This microbial community enables ruminants to convert fibrous plant materials into energy-rich compounds that would otherwise be indigestible. Under normal conditions, forage-based diets promote the dominance of fibrolytic bacteria such as Ruminococcus spp. and Fibrobacter succinogenes.

These microorganisms break down structural carbohydrates like cellulose and hemicellulose into volatile fatty acids (VFAs), primarily acetate and butyrate. These VFAs are essential for energy supply and play a critical role in milk fat synthesis in dairy cows.

In parallel, other microbial groups contribute to the stability of the rumen environment. Lactate-utilizing bacteria, including Megasphaera elsdenii and Selenomonas ruminantium, metabolize lactic acid into weaker acids, preventing its accumulation and helping maintain rumen pH within the optimal range of 6.2 to 6.5.

When balance is lost: the microbial shift behind SARA

SARA is primarily driven by dietary imbalances, particularly the excessive intake of rapidly fermentable carbohydrates such as starch. When animals are fed high-concentrate diets with insufficient effective fiber, fermentation dynamics in the rumen shift dramatically.

Amylolytic bacteria, including Streptococcus bovis and Lactobacillus spp., rapidly proliferate under these conditions. These bacteria ferment starch faster than fibrolytic microbes, producing large quantities of VFAs, especially propionate and, critically, lactic acid.

At this point, the microbial ecosystem undergoes a profound transformation:

• Fibrolytic bacteria become inhibited, reducing fiber digestion efficiency.
• Lactate-utilizing bacteria lose activity, limiting the conversion of lactic acid.
• Acid-tolerant, lactate-producing bacteria thrive, accelerating acid accumulation.

This creates a self-reinforcing cycle: lower pH favors acid-producing microbes, which in turn further decrease pH. Over time, this feedback loop destabilizes the entire rumen ecosystem and leads to persistent subacute acidosis.

Consequences of microbial dysbiosis on animal health

The microbial imbalance associated with SARA has far-reaching consequences, both locally within the rumen and systemically throughout the animal.

The suppression of fibrolytic bacteria directly affects the animal’s ability to digest fiber. This leads to reduced nutrient extraction from feed and a measurable decline in feed efficiency. Studies indicate that cattle affected by SARA may experience a 5 to 10% reduction in feed conversion efficiency.

In dairy cows, the consequences are particularly evident in milk production. Reduced acetate production, due to impaired fiber fermentation, leads to lower milk fat synthesis. As a result, milk yield can decrease by 3 to 5%, often accompanied by a significant drop in milk fat content.

Beyond digestion, prolonged exposure to acidic conditions damages the rumen lining. The epithelial tissue becomes inflamed, a condition known as rumenitis, and may develop lesions or ulcers. This compromises the barrier function of the rumen wall.

As rumen integrity deteriorates, harmful substances can cross into the bloodstream. The lysis of Gram-negative bacteria releases lipopolysaccharides (LPS), potent endotoxins that trigger systemic inflammatory responses.

These endotoxins, along with bacteria, can reach the liver and other organs, increasing the risk of liver abscesses. Additionally, systemic inflammation is closely linked to laminitis, a painful and economically significant condition affecting hoof health.

Managing SARA through microbial stability

Preventing SARA requires more than simply correcting rumen pH; it demands maintaining a stable and functional microbial ecosystem. Nutritional strategies play a key role.

• Adequate levels of physically effective fiber
• Proper feed particle size
• Gradual dietary transitions
• Balanced starch inclusion
• Support for natural rumen buffering mechanisms

Supporting microbial balance through pH stabilization

Among available nutritional solutions, magnesium oxide (MgO) has emerged as a valuable tool for managing rumen acidity. Unlike fast-acting buffers such as sodium bicarbonate, which provide immediate but short-lived effects, MgO offers a more sustained alkalizing action.

When introduced into the rumen, MgO reacts gradually with acids, releasing hydroxide ions that help neutralize acidity over time. This prolonged buffering effect is particularly beneficial in stabilizing rumen pH throughout the day, reducing the duration and severity of acidotic episodes.

Importantly, maintaining a stable pH environment supports beneficial microbial populations:

• Preserves fibrolytic bacteria, ensuring efficient fiber digestion.
• Sustains lactate-utilizing bacteria, helping prevent lactic acid buildup.
• Limits the proliferation of acid-tolerant pathogens.

Not all MgO is equal: the importance of reactivity and solubility

The effectiveness of MgO depends heavily on its physical and chemical properties. Factors such as raw material origin, calcination temperature, processing methods, and particle size all influence its solubility and reactivity in the rumen.

Standard industrial MgO sources may vary widely in performance, leading to inconsistent results in practice. Some products may dissolve too slowly to provide adequate buffering, while others may lack sustained activity.

pHix-up®: a targeted approach to rumen microbial stability

To address these challenges, Terresis has developed pHix-up®, a proprietary magnesium oxide blend designed to deliver both rapid and sustained buffering effects in the rumen.

Unlike conventional MgO products, pHix-up® is formulated to ensure optimal solubility and reactivity, allowing it to respond quickly to acid production while maintaining long-lasting pH stabilization. This dual-action profile is particularly valuable in high-starch feeding conditions, where acid production is continuous and dynamic.

The efficacy of pHix-up® has been evaluated through multiple trials, including a large-scale meta-analysis involving 84 cows equipped with intraruminal pH sensors. Over a three-month period, rumen pH was continuously monitored to assess the impact of different buffering strategies.

Results demonstrated that cows supplemented with pHix-up® spent significantly less time below the critical pH threshold of 5.8, by an average of 85 minutes per day compared to other buffering solutions.

Further evidence comes from a controlled study conducted in collaboration with Alex Bach (Journal of Dairy Science, 2023), where SARA was experimentally induced through a high-starch diet.

In this challenging context, pHix-up® not only stabilized rumen pH but also preserved key fibrolytic bacteria, including Fibrobacter succinogenes. Additionally, improvements in total-tract neutral detergent fiber (NDF) digestibility were observed, along with support for other beneficial microbial populations such as Treponema.

Conclusion: a microbial-centered approach to preventing SARA

Subacute Ruminal Acidosis is not merely a question of pH; it is fundamentally a disorder of microbial imbalance. Understanding and managing the rumen microflora is therefore essential for effective prevention and control.

By focusing on strategies that support microbial stability through balanced nutrition, proper feeding management, and targeted buffering solutions, producers can mitigate the risks associated with SARA.

Innovations such as pHix-up® represent a step forward in this approach, offering a more consistent and sustained means of maintaining rumen pH and preserving microbial function. In doing so, they help safeguard digestion, animal health, and overall production efficiency.