The rumen and reticulum (reticulo-rumen) is the largest compartment of the cow’s stomach. Fiber, starch, sugar, and proteins are fermented by the microbes to form volatile fatty acids and microbial protein. Rumen papillae absorb volatile fatty acids.
The omasum has many “leaves” of tissue that absorb water and serve as a filter.
The abomasum is the true stomach where the cow’s own enzymes and acids break down ingested feed.
Rumination is the process by which the cow regurgitates previously consumed feed and masticates it a second time.
The ruminant stomach has four compartments: rumen, reticulum, omasum, and abomasum. Ingested feed flows through the rumen and reticulum first, then through the omasum, and lastly, through the abomasum.
1. Rumen & Reticulum (also called the “Reticulo-rumen” or simply the “Rumen”):
The capacity of an adult dairy cow’s rumen is about 184 liters (49 gallons) and the reticulum is about 16 liters (4.25 gallons). It is one of the most dense microbial habitats in the world. Microscopic organisms called rumen microbes break down (or digest) ingested feed by a fermentation process. The rumen is the major site of fermentation in the cow. There are1,000,000,000 to 10,000,000,000 bacteria per ml, 1,000,000 protozoa per ml, and a variable amount of fungi. The cow does not secrete any of her own acids or digestive enzymes in the rumen. Rather, all rumen digestion is done by the microbes.
It is because of the rumen and its microbes that dairy cows can digest plant fibers, such as hemicellulose and cellulose. This allows the dairy cow to convert forages and industrial byproducts that humans cannot digest into nutritious foods for humans. Mammalian enzymes and acids cannot digest fiber. The rumen microbes ferment fiber, starch, sugar, and protein to form volatile fatty acids and microbial protein.
The rumen wall (or mucosa) is a major site of nutrient absorption. It is convoluted to give it tremendous surface area for absorption. Rumen papillae (thousands of finger-like projections on the inside surface of the rumen) are responsible for absorbing the volatile fatty acids from the rumen for use by the cow. Volatile fatty acids (VFA’s) are absorbed from the rumen and used as an energy source for the cow. Microbial protein (the actual bodies of the microbes) is not absorbed from the rumen. It absorbed from the cow’s intestine and is used as a source of quality protein for the cow.
The reticulum is located in front of the rumen and it is known for its characteristic “honeycomb” surface. If a cow accidentally eats hardware, such as screws or nails, it usually ends up lodged at the bottom of the reticulum. If the hardware punctures through the reticulum wall, it causes the often deadly “hardware disease”.
The omasum is the third stomach compartment. It holds only about 8 liters (2 gallons) in the adult cow. It is made of many “leaves” of tissue which serve to absorb water for the cow. These leaves also work as a filtration system for the cow to only allow fine particles of digested feed and some fluid to pass on to the abomasum.
The abomasum is the fourth stomach compartment. It is also called the “true stomach”. It holds about 27 liters (7 gallons). This compartment has basically the same function as the stomach in simple stomached (monogastric) animals, such as pigs and humans. It is here that the cow’s own stomach acids and enzymes are used to further breakdown ingested feed before it passes into the small intestine.
Characteristics of the Rumen Environment
A number of characteristics about the rumen allow for the growth of the rumen microbes and for their fermentation process that digests feed and forages for the cow. Any disruption to the delicate balance of the rumen environment will result in poor growth of the microbes, poor digestion, and ultimately, lower milk production.
First of all, the rumen is anaerobic. This means that there is little or no oxygen in it. The rumen microbes cannot grow in outside air. They will tolerate a small amount of oxygen so long as the fermentation is going strong enough so that they can get rid of the oxygen quickly. Some oxygen does, of course, get into the rumen through feed and water. Secondly, the rumen temperature is one degree above body temperature at 39°C (102.5°F). Luckily, it’s fairly easy for us as farmers and nutritionists to maintain an anaerobic, warm environment deep in the belly of the cow!
Rumen pH ranges between 5.7 and 7.3. The high side of this pH range (> 7) will be seen on poor quality forage diets supplemented with urea. In high-producing dairy cows, acidosis (rumen pH<6.0) is a common problem. This occurs when the cow eats too much rapidly digestible starch or sugar that creates acid and overwhelms the rumen’s buffering system. Most of the buffer in the rumen comes in the form of saliva that is generated when the cow chews her cud. Inadequate intake of long fiber that promotes rumination (cud-chewing) can also result in acidosis because it provides less salivary buffer to counteract the acid produced by grain fermentation. The rumen microbes, especially those that primarily digest fiber, are acid intolerant. They do not grow well in acid and they don’t digest feed, especially forages, well under acid conditions.
End Products of Rumen Fermentation
1. Volatile Fatty Acids (VFA’s):
The rumen microbes make three primary volatile fatty acids: acetate (CH3COOH), propionate (C2H5COOH), and butyrate (C3H7COOH). Acetate is formed primarily from the fermentation of fiber. Large amounts of propionate are formed from grain fermentation. Butyrate is produced to a lesser extent than acetate and propionate. Sometimes lactate is also formed, especially under acidic conditions in the rumen. VFA are actually waste products from the rumen microbes but the cow absorbs them from her rumen and uses them as major source of energy.
Dairy cows produce 30-50 liters (8-13 gallons) of gas per hour. Carbon dioxide (CO2) (about 60%) and methane (CH4) (about 40%) are the main waste gases produced by the rumen microbes. Hydrogen (H2) usually makes up less than 0.05% of the total rumen gases. Gas is primarily found on top of the solid and liquid contents of the rumen. Methane bacteria actually grow on CO2 and H2 and other byproducts from the other bacteria in the rumen. The methane bacteria commonly produce methane in this way:
4H2 + CO2 ---> CH4 + 2H2O
Cows get rid of these excess gases by a process called eructation (a kind of silent belching) and by absorption through the rumen wall and lung exhalation. The cow eructates using a process which is slightly different from the normal contractions of the reticulum. Once gas pressure stimulates pressure receptors, the reticulo-ruminal fold and the cranial pillar stop the rumen digesta from getting into the esophagus. The cardia and diaphragmatic sphincters relax, gas enters the esophagus and is released through the cow’s mouth.
If a cow is unable to get rid of excess gases, she can bloat. When the rumen contents foam excessively, the cow has difficulty getting rid of the gas because it is trapped inside the foam. The rumen keeps expanding. It creates pressure on the cow’s lungs and the cow can die of asphyxiation. Many legumes (white clover and alfalfa), especially those that are young and freshly cut, can cause bloat. A chronic type of bloat also occurs in beef cattle on feedlots when they are fed a lot of grain. Oils or low-foam detergents have been used to reduce the stability of foam in the rumen and prevent bloat.
3. Microbial Mass
The rumen microbes grow in the rumen and their bodies are passed down to the intestine of the cow. Thus, they make up a large portion of the cow’s diet and the largest portion of the cow’s protein supply. This microbial protein is high quality, meaning that the amino acid profile is fairly similar to that of milk and meat. Therefore, the cow can easily and efficiently convert microbial protein into milk and meat.
Amino Acid Composition of Milk Protein and Rumen Microbes (%):
Approximate Composition of the Microbial Mass (Van Soest, 1982):
As the microbes digest ingested feed and forages and grow, they release heat. This is called the “heat of fermentation”. Except for when the cow needs this heat to warm her body, this heat is a waste of energy to the cow. The fermentation of forages creates more heat than the fermentation of concentrate feeds.
Rumen Structure and Movements:
Rumination, also called cud-chewing, is the process by which the cow regurgitates (casts up) previously consumed feed and masticates it a second time. The re-chewed feed with saliva is formed into a bolus and swallowed a second time. It is the floating, large particles on top of the rumen which are re-chewed. One purpose of rumination is to decrease particle size and increase surface area of the feed. This results in an increase in digestion rate of feed and a decrease in the lag time prior to fermentation. The fibrous particles will stay in the rumen longer causing the rumen to feel more full if the cow is not ruminating enough. This will reduce the total intake of the cow and negatively impact milk production. Another purpose of rumination is to make saliva (98 to 190 liters (or 26-50 gallons) per day) to buffer the rumen and decrease rumen acidity.
Cows usually spend more time chewing during rumination than they do when they eat. Pressure of coarse material (or “scratch factor”) against the rumen wall stimulates the cow to ruminate. Therefore, the amount of time a cow spends ruminating is diet dependent. Feeding a lot of concentrates and/or finely ground feeds reduces rumination. It is critical, especially in high-producing dairy cows that consume considerable amounts of concentrate, that there is an adequate amount of long fiber present in the diet to stimulate rumination.
Stratification of Rumen Contents
Dairy cows who are fed a diet containing long fiber have a large, dense, floating layer (or floating mat) located just under the gas in the top portion of the rumen. This mat contains the more recently consumed feed, especially the fibrous portion of the diet. As the ingested feed ferments and is ruminated, it becomes more water-logged and dense and gradually sinks toward the bottom of the rumen. Smaller, dense particles and liquid is located in the bottom portion of the rumen. This bottom layer is material that is nearly ready for passage out of the rumen, into the omasum, and on down the cow’s digestive tract. Diets high in concentrates or containing only very fine fibrous material may either have no floating mat or a very small one.
Mixing of rumen contents is very important to the cow. Contraction and relaxation of the entire rumen, especially the wall between the rumen and the reticulum, helps to mix contents. Also, ridged structures called rumen pillars act like baffles on the inside of a clothes washing machine and help to set up mixing motion in the rumen.
Rumen mixing helps to inoculate feed with microbes. It also aids VFA in reaching the rumen wall for absorption. The combination of mixing and rumination allows the cow to pass indigestible matter that might otherwise stay in the rumen and decrease total feed intake of the cow. It is interesting to note that finely processed diets that result in little mat formation actually increase the rate at which feed passes and increase the size of particles found in the manure.
Adding just a small amount of long hay to a diet, such as 1-2 kg (2-4 pounds) can greatly increase the size of the mat, stimulate rumen mixing, help VFA to reach the rumen wall for absorption and help to properly sort rumen contents. At same time, hay promotes rumination and the production of saliva to buffer the rumen.
Hungate, R.E. 1966. The rumen and its microbes. Academic Press, NY.
Russell, J.B. 1988. Microbiology of the Rumen. Animal Science 607 Class Notes, Cornell University.
Van Soest, P.J. 1982. Nutritional ecology of the ruminant. O&B Books, Inc., Corvallis, OR.
Ruminant Anatomy and Physiology In: Feeding the Dairy Herd North Central Regional Extension Publication
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