What is Ruminal CO₂ Holdup?

The simplest way to visualize CO₂ holdup is to observe a glass of beer. Like ruminal fluid, beer is a non-ideal solution. Dissolved CO₂ escapes slowly, forming small columns of bubbles rising from the bottom to the top of the glass. By contrast, in an ideal solution such as carbonated water, the CO₂ bubbles are larger and released much faster.

In ideal fluids, dCO₂ concentrations quickly equilibrate with the CO₂ partial pressure above the liquid — as predicted by Henry’s law. In non-ideal solutions, however, dCO₂ does not escape easily, so concentrations remain higher in the liquid phase, and equilibrium with the gas cap occurs much more slowly.

The physicochemical properties of a liquid can alter this behavior. For example, adding a small amount of soap to carbonated water changes its surface tension: gently stirring the mixture produces smaller bubbles that rise more slowly. This is analogous to how diet composition affects ruminal fluid, either facilitating CO₂ release or trapping it (CO₂ holdup).

Once CO₂ holdup develops, large amounts of dCO₂ can accumulate in the ruminal fluid, potentially leading to CO₂ poisoning during subacute ruminal acidosis (SARA). Symptoms include dizziness, rapid breathing, elevated heart rate, confusion, apathy, and recumbency.

Importantly, any factor that suddenly alters ruminal physicochemical properties can trigger rapid release of trapped dCO₂, producing frothy foam at the surface. This mechanism underlies bloat and abomasal displacement.

This phenomenon can be simulated in a benchtop experiment:

• Add table salt or baking soda to three glasses (beer, carbonated water, and carbonated water with soap), stirring gently.

• The salts promote nucleation, enhancing CO₂ bubble formation and release.

• Beer and carbonated water with soap (non-ideal solutions) will produce stable foam.

• Carbonated water (ideal solution) will fizzle without forming persistent foam.

• Shaking the glasses produces the same effect through physical agitation.

There is much more to ruminal dCO₂ — but one key message is clear: if we can measure it, we can prevent many nutritional diseases in ruminants.

Figure 1. As the effervescence of CO2 is disrupted, dCO2 will increase leading to the nutritional and physiological changes that we observed during ruminal acidosis.