Rumen

The rumen, also known as a paunch, is the largest stomach compartment in ruminants. The rumen and the reticulum make up the reticulorumen in ruminant animals. The diverse microbial communities in the rumen allows it to serve as the primary site for microbial fermentation of ingested feed, which is often fiber-rich roughage typically indigestible by mammalian digestive systems. The rumen is known for containing unique microbial networks within its multiple sac compartments to break down nutrients into usable energy and fatty acids.

Brief anatomy

The rumen is composed of five muscular sacs: cranial sac, ventral sac, dorsal sac, caudodorsal sac, and caudoventral blind sac. Each of these areas contain unique microbial communities, environments, and physical abilities that influence digestion. The outer lining of the rumen, known as the epithelium, serves as a protective layer and contributes to the metabolic processing of fermentation products. The inner lining of the rumen wall is covered in small fingerlike projections called papillae, which aid in nutrient absorption. The reticulum is lined with ridges that form a hexagonal honeycomb pattern. These features increase the surface area of the reticulorumen wall, facilitating the absorption of volatile fatty acids and capture of smaller digesta particles. The rumen and the reticulum differ with regard to the makeup of the lining but account for approximately 80% of total ruminant stomach volume.

Digestion Digestion in the rumen and reticulorumen occurs through fermentation by diverse microbe communities to optimize resources from nutrient dense feed. Millions of microorganisms, including bacteria, archaea, viruses, fungi, and protozoa, are known to reside in the reticulorumen and are essential to digest structural carbohydrates, like lignocellulose (hemicellulose and cellulose), non-structural carbohydrates (starch, sugar, and pectin), lipids, and nitrogenous compounds (proteins, peptides, and amino acids). Both non-structural and structural carbohydrates are hydrolysed to monosaccharides or disaccharides by microbial enzymes. The resulting mono- and disaccharides are transported into the microbes. Once within microbial cell walls, the mono- and disaccharides may be assimilated into microbial biomass or fermented to volatile fatty acids (VFAs), such as acetate, propionate, butyrate, lactate, valerate and other branched-chain VFAs via glycolysis and other biochemical pathways to yield energy for the microbial cell. Most VFAs are absorbed across the reticulorumen wall, directly into the bloodstream, and are used by the ruminant as substrates for energy production and biosynthesis. Some branched chain VFAs are incorporated into the lipid membrane of rumen microbes. VFAs provide large amounts of energy for ruminants and are critical to the health of the rumen and its microbiome. Lipids, lignin, minerals, and vitamins play a less prominent role in digestion than carbohydrates and protein, but they are still critical in many ways. Lipids are partly hydrolysed and hydrogenated, and glycerol, if present in the lipid, is fermented. Lipids are otherwise inert in the rumen. Some carbon from carbohydrate or protein may be used for de novo synthesis of microbial lipid. High levels of lipid, particularly unsaturated lipid, in the rumen are thought to poison microbes and suppress fermentation activity. Lignin, a phenolic compound, is recalcitrant to digestion, through it can be solubilized by fungi. Lignin is thought to shield associated nutrients from digestion and hence limits degradation. Minerals are absorbed by microbes and are necessary to their growth. Microbes in turn synthesize many vitamins, such as cyanocobalamin, in great quantities—often great enough to sustain the ruminant even when vitamins are highly deficient in the diet. The protein ingested is either degradable intake protein or undegradable intake protein, or rumen bypass protein. Protein is hydrolysed to peptides and amino acids by microbial enzymes, which are subsequently transported across the microbial cell wall for assimilation into cell biomass, primarily. Peptides, amino acids, ammonia, and other sources of nitrogen originally present in the feed can also be used directly by microbes with little to no hydrolysis. In situations in which nitrogen for microbial growth is in excess, protein and its derivatives can also be fermented to produce energy, yielding ammonia. Excess ammonia is absorbed by the rumen and converted into urea in the liver. Non-amino acid nitrogen is used for synthesis of microbial amino acids. Ruminants have access to food-sourced protein and microbial proteins produced by the microbes in the rumen. This creates a symbiotic relationship between the ruminant and the microbial communities, as the microbes can be used as a protein source when washed into the abomasum section of the digestive tract.

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