During the early phase, after ingesting large quantities of HCD, ruminants show a decline in both the frequency and duration of their daily rumination. The phenomenon occurs due to the lower fiber content and higher starch levels in HCD. This results in reduced saliva production, which decreases the supply of buffers needed to neutralize acids in the rumen, thereby impairing the rumen’s buffering capacity (17, 18). Moreover, the fermentation substrates in HCD are mainly composed of soluble carbohydrates, which leads to the production of substantial amounts of VFAs, CO₂, and other acids that start to accumulate. Consequently, the rumen pH experiences its first decline, dropping below the normal range of 6.2–6.8 to below 6.0. A stable and balanced ecosystem is maintained in the rumen, with microbial communities thriving within an optimal pH range, typically between 6.0 and 7.0. Under normal pH conditions, the rumen microbial community is both diverse and stable, encompassing fiber-degrading bacteria (such as Fibrobacter succinogenes, Ruminococcus flavefaciens), starch-degrading bacteria (such as Streptococcus bovis, Ruminobacter amylophilus), and methanogens (such as Methanobrevibacter ruminantium). However, the microbial community in the rumen undergoes significant changes when the pH falls due to the intake of HCD. This alteration in the composition of the microbial community disturbs the rumen’s equilibrium, leading to further acid build-up and metabolic disturbances (7). Thus, this phase marks the onset of rumen acidosis development.

After the rumen pH falls below 6.0, the environment for rumen microorganisms changes, resulting in a decrease in the growth of fiber-degrading bacteria that are highly sensitive to pH fluctuations. This sensitivity is evident in a reduction of both their activity and numbers (19). In contrast, the population of amylolytic bacteria increases, particularly the lactic acid-producing species like Streptococcus bovis and Lactobacillus spp., which flourish at lower pH levels and quickly ferment starch. The increased fermentative action raises the production of VFAs, which in turn causes the rumen pH to drop further, to below 5.8 (17, 20). In this scenario, the proliferation of lactic acid-producing bacteria (e.g., Streptococcus bovis) leads to enhanced lactic acid production, causing the rumen pH to decrease further to below 5.6. This dip creates a markedly acidic rumen environment, disrupting its normal function (7, 17), marking this stage as the second phase of rumen acidosis.

When the rumen pH falls below 5.6, the growth and reproductive abilities of most rumen microorganisms are compromised due to the overly acidic environment. This heightened acidity promotes the proliferation of lactate-producing bacteria, such as Streptococcus bovis and Lactobacillus lactis, within the rumen flora. This leads to the excessive production and accumulation of lactate. As a result, the rumen pH drops rapidly to below 5.0 (7, 20). Under such conditions, many bacteria die and release endotoxins such as lipopolysaccharides (LPS), which can further impair rumen health and potentially triggering systemic inflammation (21, 22). This stage is considered the third phase of rumen acidosis.

In ruminant digestion, saliva plays a crucial role as one of the primary fluids that enter the rumen. After ingesting roughage, ruminants such as cattle can produce a daily salivary output exceeding 150 liters (24), whereas in sheep, it generally ranges between 1.2 and 10.2 liters per day (25). The concentration of dry matter in ruminant saliva is approximately 1.0–1.4 mg/mL, consisting mainly of inorganic substances like sodium, bicarbonate, potassium, phosphate, and a smaller amount of chloride (26, 27). Due to high levels of bicarbonate and phosphate, the saliva is alkaline (28). J. T. Reid et al. (29) documented the salivary pH for various ruminant species: buffaloes at a pH of 8.8; sheep between 8.12 and 8.32; goats from 8.2 to 8.8; calves with a pH of 8.1–8.23; and cattle ranging from 8.55 to 8.90, averaging 8.8. The alkaline nature of saliva helps to neutralize acids and, combined with its lubricating effects on the food bolus, aids in chewing and swallowing. This action helps to minimize the accumulation of VFAs and maintains the rumen’s pH balance (30). It is estimated that a cow’s saliva can neutralize about 30 to 40% of the VFAs produced in the rumen (24). Additionally, research indicates that the 24-h salivary output of an adult ruminant contains 300–350 grams of sodium carbonate, which can neutralize 56.6–65.0 liters of a 0.1 mol/L hydrochloric acid solution (29). Moreover, the alkalinity of substances is commonly assessed based on the pKa of their conjugate acid, with higher pKa values denoting greater alkalinity. In ruminant saliva, the pKa values for bicarbonate and phosphate are 10.25 and 12.67, respectively, emphasizing their significant acid-neutralizing ability.

Research suggests that in ruminants, there is a direct relationship between the frequency of chewing and the production of saliva (31). Conversely, feeding ruminants diets high in concentrates reduces fiber content and results in smaller feed particle sizes, leading to fewer chewing events and decreased salivary secretion, thus diminishing their capacity to counteract rumen acidity (31–33). K. Jacques and colleagues (34) found that the proportion of roughage in the diet influences salivary secretion, noting an increase in salivary outflow when the hay content in the diet was increased from 50 to 90%, even with a constant intake of ruminal fluid. An increase in hay proportion or intake directly enhanced salivary flow. Furthermore, G.E. Chibisa (35) and his team observed that cows on low-fiber, high-grain diets consumed their food more rapidly, exhibited significantly less salivary secretion during meals, and secreted less saliva at rest. Ezequias Castillo-Lopez (36) and his team discovered that prolonged consumption of HCD led to a decrease in rumen pH, an effect worsened by the reduction in feed-induced salivation and chewing activity. Consequently, the reduced fiber content and increased concentrate in HCD decrease the stimulation for salivary secretion, leading ruminants to chew and ruminate less. This results in a further reduction in salivary secretion, depleting salivary buffer components like bicarbonate, which are vital for neutralizing acids (18, 35, 37).

VFAs are critical end-products of rumen fermentation in ruminants, primarily synthesized through microbial degradation of carbohydrates like fiber, starch, and sugar (24). The main VFAs - acetic, propionic, and butyric acids - make up about 40–70%, 15–40%, and 10–20% of the total ruminal VFAs, respectively (38). Rumen acid buffering relies on bicarbonate from saliva, epithelial VFA absorption (which removes 50–80% of acid, with each VFA mole eliminating ~0.5 H + moles (39)), and proton efflux. Extra acid is neutralized by saliva or sent to the small intestine (24, 40). When ruminants eat HCD, the non-structural carbs boost microbial fermentation, causing VFA production to exceed clearance and leading to accumulation. (7, 41). E. J. Carroll et al. found that VFA production rates go in this order: grain-fed > hay-fed > pastured animals. (42). As weak acids with similar pKa values (~4.80), VFAs mostly dissociate in the rumen, lowering pH (43). Henderson-Hasselbalch calculations (Equation 1) suggest that over 95% of acetate, propionate, and butyrate are present in dissociated form at a pH of 6.0 (43). After the ingestion of HCD, rapid VFAs production initially decreases rumen pH. However, as H⁺ ion concentration reaches equilibrium dictated by the pKa values, the dissociation equilibrium stabilizes, limiting further pH decline. This buffering effect generally restricts the reduction of rumen pH to approximately 5.8 during prolonged VFAs accumulation.

Ramos et al. (44) demonstrated that shifting the dietary concentrate-to-forage ratio from 2:8 to 8:2 elevated total VFAs concentration from 92.39 to 154.59 mmol/L. During sustained HCD feeding, VFAs levels stabilized around 150 mmol/L, concurrent with a marked decline in rumen pH from 6.87 to 5.58. Similarly, Chibisa et al. (35) observed significantly higher total VFAs (118 mM vs. 95 mM), osmolality, and molar proportions of propionate/isovalerate in cattle fed low-fiber, high-grain (LF) diets compared to high-fiber (HF) diets. Despite equivalent VFAs absorption rates by rumen epithelium between dietary groups, LF-fed cattle exhibited exacerbated ruminal acidosis indices. This suggests that accelerated VFAs production rates under LF conditions exceeded clearance capacity, leading to VFAs accumulation and subsequent pH decline (35). Shen et al. (45) further corroborated these findings, reporting that increased dietary concentrate proportion induced elevated rumen osmolality alongside significant rises in propionate, butyrate, and total VFAs concentrations. Notably, their study revealed a positive correlation between rumen osmolality and total VFAs (LC: 90.34 mM; MC: 101.96 mM; HC: 113.95 mM). Elevated osmolality impaired paracellular resistance, suppressed apical Na⁺-H⁺ exchange activity, and compromised epithelial integrity. Critically, osmotic overload was shown to inhibit VFAs absorption, exacerbating acid retention (45). In summary, these studies support the hypothesis that HCD-induced microbial fermentation rapidly generates VFAs exceeding epithelial absorption capacity. This imbalance results in ruminal VFAs accumulation and subsequent acidotic pH depression (46).

Current understanding of ruminal acidosis pathogenesis predominantly attributes pH reduction to VFA or lactic acid accumulation, while existing studies largely neglect the contributory role of carbon dioxide (CO₂) (47). In the rumen ecosystem, CO₂ primarily originates from microbial decarboxylation of VFAs (e.g., acetate, propionate, butyrate) during feed fermentation. HCD, rich in rapidly fermentable carbohydrates, intensify microbial activity, thereby amplifying CO₂ production. Notably, elevated ruminal dissolved CO₂ (dCO₂) concentrations may independently drive pH depression through acid–base perturbations (47). CO₂ exhibits high aqueous solubility, existing in three equilibrium states within ruminal fluid: Hydrated form (dCO₂: CO₂·H₂O), Acidic species (carbonic acid, H₂CO₃) and Conjugate base (bicarbonate, HCO₃⁻) (Equation 2) (47). Mechanistically, dCO₂ forms labile associations with hydronium ions (H₃O⁺). During active fermentation, excessive CO₂ generation increases H₃O⁺ activity through dCO₂ protonation, thereby acidifying ruminal fluid (48). This CO₂-mediated proton release establishes an underappreciated pathway for pH reduction in HCD-induced acidosis.

In addition, rumen pH can also be better explained by the balance between dCO₂ and HCO₃⁻ concentrations (Equation 3). Equation 3 illustrates that HCO₃⁻ comprises the principal form of CO₂ at pH levels exceeding the equilibrium point (pKa = 6.37). For instance, within blood (pH 7.4), CO₂ is largely present as HCO₃⁻ (88%), with a minimal quantity (5%) existing as dCO₂ (49). Conversely, during the progression of rumen fermentation, dCO₂ becomes the predominant CO₂ form in the rumen fluid, causing a drop in pH. HCD are typically rich in rumen-degradable starch (RDS), enhancing CO₂ production and increasing the viscosity of rumen fluid (47). Moreover, HCD diets exhibit a reduced fraction of physically effective neutral digestible fiber (pe NDF) (50), leading to diminished rumen chyme particle sizes and elevated rumen gas content (51). These alterations in the physicochemical characteristics might decrease CO₂ volatility, elevate CO₂ retention, and augment the concentration of dCO₂, potentially influencing rumen pH. Laporte-Uribe’s (48) study have demonstrated that rumen CO₂ retention is linked with substantial dCO₂ levels and modifications in HCO₃⁻ activity, rumen fluid viscosity, and surface tension, which are evident during the CO₂ retention phase in SARA.

The elevated levels of non-structural carbohydrates in high concentrate diets enhance the rate of rumen microbial fermentation, resulting in a considerable production of VFAs and CO₂. The pace of acid production surpasses the rate of absorption, leading to the accumulation of acid within the rumen (52, 53). This phenomenon is commonly attributed by many scholars to the cause of ruminal acidosis. Nevertheless, due to concurrent processes of VFAs production and absorption, along with CO₂ production and emission in the rumen, it is challenging to ascertain precise data on these rates. Some scholars’ studies could demonstrate that feeding HCD resulted in faster rates of VFAs production and higher yields. For example, some scholars (42) have indirectly gaged VFAs production rates through in vitro fermentation experiments, discovering that cereal-fed animals exhibit the highest VFAs production, while hay-fed animals have a moderate rate, and pasture-fed animals display a slightly lower rate. The lowest VFAs production was observed in pasture-fed animals, partially affirming the conjecture that VFAs production is excessively rapid with HCD feeding. However, there is little direct evidence on the rate of uptake of VFAs in the rumen, and the majority of studies have only assessed the rumen VFAs concentration post high concentrate diet consumption, exemplifying an increase that may lead to VFAs accumulation. Whether this increase results from a significantly higher production rate than absorption rate awaits confirmation from more direct studies.

It has been noted that the ingestion of HCD may result in the formation of numerous stable foams within the rumen, leading to foam-type flatulence (54). The causes of rumen foam associated with HCD intake include diminished salivary output, augmented rumen fluid viscosity, decreased surface tension, and an elevated presence of polysaccharides and proteins acting as ‘foaming agents’ in the rumen content, promoting bubble formation and stability. Research indicates that animals fed HCD exhibit a marked increase in foam stability, production, and rumen fluid viscosity compared to those on low concentrate diets (55), predisposing them to the development of extensive rumen foams, thereby precipitating flatulence. This can be due to several factors. High levels of carbohydrates, particularly non-structural carbohydrates such as starch and sugar, are commonly associated with increased foam production. These carbohydrates also lead to rapid fermentation by rumen microorganisms, producing gasses such as carbon dioxide and methane, which can lead to foam formation. Additionally, certain proteins and peptides in feeds may act as surfactants, stabilizing foam. These foams might hinder the rumen epithelium’s absorption of VFAs, leading to VFAs accumulation and subsequent reduction in rumen pH. Qy and colleagues (56) observed that incorporating an antifoam agent, dimethylsiloxane oil, into HCD diets effectively dissipated rumen foams, lowered VFAs concentrations in rumen fluid, and elevated rumen pH. Zhang (57) found a close link between the amount of polysaccharides in rumen fluid and its viscosity, foaming ability, and foam stability. This points to a key factor in rumen foam formation. Later studies showed that non-starch polysaccharidases in the diet can greatly cut down polysaccharide levels. This in turn reduces foam traits such as bubble stability, formation ability, and viscosity. Notably, these enzymes increased rumen pH and decreased the levels of acetic and propionic acids. It was postulated that rumen foam could obstruct VFAs contact with the rumen wall, diminishing VFAs absorption and causing rumen VFAs build-up. During HCD consumption, rumen liquid viscosity and surface tension are reduced, conditions favorable for foam generation. Foam presence may also inhibit rumen motility, disrupting normal gas flow and leading to CO₂ accumulation. Moreover, abundant rumen foam raises internal pressure, enhancing CO₂ solubility and dCO₂ concentration, contributing to pH decline. Further studies are needed to substantiate these theories. Additionally, saliva contains minor foam inhibitors (58), and HCD-induced reduction in saliva reduces the availability of these inhibitors, failing to prevent foam formation. The cumulative effect of foam formation due to HCD, with its high starch and soluble carbohydrate content, creates an environment fostering stable foam presence, leading to VFAs and dCO₂ accumulation, a drop in rumen pH, and the onset of rumen acidosis.