Ruminant Forestomach
Fermentative Digestion
No mammal can directly digest the complex carbohydrates that constitute plant cell walls (cellulose and hemicellulose), because mammals do not produce the enzyme cellulase, which is necessary to break the unique chemical bonds in these compounds.
The ruminant forestomach provides an excellent environment for the growth of bacteria, protozoa, and possibly other microbes that do produce cellulase. The action of cellulase on cellulose and hemicellulose produces monosaccharides and simple polysaccharides, which are available for further microbial digestion.The microbial digestion in the forestomach occurs in an anaerobic environment and is termed fermentative digestion. Volatile fatty acids (VFAs) are produced by fermentation of carbohydrates consumed by ruminants, including carbohydrates produced by the actions of microbial cellulase. The primary VFAs are acetic acid, propionic acid, and butyric acid. The VFAs are absorbed directly from the forestomach and are the major energy source for ruminants. VFAs are also used for synthesis of milk fat in lactating animals.
Methane and carbon dioxide are produced by fermentative digestion and accumulate as a gaseous layer above the ingesta in the rumen and reticulum. Bloat (acute tympany) results in enlargement of the rumen and reticulum, which in turn press on the thorax, inhibiting function of the heart and lungs. Bloat results from more gas being produced than is eliminated by eructation (belching). Eructation is particularly difficult if foam forms. A stomach tube may be passed into the rumen by mouth to remove the gas. If that is not possible, a trocar (sharp tube) may be passed into the rumen through the left flank.
Dietary protein consumed by ruminants is available first to the microbes in the forestomach. The microbes may use the dietary protein to produce microbial proteins and promote microbial growth or to produce VFAs by fermentative digestion.
Microbes can also produce microbial proteins from nonprotein nitrogen sources, such as urea and ammonia.The microbes and by-products of microbial metabolism other than VFAs continue down the ruminant gastrointestinal tract from the forestomach. The microbial organisms, which grew in the forestomach, are a major source of dietary protein for ruminants. When the organisms enter the abomasum (true or glandular stomach of the ruminant) and the remainder of the tract, they are digested in a manner similar to digestion of protein sources in nonruminants. Beneficial by-products of microbial metabolism include many water-soluble vitamins.
Forestomach Motility
The rumen and reticulum of the adult cow normally undergo complicated sequences of contractions that are repeated at varying frequencies up to several times per minute. One pattern of contractions begins in the reticulum and spreads over both the dorsal and ventral sacs of the rumen (see Fig. 20-10). This series of contractions mixes the contents to promote fermentation and provide force to move liquified digesta out of the forestomach and into the abomasum. A second pattern of contractions begins in the caudal portion of the dorsal sac and moves cra- nially. These contractions move gases toward the cranial part of the rumen for eructation. Rumen contractions can be felt by forcing the fist into the upper left flank (paralumbar fossa). Pathologic condition of the rumen or morbidity associated with systemic diseases usually results in a decreased rate or complete cessation of rumen movements.
Rumination permits an animal to forage and ingest food rapidly and finish chewing later. it entails regurgitation of the food (returning it to the mouth) from the forestomach, remastication (rechewing), reinsalivation (mixing with more saliva), and finally reswallowing.
Regurgitation is the only step of rumination that differs markedly from the initial mastication, insalivation, and swallowing. Regurgitation is preceded by contraction of the reticulum, which presumably brings some of the heavier ingesta into proximity to the cardia.
The sphincter at the junction of the esophagus and forestomach (lower esophageal sphincter) relaxes as the bolus of food reaches it. An inspiratory movement with closed glottis follows. The negative pressure produced in the thorax by this movement is transmitted to the relatively thinwalled esophagus, dilating the thoracic esophagus and cardia. The lower pressure in the esophagus than in the rumen coupled with reverse peristalsis causes a quantity of material (semifluid ingesta) to pass through the cardia into the esophagus and up to the mouth. The regurgitated material consists largely of roughage and fluid, with little if any concentrate. it is well known that whole kernels of corn may pass through the entire digestive tract with little change in physical appearance.Cattle average about 8 hours a day ruminating, with periods of activity scattered throughout the entire day. One rumination cycle requires about 1 minute, of which 3 to 4 seconds is used for both regurgitation and reswallowing. Rumination appears to be largely reflexive, although the process can be interrupted or stopped voluntarily. Both afferent and efferent portions of the reflex are probably carried in the vagal nerves. Contact of roughage with the wall of the reticulum and near the cardia is likely the major stimulus for rumination.
Reticular, or Esophageal, Groove
in young ruminants, nursing and afferents from the pharynx appear to stimulate reflex closure of the groove, which causes milk to bypass the rumen and reticulum and pass through the omasum directly to the abomasum. The paunchiness of bucket-fed calves usually is attributed to milk entering the rumen, where it is not properly digested. The use of buckets with nipples tends to prevent appreciable amounts of milk from entering the rumen. After weaning, fluid drunk from open containers largely passes into the rumen and reticulum. The groove has no known function in adult animals, but reflex closure in adults has been produced with sodium salts in cattle and copper sulfate in sheep.
Omasum
The omasum is nearly always found packed tightly with rather dry roughage in animals examined after death. The appearance of the omasal leaves, studded with short horny papillae, suggests a burr type of grinder. Experimental vagal stimulation elicits strong contractions of the omasal wall, but movement of the leaves is limited.