Animals in this group of ruminants select plants and plant parts high in easily digestible, nutrient dense substances such as plant starch, protein, and fat. For example, deer prefer legumes over grasses. Concentrate selectors are very limited in their ability to digest the fibers and cellulose in plant cell walls. These ruminants depend on diets of grasses and other fibrous plant material. They prefer diets of fresh grasses over legumes but can adequately manage rapidly fermenting feedstuffs.
Goats are classified as intermediate types and prefer forbs and browse such as woody, shrubby type plants. They have a fair though limited capacity to digest cellulose in plant cell walls. On high-forage diets ruminants often ruminate or regurgitate ingested forage.
As ruminants are transitioned to higher concentrate grain-based diets, they ruminate less. Once inside the reticulorumen, forage is exposed to a unique population of microbes that begin to ferment and digest the plant cell wall components and break these components down into carbohydrates and sugars.
Rumen microbes use carbohydrates along with ammonia and amino acids to grow. The microbes ferment sugars to produce VFAs acetate, propionate, butyrate , methane, hydrogen sulfide, and carbon dioxide.
The VFAs are then absorbed across the rumen wall, where they go to the liver. Once at the liver, the VFAs are converted to glucose via gluconeogenesis.
Because plant cell walls are slow to digest, this acid production is very slow. Coupled with routine rumination chewing and rechewing of the cud that increases salivary flow, this makes for a rather stable pH environment around 6. When ruminants are fed high-grain or concentrate rations, the digestion process is similar to forage digestion, with a few exceptions.
Additionally, most grains have a high concentration of readily digestible carbohydrates, unlike the more structural carbohydrates found in plant cell walls. This readily digestible carbohydrate is rapidly digested, resulting in an increase in VFA production. The relative concentrations of the VFAs are also changed, with propionate being produced in the greatest quantity, followed by acetate and butyrate.
Less methane and heat are produced as well. The increase in VFA production leads to a more acidic environment pH 5. It also causes a shift in the microbial population by decreasing the forage using microbial population and potentially leading to a decrease in digestibility of forages. Lactic acid, a strong acid, is a byproduct of starch fermentation. The acidic environment leads to tissue damage within the rumen and can lead to ulcerations of the rumen wall.
Take care to provide adequate forage and avoid situations that might lead to acidosis when feeding ruminants high-concentrate diets. Two sources of protein are available for the ruminant to use: protein from feed and microbial protein from the microbes that inhabit its rumen.
A ruminant is unique in that it has a symbiotic relationship with these microbes. Like other living creatures, these microbes have requirements for protein and energy to facilitate growth and reproduction. Each feedstuff such as cottonseed meal, soybean hulls, and annual ryegrass forage has different proportions of each protein type. Rumen microbes break down the DIP into ammonia NH3 amino acids, and peptides, which are used by the microbes along with energy from carbohydrate digestion for growth and reproduction.
Excess ammonia is absorbed via the rumen wall and converted into urea in the liver, where it returns in the blood to the saliva or is excreted by the body. Urea toxicity comes from overfeeding urea to ruminants. Ingested urea is immediately degraded to ammonia in the rumen. When more ammonia than energy is available for building protein from the nitrogen supplied by urea, the excess ammonia is absorbed through the rumen wall.
This can kill the animal. However, with sufficient energy, microbes use ammonia and amino acids to grow and reproduce. The rumen does not degrade the UIP component of feedstuffs. In the abomasum, the ruminant uses UIP along with microorganisms washed out of the rumen as a protein source. The digestive system of ruminants optimizes use of rumen microbe fermentation products. This adaptation lets ruminants use resources such as high-fiber forage that cannot be used by or are not available to other animals.
Ruminants are in a unique position of being able to use such resources that are not in demand by humans but in turn provide man with a vital food source. Ruminants are also useful in converting vast renewable resources from pasture into other products for human use such as hides, fertilizer, and other inedible products such as horns and bone. One of the best ways to improve agricultural sustainability is by developing and using effective ruminant livestock grazing systems.
Rumen development occurs following a change in diet and microbial growth. The rumen on the left side of the animal is the largest stomach compartment and consists of several sacs. It can hold 25 gallons or more of material depending on the size of the cow. Because of its size, the rumen acts as a storage or holding vat for feed. Aside from storage, the rumen is also a fermentation vat. These microbes digest or ferment feed within the rumen and make volatile fatty acids VFAs.
The rumen absorbs most of the VFAs from fermentation. A good blood supply to the rumen walls improves absorption of VFAs and other digestion products. The reticulum is a pouch-like structure in the forward area of the body, close to the heart.
The tissues in the reticulum form a network similar to a honeycomb. Heavy or dense feed and metal objects eaten by the cow drop into this compartment.
Leaving it untreated may lead to infection and possibly death. The omasum is a globe-shaped structure containing leaves of tissue like pages in a book. It absorbs water and other substances from digestive contents.
Feed material ingesta between the leaves will be drier than ingesta found in the other compartments. The abomasum is the only compartment lined with glands. These glands release hydrochloric acid and digestive enzymes, needed to breakdown feeds. The abomasum is similar to a nonruminant stomach. The small intestine consists of three sections: the duodenum, jejunum and ileum. It measures about 20 times the length of the animal.
Secretions from the pancreas and gallbladder aid in digestion within the small intestine. The small intestine completes most of the digestive process and absorbs many nutrients through villi small finger-like projections.
From the villi the nutrients enter into the blood and lymphatic systems. The cecum is the large area where the small and large intestine meet. The cecum breaks down some previously undigested fiber, but the exact importance of the cecum remains unknown. The large intestine is the last section of the tract that undigested feedstuffs pass through. Microbes digest some undigested feed here, but the main digestive function of the large intestine is to absorb water.
Digestion produces 30 to 50 quarts of gas per hour in the rumen. Carbon dioxide and methane are the main gases present. However, water flows through the rumen rapidly and appears to be critical in flushing particulate matter downstream.
As fermentation proceeds, feedstuffs are reduced to smaller and smaller sizes and microbes constantly proliferate. Ruminal contractions constantly flush lighter solids back around the reticulorumen while denser particles feedstuffs that have been there longer proceed to the omasum. The function of the omasum is rather poorly understood. It may function to absorb residual volatile fatty acids and bicarbonate.
The tendency is for fluid to pass rapidly through the omasal canal, but for particulate matter to be retained between the omasal leaves. Periodic contractions of the omasum knock flakes of material out of the leaves for passage into the abomasum. The abomasum is a true, glandular stomach which secretes acid significantly lowering the pH and otherwise functions very similarly to the stomach of a monogastric.
One fascinating specialization of this organ relates to its ability to process large masses of bacteria. The cow has four stomachs and undergoes a special digestive process to break down the tough and coarse food it eats. When the cow first eats, it chews the food just enough to swallow it.
The unchewed food travels to the first two stomachs, the rumen and the reticulum, where it is stored until later.
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