Ruminant Rumen Physiology and Acidosis (cattle／sheep／goats; alpacas are pseudoruminants)

This content is compiled from the farm animals and horses chapter of the veterinary animal encyclopedia. The same sign calls for different handling in different species, and copying what is done for another species can be harmful — first confirm which animal you are dealing with.

• Normal structure: cattle, sheep and goats are true ruminants, with a stomach divided into 4 compartments — the rumen (the largest, a microbial fermentation vat), the reticulum (a honeycomb structure in which swallowed metal foreign bodies readily lodge, giving rise to "hardware disease／traumatic reticuloperitonitis"), the omasum (a many-layered structure, mainly absorbing water), and the abomasum (the true glandular stomach, functionally similar to the stomach of a monogastric animal). Alpacas／camelids have only 3 compartments (C1/C2/C3) and are "pseudoruminants"; the mechanism is similar but the structure is not the same, and the two must not be conflated.
• Mechanism of acidosis: the symbiotic microbial population in the rumen needs time to adapt to a change of feed. If a large amount of high-starch／high-sugar concentrate is given suddenly (excess grain, an abrupt change of feed formulation, escaping the fence and raiding the grain store), the microbes ferment rapidly and produce large quantities of volatile fatty acids and lactic acid at a rate exceeding the buffering and absorptive capacity of the rumen wall; pH falls rapidly, the rumen wall is damaged and endotoxin is released, which can go on to cause systemic toxaemia, laminitis (see 3.3), liver abscesses and even death.
• Prevention principles a keeper can act on: any change of feed must always be "gradual" rather than "all at once"; grain／concentrate must be limited in amount and given in divided portions; pasture／hay must always remain the basis of the staple diet; and animals must be prevented from accidentally getting into the grain store or feed room.
• Specific acidosis pH figures: the diagnostic criterion for acute grain overload is ruminal fluid pH < 5.5 (often < 5.0) (Merck Veterinary Manual, "Grain Overload in Ruminants": "Ruminal pH < 5.5 (often < 5.0)" <https://www.merckvetmanual.com/digestive-system/diseases-of-the-ruminant-forestomach/grain-overload-in-ruminants>, verified 2026-07-30); subacute ruminal acidosis (SARA) is defined as repeatedly occurring, prolonged periods in which ruminal pH falls to between 5.6 and 5.2 (Merck Veterinary Manual, "Subacute Ruminal Acidosis in Cattle and Sheep": "Subacute ruminal acidosis is most commonly defined as repeatedly occurring, prolonged periods of decrease of the ruminal pH to between 5.6 and 5.2." <https://www.merckvetmanual.com/digestive-system/diseases-of-the-ruminant-forestomach/subacute-ruminal-acidosis-in-cattle-and-sheep>, verified 2026-07-30). The two are classifications of different degrees of severity (acute vs subacute) and are not the same figure; compromising them into a single threshold is forbidden. Details of fermentation rate [No single authoritative figure for this item: the whitelisted sources give no fermentation rate reference value specific to ruminants].

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