The four compartments of the cow's stomach are the rumen, reticulum, omasum, and abomasum.
The rumen:
The rumen is the largest compartment, and it contains billions of bacteria, protozoa, molds, and yeasts. These microorganisms live in a symbiotic manner with us, and they are the reason we can eat and digest large amounts of roughage. The rumen microorganisms are adaptable enough that cows can digest a large variety of feeds from grass, hay, and corn to brewer’s grains, corn stalks, silage, and even urea. (That may be gross to you humans, but hey, we're still eating it!)
The bacteria and protozoa do most of the digestion of food for us. There are 25 to 50 billion bacteria and 200 to 500 thousand protozoa in every milliliter of rumen fluid. The microorganisms digest the plant fiber and produce volatile fatty acids. These fatty acids are absorbed directly through the rumen wall and supply 60 to 80 % of the energy needed by the cow. In addition to energy, the microorganisms produce protein including essential amino acids from the protein and nitrogen we ingests. Because the microbes can use nitrogen to make protein, cows can eat urea and other sources of non-protein nitrogen that would kill non-ruminants. The microbes also make vitamins B and C.
When cows ruminate, or chew their cud, they are regurgitating a bolus of incompletely chewed feed. In order for the microbes to digest fiber rapidly and efficiently it must be in small pieces, so we re-chew our food several times. Cows also belch giving off carbon dioxide and methane. When cows “lose their cud” or stop ruminating, it is an indication that they have a digestive upset, and their rumen is not functioning properly.
Bloat is another condition that occurs when cows can’t eructate. It is caused by a rapid change in food or overeating grain (gaseous bloat) or grazing pure stands of clover or alfalfa (frothy bloat). Gaseous bloat is a result of improper digestion or fermentation of grain. It is treated by passing a tube into the rumen or using a trocar and cannula to make an external opening in the rumen to release the gas pressure. The procedure may have to be repeated. Frothy bloat is a result of surfactants in legumes causing gas to be trapped in a bubbly foam. Large amounts of mineral oil must be forced into the rumen via a tube to break up the bubbles as a treatment for frothy bloat. Bloat must be treated quickly as the increased rumen size and pressure interferes with normal breathing. The incidence of bloat in cows grazing legumes can be reduced by maintaining at least 50% of the stand as grass. Also, cows should not be turned out onto a pasture with a high percentage of legumes when cows are hungry or the pasture is wet. Once the cows are adapted to legume/grass pastures, they can graze it even when wet. A final option is to use “bloat guard” blocks which contain poloxolene.
Although rumen microbes can digest a great variety of different feeds, they are very sensitive to drastic changes in feeds. Some groups of microbes are better at digesting fiber (forages), whereas others are better at digesting starch (grains). Changing rapidly from a forage-based diet to a grain-based diet causes millions of fiber-digesting microbes to die-off as they cannot digest the starch, and there are too few starch-digesting microbes to use the grain so the grain sours in the rumen. As a result, rumen pH decreases, the rumen stops working, and the animal becomes ill. In severe cases, the cow can develop acidosis and founder or die.
The reticulum:
The reticulum, with its honeycomb-like lining, is a compartment of the stomach that is involved with rumination. It also acts as a trap for foreign objects taken in by the cow. It is not unusual to find rocks, nails, and pieces of wire and metal in our reticulum. (Not that we're particularly stupid to eat these; it's just that sometimes these things get mixed up with the grass) If wire or metal punctures the side of the reticulum, it can cause “hardware disease.” Hardware disease is actually an irritation or infection of the diaphragm, heart or lungs. It is hard to treat, but can be prevented by keeping metal trash out of pastures. Specially shaped magnets can be administered to cows to decrease the possibility that ingested metal will pierce the digestive tract. These magnets stay in the reticulum for the life of the animal. Fortunately, Mr. Farmer had inserted one of these magnets into my reticulum, so I wouldn't encounter an infection any time soon!
The omasum:
The omasum is also known as “the book” or many piles because of its many leaf-like folds. It functions as the gateway to the abomasum, filtering large particles back to the reticulorumen and allowing fine particles and fluid to be passed to the abomasum. Though the complete function of this compartment is unknown, it does aid in water resorption and recycling of buffers for the saliva. The omasum may also absorb some volatile fatty acids.
The abomasum:
The abomasum is also known as the “true stomach.” It functions much like the human stomach producing acid and some enzymes to start protein digestion. Animals that go off feed or have acidosis can develop a displaced abomasum or “twisted stomach.” The abomasum will actually float out of place and become torsioned stopping the flow of digesta. Surgery is the only cure for a displaced abomasum. Although displaced abomasum is more common in dairy cattle than beef cattle, producers should be aware of the possibility of this problem in cows that have had severe digestive upsets.
Well, anyway, it's grazing time already. I'll be right back with more information on the other parts of a cow's digestive system!
-MooMoo
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