Is ruminal fluid (digesta) an ideal solution?

I first discussed the significance of dissolved carbon dioxide (dCO₂) and CO₂ holdup in ruminant health and function in my original work (Laporte-Uribe, 2016). At that time, I used the findings of Moate et al. (1997) to illustrate the link between diet composition, ruminal dCO₂ concentrations, and the associated risk of bloat.

In this updated version, I focus on the physicochemical characteristics of ruminal fluid, particularly the relationship between dCO₂ and the ruminal partial pressure of carbon dioxide (pCO₂). According to Henry’s law, in ideal solutions the concentration of a dissolved gas is directly proportional to the partial pressure of that gas above the liquid. However, Moate et al. (1997) demonstrated a logarithmic relationship between ruminal pCO₂ (%) and dCO₂ (mM/min), showing that ruminal fluid behaves as a non-ideal solution. This means that conventional calculations based on Henry’s law do not accurately reflect true ruminal dCO₂ concentrations.

Modern feeding practices further exacerbate these non-ideal conditions, promoting CO₂ holdup and elevated dCO₂ levels. This, in turn, increases the risk of CO₂ poisoning (SARA) and other nutrition-related disorders.

By directly monitoring CO₂ holdup, we can shift from reactive to proactive rumen management — preventing disease, improving animal welfare, and optimizing ruminant health through precision control of ruminal fermentation.

Figure 1. Relationship between ruminal pCO₂ (%) and the rate of CO₂ entry (ROE; mM/min), an indirect method for estimating dissolved CO₂ (dCO₂). Data extrapolated from Moate et al. (1997); see Laporte-Uribe (2016) for details.