The rumen is the first stomach of ruminants, lying next to the reticulum. When a herbivore is ruminating, the food returns from here to the mouth. The rumen is located on the left side of the abdominal cavity and occupies almost the entire left abdominal cavity.
The rumen is a major compartment of ruminant stomachs and is well established as a natural and efficient system for crude fiber fermentation. Rumen microbes anaerobically ferment complex lignocellulosic plant materials which cannot be directly utilized by a host, into monomers which are further degraded into different microbial end-products, including volatile fatty acids, hydrogen, carbon dioxide, methane, and other fermentation products necessary for essential metabolic pathways. The complex rumen microbiome network is dominated by bacteria, archaea, protozoa, and anaerobic fungi.
This entry is adapted from 10.3390/microorganisms9010190