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Ruminal CO2 Holdup: A New Perspective on Rumen Function

Dr. José A. Laporte Uribe, DVM, PhD Animal Science

My research on the cattle rumen in New Zealand pastures led me to a significant discovery that has changed my understanding of rumen function (Laporte-Uribe and Gibbs, 2009).

I was aiming to determine how quickly an indwelling pH sensor could respond to changes in SCFA concentrations within the rumen. The prevailing belief was that SCFA production directly controlled ruminal pH decline (Dijkstra et al., 1993). This implied that monitoring SCFA formation during fermentation could indicate the accuracy of pH sensor and the suitability of pH as a fermentation indicator.

To accelerate fermentation, I added liquid molasses directly to the rumen content of a dry, fistulated cow, while monitoring pH in the ventral sac using a real-time interface. The results were surprising! Ruminal pH dropped rapidly, by nearly 1 unit (from 6.8 to 5.8) within seconds. This swift decline could not be attributed to SCFA production, which might require at least 15 minutes for significant bacterial generation. Not only pH was ineffective to monitor fermentation, it cannot follow SCFA formation, but something else was changing the pH! I did not know at that time what was responsible for this effect.

The overlooked role of dissolved CO2 (dCO2)

The answer came by changing my focus on a forgotten component of the ruminal buffering system, dissolved CO2, the primary acid in the rumen's buffering system (Turner and Hodgetts, 1955; Laporte-Uribe, 2016), Figure 1. The addition of molasses altered the rumen fluid's physicochemical properties, increasing its viscosity and hindering dCO2 release (effervescence). This phenomenon is known as CO2 holdup. As dCO2 formation rises and bicarbonate declines, ruminal pH falls, the Henderson-Hasselbalch equation (Table 1).

Figure 1. The ruminal buffering system: Protons (H3O+) are neutralized in the rumen by bicarbonate (HCO3-). This reaction is facilitated by ruminal carbonic anhydrase (enzyme). The process results in the formation of dissolved carbon dioxide (dCO2). As water (H2O) is released and CO2 gas escapes to the gas cap, the system's buffering capacity is maintained. Remarkably, ruminal dCO2 is the key component and its concentrations in the fluid promote optimal fermentation. This principle is similar to the fizz you see in carbonated beverages like beer or soda.

CO2 Holdup: A Dynamic Process

Ruminal CO2 holdup is a natural consequence of the constantly changing physicochemical properties of the rumen fluid. These properties are influenced by feeding, water intake, metabolic activity, and dietary composition, rendering the rumen fluid a non-ideal solution (Laporte-Uribe, 2016, 2019).

An analogy can be drawn to carbonated beverages. In a glass of carbonated water, large bubbles readily form and escape, whereas in a glass of beer, smaller bubbles are less readily released. This difference arises from the beer's higher density (non-ideal solution), which hinders bubble formation and leads to a higher concentration of dCO2 compared to the water (ideal solution). Upon exposure to air, the carbonated water loses its fizz quicker than the beer. Similarly, the rumen fluid, particularly when cattle are fed feeds containing fine particles and highly fermentable products like corn, behaves more like the beer and the dCO2 generated during fermentation or as product of rumen buffering cannot escape easily and it is retained.

Benefits and Drawbacks of CO2 Holdup

This phenomenon offers a crucial benefit. Increased dCO2 availability provides beneficial bacteria with more substrate to produce lactate and succinate, ultimately leading to higher propionate production – the primary energy source for cattle milk yield (Aschenbach et al., 2010). Additionally, this favourable environment for propionate-producing bacteria leads to higher hydrogen retention, outcompeting methanogens and reducing methane emissions (Russell, 1998), Figure 1.

However, CO2 holdup also has drawbacks. High dCO2 concentrations can precipitate ruminal acidosis, a prevalent disease in ruminants (Laporte Uribe, 2023). Moreover, numerous conditions associated with ruminal acidosis appear to be linked to CO2 holdup or the prolonged presence of critical dCO2 concentrations (Figure 2).

Figure 2. The negative consequence of CO2 holdup is the disruption of the rumen buffering system leading to dissolved CO2 (dCO2) accumulation, prevent or enhance bicarbonate (HCO3-) formation creating an acidic environment, Ruminal acidosis. The osmolarity of the ruminal fluid increases and disrupts the normal absorption of nutrients, including water (H2O), sodium (Na+), and short-chain fatty acids (SCFAs). Bacteria alter their metabolism and produce lipopolysaccharides (LPS) and lactate. These changes are all signs of ruminal acidosis.

Ruminal pH monitoring cannot detect CO2 holdup.

The importance of monitoring dissolved carbon dioxide (dCO2) in the rumen requires a better approach than only monitoring ruminal pH. While rumen pH reflects the balance between acids and bases, it only provides a relative value, not the total dCO2 concentration. As we observed in Figure 2, diets high in dCO2 can also be high in bicarbonate, which can mask CO2 Holdup formation (Table 1).

Table 1. The ruminal pH scale cannot determine the concentrations of individual components. The pH scale reflects the “quotient” between acids and bases, not absolute concentrations. The Henderson-Hasselbalch equation demonstrates this concept. Two solutions can have similar pH values despite different concentrations of specific components.

Developing a Tool for improved rumen monitoring.

With this in mind, we are developing a patented rumen bolus (Laporte Uribe, 2024) that can be dosed in cattle to continuously measure ruminal dCO2 concentrations. This data will be used for real-time monitoring of ruminal conditions and will enable us to optimize diets, feeding practices, and feed ingredients. Additionally, farmers will receive real-time feedback on how their management strategies impact their cattle via our Artificial intelligence algorithms.

This technology has the potential to reduce diseases associated with acidosis, enhance productivity, lower methane emissions, and ultimately ensure optimal growth conditions for cattle.


Aschenbach, J. R., N. B. Kristensen, S. S. Donkin, H. M. Hammon, and G. B. Penner. 2010. Gluconeogenesis in dairy cows: the secret of making sweet milk from sour dough. IUBMB Life 62(12):869-877. doi: 10.1002/iub.400

Dijkstra, J., H. Boer, J. Van Bruchem, M. Bruining, and S. Tamminga. 1993. Absorption of volatile fatty acids from the rumen of lactating dairy cows as influenced by volatile fatty acid concentration, pH and rumen liquid volume. Br. J. Nutr. 69(02):385-396.

Laporte-Uribe, J., and J. Gibbs. 2009. Rumen pH and function in dairy cows of the South Island of New Zealand. In: Proceedings of the XIth International Symposium on Ruminant Physiology. Ruminant Physiology: Digestion, Metabolism and Effects of Nutrition on Reproduction and Welfare., Clermont-Ferrand, France. p 250-251.

Laporte-Uribe, J. A. 2016. The role of dissolved carbon dioxide in both the decline in rumen pH and nutritional diseases in ruminants. Anim. Feed Sci. Technol. 219:268-279. doi: 10.1016/j.anifeedsci.2016.06.026

Laporte-Uribe, J. A. 2019. Rumen CO2 species equilibrium might influence performance and be a factor in the pathogenesis of subacute ruminal acidosis. Translational Animal Science 3(4):1081-1098. doi: 10.1093/tas/txz144

Laporte Uribe, J. 2023. CO2 holdup monitoring, ruminal acidosis might be caused by CO2 poisoning. doi:

Laporte Uribe, J. 2024. Apparatus for monitoring nutrition, especially fermentation in the rumen of a ruminant No. US11857347, US.

Russell, J. B. 1998. The Importance of pH in the Regulation of Ruminal Acetate to Propionate Ratio and Methane Production In Vitro. J. Dairy Sci. 81(12):3222-3230. doi:

Turner, A. W., and V. E. Hodgetts. 1955. Buffer systems in the rumen of sheep. I. pH and bicarbonate concentration in relationship to pCO2. Crop Pasture Sci. 6(1):115-124.

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