminal pH during fermentation reflects the dissociation of water driven by acid formation, as described by Arrhenius theory. This phenomenon can be simulated by calculating the increase in acid concentrations at ruminal pH values typically observed during subacute ruminal acidosis (SARA).

Please note which “acid” rises to levels that may exert significant physiological effects.

On average, the concentration of dissolved CO₂ (dCO₂) in the rumen is ~60 mM at standard temperature and pressure, at a pH of 6.1 (Kohn & Dunlap, 1998). By contrast, the total concentration of acetate/acetic acid is typically ~50 mM, of which only ~7 mM is present as acetic acid, with the remainder existing as acetate. Thus, the largest concentration of acid in the rumen is dCO₂.

In vitro studies have shown that VFAs are toxic to the rumen epithelium (Gabel et al., 1991). However, in vivo, it is sustained ruminal dCO₂ accumulation — exceeding 80 mM for extended postprandial periods (CO₂ holdup) — that poses the greater risk, potentially leading to CO₂ poisoning (Laporte-Uribe, 2024).

Figure 1: Proportion of acid/base ratios for each Lewis pair of molecules that hold significant importance in the composition and functionality of the ruminal fluid at rumen pH thought to be the threshold for ruminal acidosis (pH 5.5).

Below is a detailed example from the scientific literature illustrating the average total concentrations of various molecules and their fluctuations at different ruminal pH levels, each governed by its specific dissociation constant (pKₐ). This simulation was performed using the Henderson–Hasselbalch equation, which describes the relationship between pH and the ionization state of weak acids and bases.

Figure 2. Model of concentrations of VFAs, ammonia/ammonium and bicarbonate/dissolved CO2 using concentrations described in the literature. The ruminal pH scale described the equilibrium between Lewis pairs as described by the Henderson-Hasselbalch equation.

The clear conclusion from this model is that the primary acid in the rumen is dissolved CO₂ (dCO₂), particularly when pH drops below the equilibrium threshold, with a ruminal pKₐ of 6.1 (Hille et al., 2017). By contrast, volatile fatty acids (VFAs) exist predominantly in their dissociated form and are therefore present mainly as bases, not acids.