1) digestion in the dairy cow - Babcock Institute - University of ...

Babcock Institute for InternationalDairy Research and DevelopmentUniversity of Wisconsin-MadisonDairyEssentials1) DIGESTION IN THE DAIRY COWMichel A. WattiauxBabcock InstituteW. Terry HowardDepartment of Dairy ScienceINTRODUCTIONDairy cows and other animals such assheep, goats, buffaloes, camels and giraffesare herbivores because their diets arecomposed primarily of plant material.Many herbivores also are ruminants.Ruminant animals can be recognized easilybecause they chew frequently even whenthey are not eating. This chewing activitycalled rumination, is part of the processesthat allow a ruminant to obtain energy fromplant cell walls also called fiber.Large IntestineSmall IntestinePelvicboneBladderRumenAbomasumOmasumEsophagusADAPTATION TO UTILIZE FIBER ANDNON-PROTEIN NITROGENFiber is the structure than gives strengthand rigidity to plants and is the mainconstituent of grass stems and other plantstems. Complex sugars (cellulose,hemicellulose) are locked within the plantcell wall and remain inaccessible for nonruminant animals. However, the microbialpopulation that lives in the reticulum andthe rumen (Figure 1) allows the cow toobtain energy from the fiber.The nitrogen requiredin the rationof a cow comes fromthe amino acidsfound in proteinsReticulumMouthFigure 1: The digestive system of a cow includes four stomachs. Therumen is similar to a lake with a river running through one corner.and other sources ofnon-protein nitrogen(NPN). Nonproteinnitrogencompounds cannotbe used by nonruminants,but maybe used by ruminalbacteria for thesynthesis of proteins.The majorityof the amino acidsavailable to the cowcomes from thebacterial proteinsynthesized in therumen.240 Agriculture Hall, 1450 Linden Dr., Madison, WI 53706 USA, phone: 608-265-4169, babcock@calshp.cals.wisc.edu 1

Dairy Essentials—Nutrition and FeedingExampleof feedTable 1: Utilization of various energy andnitrogen sources in ruminants and nonruminantsNonruminant(hog, bird)Ruminant(cow,sheep)ENERGYSugars Molasses + +Starch Roots + +Cellulose Straws 0 ±NITROGENNPN 1 Urea 0 +Protein Soybean + +1 NPN = non protein nitrogen+ completely available, ± partially available,0 not availableTHE FOUR STOMACHSReticulum and rumenThe reticulum and the rumen are the firsttwo stomachs of ruminants. The contentsof the reticulum is mixed with that of therumen almost continuously (once everyminute). Both stomachs, often referred toas the reticulo-rumen, share a densepopulation of micro-organisms (bacteria,protozoa, and fungi).The rumen is a large fermentation vesselthat can contain as much as 100 to 120 kg ofdigesting material. Fiber particles remainin the rumen from 20 to 48 hours becausebacterial fermentation of fiber is a slowprocess. However, particles that digestfaster tend to stay in the rumen for a shorterperiod of time.The reticulum is the "crossroad" whereparticles entering or leaving the rumen aresorted. Only particles that are small is size(< 1-2 mm) and dense (> 1.2 g/ml) maymove on to the third stomach.OmasumThe third stomach or omasum is round(Figure 1) and has a capacity of about 10liters. The omasum is a small organ withgreat absorption capacity. It allows therecycling of water and minerals such assodium and phosphorus which return tothe rumen through the saliva. Since themodes of digestion in the rumen and theabomasum differ drastically, the omasumacts as an organ of transition between thesetwo organs. The omasum is not essential,however, as it is absent in camels, llamasand alpacas (pseudoruminants).AbomasumThe fourth stomach is the abomasum. Thisstomach is like the stomach of nonruminants.It secretes a strong acid andmany digestive enzymes. In nonruminants,ingested feeds are first digestedin the abomasum. However, the materialentering the abomasum of a ruminant ismade up primarily of unfermented feedparticles, some end-products of microbialfermentation and microbes which grew inthe rumen.THE RUMINAL BACTERIAThe rumen provides a suitableenvironment with generous food supply formicrobes to grow and reproduce. Theabsence of air (oxygen) in the rumen favorsthe growth of some particular species ofbacteria, among them are those that candegrade plant cell walls (cellulose) intosimple sugars (glucose). The microbesferment glucose to obtain energy to growand they produce volatile fatty acids (VFA)as end-products of fermentation. The VFAcross the rumen wall and become the majorsources of energy to the cow.As ruminal microbes grow, theysynthesize amino acids, the building blocksof proteins. Bacteria can use ammonia orurea as nitrogen sources to build aminoacids. Without bacterial conversion,ammonia and urea would be useless to thecow. However, bacterial proteinssynthesized in the rumen are digested inthe small intestine and constitute the majorsource of amino acids for the cow.2 The Babcock Institute

1 - Digestion in the Dairy CowTHE ORGANS OF THE DIGESTIVE TRACT AND THEIR FUNCTIONS1 - Rumination (break down of particles) and saliva (buffers) production• Rumination reduces particle size of fiber and exposes sugars tomicrobial 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 notstimulated (too much concentrate in the diet).• Buffers in the saliva (bicarbonate and phosphates) neutralize theacids produced by microbial fermentation to maintain a neutralacidity which favors fiber digestion and microbial growth in therumen.GasesFibrous mat oflong particles;;;;;;;Small particles in;a ;liquid suspension;;; ;;2 - Reticulo-rumen (fermentation)• Retention of long forage particles thatstimulate rumination.• Microbial fermentation produces: 1) volatilefatty acids (VFA) as end-products of thefermentation of cellulose and other sugars and2) a microbial mass rich in a high qualityprotein.• Absorption of VFA through the rumen wall.The VFA are used as the major energy sourcefor the cow and also for the synthesis of milkfat (triglycerides) and milk sugar (lactose).• Production and expulsion through belching ofas many as 1000 liters of gases per day.3 - Omasum (recycling of some nutrients)• Absorption of water, sodium, phosphorus and residual VFA.4 - Abomasum (acid digestion)• Secretion of strong acids and digestive enzymes.• Digestion of feed fractions not fermented in the rumen (someproteins and lipids).• Digestion of bacterial proteins produced in the rumen (0.5 to 2.5kg per day).5 - Small intestine (digestion and absorption)• Secretion of digestive enzymes by the small intestine, the liverand the pancreas.• Enzymatic digestion of carbohydrates, proteins and lipids.• Absorption of some water, minerals and products of digestion:glucose, amino acids and fatty acids.6 - Cecum (fermentation) and large intestine• A usually small microbial population ferments the unabsorbedproducts of digestion.• Absorption of water and feces formation.University of Wisconsin-Madison 3

Dairy Essentials—Nutrition and FeedingSOME DEFINITIONSAbsorption is the passage of the productsof digestion and other simple compoundsfrom the digestive tract into the blood.Buffers are compounds secreted by thesaliva or added to the diet that help tomaintain a stable rumen, and promote feeddigestion and bacterial growth.Digestion is the first step in a series ofprocesses that break down complexparticles (feed or microbes) into simplesubstances that can be used by the body. Astrong acid and many digestive enzymesare secreted into the digestive tract to digestfood.Metabolism refers to the changes that theabsorbed products of digestion (nutrients)undergo during their utilization by thebody. Nutrients may be degraded bytissues of the body to obtain energy tomaintain vital functions, and to accomplishwork (eating, walking, ruminating, etc.).Nutrients also may be used as precursorsfor the synthesis of tissues (muscle, fat) andin the case of a dairy cows the synthesisof milk.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 to convert fibrous feed(forages, crop residues and industrial by-products) and non-protein nitrogen (ammonia,urea) into highly nutritious and palatable food for humans (milk, meat).• Fibrous feed is necessary for the health of the cow because it maintains rumination andsaliva production which are necessary for the proper function of the rumen and to obtainthe 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 offour 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) andinorganic matter (Nitrogen, Phosphorus and Potassium), which are excellent fertilizers.4 The Babcock Institute