RUMINANT DIGESTIVE SYSTEM

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Pengantar: Herbivorous Strategies• Sekitar 50% dari karbon organik yg ada di bumi ini terikat dalam bentuk

selulosa. Namun sel tubuh hewan vertebrata tidak menghasilkan enzim ygdapat mencerna selulosa ini.

• Untunglah ada sejumlah mikroba yg memproduksi selulase sehingga merekadapat menggunakan sesulosa sebagai sumber energi dan beberapa darimikroba selulolitik ini hidup di dalam saluran pencernaan hewan. Hewan-hewan tsb telah beradaptasi sedemikian rupa sehingga mampumemanfaatkan produk fermentasi mikrobial yg berlangsung di dalam saluranpencernaan mereka.

• Manfaat relatif fermentasi mikrobial bagi nutrisi hewan sangat tergantungukuran wadah di mana fermentasi mikrobial tsb berlangsung. Pd anjing danmanusia fermentasi mikrobial terjadi di dalam kolon yang berukuran sangatkecil sehingga manfaatnya tidak signifikan. Sebaliknya, pada hewan herbivorafermentasi terjadi secara masif (besar) sehingga sumbangannya sangatsignifikan.

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Di antara hewan herbivora (professional fermentors), ada duastrategi fermentasi yg berkembang, di mana perbedaan utamanyaterdapat pd posisi relatif lokasi fermentasi, yaitu :

• Cranial fermentors or ruminants: saluran pencernaan antaraesofagus dan lambung berukuran besar dan memiliki banyakkompartemen (ruangan), disebut forestomachs, yg menyediakanekosistem kompleks yg mendukung fermentasi. Contoh: sapi,domba, rusa dll.

• Caudal fermentors: disebut juga cecal digestors, memiliki lambung dan usus halus yang mirip dgn anjing dan manusia, namun usus kasarnya berkembang sangat besar dan mendukung fermentasi mikrobial. Contoh: kuda dan kelinci.

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• Proses dan produk fermentasi pada rumen sapi dan cekum kudapd dasarnya adalah identik. Namun, posisi relatif wadahfermentasi terhadap usus halus memiliki implikasi pentingterhadap fisiologi dan nutrisi hewan yg bersangkutan. Kemiripandan perbedaannya diringkas sebagai berikut:

Function RuminantsCecal

Digestors

Kemampuan mencerna dan mengekstrak energi

secara efisien dari selulosaYa Ya

Kemampuan menggunakan karbohidrat heksosa (a.l

glukosa) pakan secara langsung

Tidak Ya

Kemampuan menggunakan protein yg berasal dari

mikroba fermentatif

Ya Tidak

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THE FOUR STOMACHS

The ruminant stomachs, as seen from the right side 5

Reticulum and rumen

• The reticulum and the rumen are the first two stomachs of ruminants. The contents of the reticulum is mixed with that of the rumen almost continuously (once every minute).

• Both stomachs, often referred to as the reticulo-rumen, share a densepopulation of micro-organisms (bacteria, protozoa, and fungi).

• The rumen is a large fermentation vessel that can contain as much as 100 to 120 kg of digesting material. Fiber particles remain in the rumen from 20 to 48 hours because bacterial fermentation of fiber is a slow process. However, particles that digest faster tend to stay in the rumen for a shorter period of time.

• The reticulum is the "crossroad" where particles entering or leaving the rumen are sorted. Only particles that are small is size (< 1-2 mm) and dense (> 1.2 g/ml) may move on to the third stomach.

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The interior surface of the rumen forms numerous papillae that vary in shape and size from short and pointed to long and foliate.

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Reticular epithelium is thrown into folds that form polygonal cells that give it a reticular, honey-combed appearance. Numerous small papillae

stud the interior floors of these cells.

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Omasum

• The third stomach or omasum is round (Figure 1) and has acapacity of about 10 liters. The omasum is a small organ with greatabsorption capacity.

• It allows the recycling of water and minerals such as sodium andphosphorus which return to the rumen through the saliva.

• Since the modes of digestion in the rumen and the abomasumdiffer drastically, the omasum acts as an organ of transitionbetween these two organs.

• The omasum is not essential, however, as it is absent in camels,llamas and alpacas (pseudoruminants).

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The inside of the omasum is thrown into broad longitudinal folds or leavesreminiscent of the pages in a book (a lay term for the omasum is the 'book'). Theomasal folds, which in life are packed with finely ground ingesta, have been estimatedto represent roughly one-third of the total surface area of the forestomachs.

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Abomasum

• The fourth stomach is the abomasum. This stomach islike the stomach of nonruminants.

• It secretes a strong acid and many digestive enzymes. Innonruminants, ingested feeds are first digested in theabomasum. However, the material entering theabomasum of a ruminant is made up primarily ofunfermented feed particles, some end-products ofmicrobial fermentation and microbes which grew in therumen.

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THE RUMINAL BACTERIA

• The rumen provides a suitable environment with generous foodsupply for microbes to grow and reproduce.

• The absence of air (oxygen) in the rumen favors the growth of someparticular species of bacteria, among them are those that candegrade plant cell walls (cellulose) into simple sugars (glucose).

• The microbes ferment glucose to obtain energy to grow and theyproduce volatile fatty acids (VFA) as end-products of fermentation.The VFA cross the rumen wall and become the major sources ofenergy to the cow.

• Fermentation is supported by a rich and dense collection ofmicrobes. Each milliliter of rumen content contains roughly 10 to 50billion bacteria, 1 million protozoa and variable numbers of yeastsand fungi.

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• As ruminal microbes grow, they synthesize amino acids, thebuilding blocks of proteins.

• Bacteria can use ammonia or urea as nitrogen sources to buildamino acids. Without bacterial conversion, ammonia and ureawould be useless to the cow.

• Bacterial proteins synthesized in the rumen are digested inthe small intestine and constitute the major source of aminoacids for the cow.

The micrograph of sheep rumenal fluid, shows a Gulliver-like ciliated protozoon

in the midst of thousands of bacteria (the small specks). 13

Bacteria

• Fermentative bacteria representing many genera provide a comprehensivebattery of digestive capabilities. These organisms are often classified by theirsubstrate preferences or the end products they produce. Although there issome specialization, many bacteria utilize multiple substrates. Some of themajor groups, each of which contain multiple genera and species, are:

> Cellulolytic (digest cellulose)

> Hemicellulolytic (digest hemicellulose)

> Amylolytic (digest starch)

> Proteolytic (digest proteins)

> Sugar utilizing (utilize monosaccharides and disaccharides)

> Acid utilizing (utilize such substrates as lactic, succinic and malic acids)

> Ammonia producers

> Vitamin synthesizers

> Methane producers14

Protozoa• Protozoa, predominantly ciliates, appear to contribute

substantially to the fermentation process. Several experimentshave demonstrated that lambs and calves deprived of theirruminal protozoa show depressed growth rates and are relative"poor-doers" compared to controls with both bacteria andprotozoa. In general, protozoa utilize the same set of substrates asbacteria and, as with bacteria, different populations of protozoashow distinctive substrate preferences. Many utilize simple sugarsand some store ingested carbohydrate as glycogen.

• An interesting feature of some protozoa is their inability toregulate glycogen synthesis: when soluble carbohydrates are inabundance, they continue to store glycogen until they burst. Anadditional feature of protozoa is that many species consumebacteria, which is thought to perhaps play a role in limitingbacterial overgrowth.

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• The distribution of microbial species varies with diet. Someof this appears to reflect substrate availability; forexample, populations of cellulolytic bugs are depressed inanimals fed diets rich in grain.

• Environmental conditions in the fermentation vat also canhave profound effects on the microbial flora. Rumen fluidnormally has a pH between 6 and 7, but may fall if largeamounts of soluble carbohydrate are consumed. If pHdrops to about 5.5, protozoal populations becomemarkedly depressed due to acid intolerance. More drasticlowering of rumen pH, as can occur with grainengorgement, can destroy many species and have seriousconsequences to the animal.

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The digestive system of a cow includes four stomachs.The rumen is similar to a lake with a river running through one corner.

THE ORGANS OF THE DIGESTIVE TRACT AND THEIR FUNCTIONS

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• Rumination reduces particle size of fiber and exposes sugars to microbial fermentation.

• Production of 160 to 180 liters of saliva when a cow chews 6 to 8hours per day, but less than 30 to 50 liters if rumination is not stimulated (too much concentrate in the diet).

• Buffers in the saliva (bicarbonate and phosphates) neutralize the acids produced by microbial fermentation to maintain a neutral acidity which favors fiber digestion and microbial growth in the rumen.

1.

Rumination (breakdown of particles)and saliva (buffers)production

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• Retention of long forage particles thatstimulate rumination.

• Microbial fermentation produces:

1) volatile fatty acids (VFA) as end-products

of the fermentation of cellulose and

other sugars and

2) a microbial mass rich in a high quality

protein.

• Absorption of VFA through the rumen wall.The VFA are used as the major energysource for the cow and also for the synthesis of milk fat (triglycerides) and milk sugar (lactose).

• Production and expulsion through belching of as many as 1000 liters of gases per day.

2.

Reticulo-rumen (fermentation)

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• Absorption of water, sodium, phosphorus and residual VFA.

3.

Omasum (recycling of

some nutrients)

4.Abomasum

(acid digestion)

• Secretion of strong acids anddigestive enzymes.

• Digestion of feed fractions not fermented in the rumen (someproteins and lipids).

• Digestion of bacterial proteins produced in the rumen (0.5 to 2.5 kg per day).

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• Secretion of digestive enzymes bythe small intestine, the liver and thepancreas.

• Enzymatic digestion ofcarbohydrates, proteins and lipids.

• Absorption of some water, mineralsand products of digestion: glucose,amino acids and fatty acids.

5.

Small intestine (digestion and absorption)

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Cecum (fermentation) and large intestine • A usually small microbial population

ferments the unabsorbed productsof digestion.

• Absorption of water and fecesformation.

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Equine Gastrointestinal

Tract

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IN PRACTICE• Ruminant animals can use a greater variety of food sources than non-ruminant

animals. The microbes living within the reticulo-rumen allow ruminants toconvert fibrous feed (forages, crop residues and industrial by-products) andnon-protein nitrogen (ammonia, urea) into highly nutritious and palatablefood for humans (milk, meat).

• Fibrous feed is necessary for the health of the cow because it maintainsrumination and saliva production which are necessary for the proper functionof the rumen and to obtain the desired bacterial population within the rumen.

• A cow can eat forages (low energy feed) and concentrates (usually high energy feed). However, large additions of concentrates to a ration should be gradual (over a period of four to five days) to allow the population of bacteria in the rumen to adapt to the new diet.

• The feces of ruminants are rich in organic matter (undigested microbial debris) and inorganic matter (Nitrogen, Phosphorus and Potassium), which are excellent fertilizers.

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