forestomachs

Forestomachs are the first three compartments of the stomach in ruminant animals: the rumen, reticulum, and omasum. They house a complex microbial ecosystem that enables the fermentation of plant fiber, especially cellulose, and the production of volatile fatty acids (VFAs) that provide most of the animal’s energy. The abomasum is the true, glandular stomach that follows the forestomachs and secretes acid and enzymes for chemical digestion.

Anatomy and function

- Rumen: the largest compartment, serving as a fermentation vat. It contains papillae that increase surface area for absorption of VFAs. Ruminal contractions mix contents, deliver feed to the reticulum, and help move fines and saliva through the system.

- Reticulum: often called the hardware stomach, it has a honeycomb structure and works with the rumen to trap dense foreign objects and participate in regurgitation for rumination, a process that physically re-chews ingested material.

- Omasum: characterized by many folds, it reduces particle size further and absorbs water, electrolytes, and some VFAs before material passes to the abomasum.

Physiology and diet

The forestomachs maintain an anaerobic environment and a stable pH that supports microbes such as bacteria, protozoa, and fungi. Fermentation yields VFAs (acetate, propionate, butyrate) used for energy, microbial protein fed to the host, and gases like methane that are eructated. Diet composition influences retention time, rumination, and risk of disorders such as bloat or acidosis.

Development and clinical relevance

Young ruminants gradually develop forestomach function as solid feed is introduced. Disorders affecting forestomachs include hardware disease (reticulum), bloat, ruminal acidosis, and impaired motility, which can impact digestion and overall health.