Rumen

The rumen is the first compartment of the stomach in most ruminants, such as cattle, sheep, and goats. It is a large, muscular chamber on the left side of the abdomen that serves as the primary site of microbial fermentation. Together with the reticulum, omasum, and abomasum, the rumen forms the ruminant stomach. The interior is lined with papillae and houses a diverse microbial ecosystem including bacteria, protozoa, fungi, and archaea that break down fibrous plant material.

Food ingested is chewed and mixed by contractions, allowing microbes to ferment carbohydrates to volatile fatty acids. The main end products are acetate, propionate, and butyrate, which are absorbed across the rumen wall and supply most of the animal's energy. Microbes also synthesize amino acids and microbial protein, contributing to the animal's protein intake when they pass to the lower digestive tract.

The rumen operates as an anaerobic fermentation chamber. Gas produced during fermentation is expelled by eructation. The rumen maintains a pH generally around 6.0 to 7.0; diets high in readily fermentable carbohydrate can lower pH and lead to acidosis, while insufficient rumination and fiber can affect digestion. Adequate fiber stimulates chewing and saliva production, which helps buffer rumen fluid and stabilize pH.

In young ruminants, the rumen develops as solid feed is introduced. The esophageal groove may channel milk directly to the abomasum in suckling animals. Nutritional management and microbial colonization influence rumen development, health, and productivity. Disorders affecting the rumen include bloat, ruminal acidosis, and displaced abomasum.