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Chapter 2 Digestion and absorption.

Through painstaking research, chemists and physiologists have been able to gain much information on the various steps in the digestion of food. When the nutrients leave the digestive tract and enter the body, the difficulties of learning what becomes of them are much greater.

F.B. MORRISON, 1949

INTRODUCTION

The digestive systems of livestock are diverse in anatomy but the function of the digestive system of all animals is essentially the same: to extract and absorb the nutrients in ingested feedstuffs. To this end, digestive organs of livestock reduce feed particle size through physical and chemical action. Populations of microbes live within the digestive systems of all animals and play some role in digestion in all livestock, but especially in the ruminant, where a digestive organ has evolved specifically to accommodate the microbial population.

THE DIGESTIVE SYSTEM

The digestive system of an animal connects the animal's diet with the metabolic activities that make life possible. It is essentially a muscular tube that runs from the mouth to the anus. Animals with molar teeth begin the process of digestion by grinding the feed material. Muscles of the tube grind, mix, and move the feed. At various locations along the tube are glands that manufacture products secreted into the tube. In ruminant animals, an expanded area along the digestive tube harbors a population of microbes that assist in digestion through the fermentation of feed materials. The activities of the microbes in the rumen also include the synthesis of essential nutrients for their host. All animals have microbial populations living in their digestive tubes, and most if not all animals can potentially benefit from the activity of these microbes. In the dog, for example, the microbes inhabiting the small intestine make a contribution to overall starch utilization through their fermentation activities (Murray et al., 2001).

DIGESTION

The actions and secretions of the digestive system work to degrade (digest/ ferment) feed material in the tube into absorbable compounds and to synthesize essential nutrients from ingested feed. Absorbable products of the digestion include monosaccharides, amino acids, peptides, fatty acids, monoglycerides, glycerol, vitamins, and salts. Only if feed is reduced to these compounds will the animal benefit from being fed.

ABSORPTION

Only if the compounds produced by actions of the digestive system are actually absorbed will the consumed ration be successful in meeting the animal's nutrient requirements. Absorption involves moving these compounds from inside the lumen of the digestive tube across the wall of the digestive tube, and making them available to the body's tissues. Nutrients may be absorbed by active transport, facilitated diffusion, or diffusion. In active transport, energy is required to absorb the nutrient. In facilitated diffusion, energy is not needed, but absorption will not occur without a specific carrier molecule. In diffusion, energy is not needed and the nutrient moves from the area of high concentration (inside the digestive tube) to the area of lower concentration (the cytoplasm of the cells lining the digestive tube).

Most nutrient absorption takes place across the wall of the small intestine. However, some nutrients may be absorbed before reaching the small intestine, as in the ruminant (Figure 2-1), and some nutrients may be absorbed beyond the small intestine. The latter situation is particularly important in some monogastrics (Figure 2-2).

[FIGURE 2-1 OMITTED]

During absorption in the small intestine, nutrients move inside the cells lining the lumen of the tube. The opportunity for absorption is maximized through the arrangement of these cells in finger-like projections called villi (singular villus). Figure 2-3a is a diagram representing the villi. From inside the cells of the villus, nonfat nutrients then pass into blood capillaries of the villus and are transported via the hepatic portal system to the liver for possible modification before entering the blood circulation. Figure 2-3b illustrates this activity.

Most digested fat, however, is not absorbed in this way. Most fatty acid and monoglyceride molecules move into the epithelial cells of the villus, as do carbohydrates and amino acids. As illustrated in Figure 2-3b, instead of being picked up in the blood circulation serving the villus, these products of fat digestion enter a lacteal within the villus. The lacteal is part of the lymphatic system. Fat absorbed into the lacteal is carried in the lymphatic circulation from the digestive system directly to a major vein, bypassing the liver. The fact that absorbed fat does not pass through the liver is important when considering the nutrition-related diseases of ketosis and fatty liver.

The efficiency with which animals absorb feed nutrients is an important economic issue for livestock feeders. As the passage rate of feed material through the digestive tube increases, the efficiency of absorption declines, and more of the nutrients in the tube will pass out in manure. Given an increased feed passage rate, the various methods of feed processing become increasingly economical. These processes aid the digestive system in degrading feed into absorbable compounds so less resident time in the digestive tract is needed.

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DIGESTIVE ORGANS

The anatomy of the digestive tract determines the type of feed that is nutritionally useful for a particular species. Meat-eating animals (carnivores) have digestive tracts that are relatively short and low in volume, whereas in planteating species (herbivores), the digestive tract volume is relatively large. Diagrams of livestock digestive tracts with capacity data are shown in Figures 2-4a through 2-4i. Images and supplemental information on livestock digestive tracts are found in the file titled Images.ppt on this text's companion CD.

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[FIGURE 2-4i OMITTED]

The Mouth, Saliva Glands, and Esophagus

For those animals that chew, the initial grinding (mastication or chewing) of feed takes place in the mouth. Mastication functions to reduce feed particle size and to mix feed with saliva to form a bolus for swallowing. In ruminant animals, complete mastication of regurgitated ruminal contents (rumination) occurs while ruminants are resting.

Saliva is produced by three pairs of glands in most livestock. These include the parotid, the mandibular, and the sublingual. Saliva plays many important roles in the process of digestion. A summary of saliva functions follows:

* Moistens the feedstuffs for chewing and bolus formation

* Lubricates the bolus for easy swallowing and passage down the esophagus

* Helps provide digestive action. In some monogastric species, the saliva contains enzymes that begin the process of enzymatically digesting the carbohydrate and fat in feed.

* Helps maintain water balance. A mature dairy cow will produce upwards of 50 gallons of saliva each day, about 98.7 percent of which is water (Church, 1988).

* Contains buffering compounds. For ruminant animals, these are essential in maintaining rumen health.

* Contains recycled nitrogen, sodium, potassium, calcium, magnesium, phosphorus, and chlorine (Table 2-1) (McDougall, 1948). For ruminant animals, these serve as a source of nutrients for bacteria and protozoa in the rumen.

There are significant differences in mouth anatomy and physiology among livestock.

Chickens do not have teeth. Trout have teeth but lack molar teeth for chewing. Ruminants lack incisor teeth on the upper jaw.

The lips, tongue, teeth, and beak are organs of prehension for animals when eating. They are used to pick up feed to be eaten. The lips and tongue are responsible for animals being able to sort through a mix of feed and eat only selected portions. Among all livestock, the lips of goats are the most prehensile and enable them to select only the most palatable portions of plants when grazing. In the chicken, lips and teeth are replaced by a horny mandible on each jaw, forming the beak.

As food is chewed and mixed with saliva, a wad of moist feed material called a bolus is formed. The tongue is used to move the bolus to the back of the mouth and down the esophagus during swallowing. The muscle in the wall of the esophagus contracts reflexively in response to the bolus of feed material. Peristalsis involves alternate relaxation and contraction of rings of muscle in the wall of the esophagus, coupled with contraction of longitudinal muscles in the area of the bolus.

The Forestomachs of the Ruminant and the Crop of Poultry

The reticulum, rumen, and omasum are called stomachs because of their location, not because of their function; they lack the secretory function that characterizes a true stomach. Their function is similar to that of the crop of a chicken; both are feed storage organs, and while in storage, feed is exposed to bacterial activity.

The Crop

The crop of poultry is a pouch, formed as a specialized area of the esophagus. The primary function of the crop is storage of ingested feed material. While in storage, feed begins to be degraded (fermented) by bacteria that inhabit the crop. In terms of both qualitative and quantitative output, the bacterial activity occurring in the crop is not as productive as that occurring in the rumen, and it is usually ignored as a source of nutrients for the bird.

The Reticulum or Honeycomb

The reticulum is a compartment of the ruminant stomach. There is a reticulated or honeycomb-like pattern on the tissue of the inner wall that distinguishes the reticulum from the other compartments of the ruminant stomach. The reticulum is separated from the rumen by a low pillar of tissue. The reticulum is much smaller than the rumen and aids in rumination and particularly regurgitation. During regurgitation, a bolus of coarse feed material is sent up the esophagus to the mouth for rechewing. As a human food item, the reticulum of cattle is called tripe.

The reticulum is located where heavy materials, such as pieces of metal, often become lodged. Ingested bits of wire and nails may penetrate the wall of the reticulum and slowly move through the diaphragm into the pericardial sac causing a condition described as pericarditis or "hardware disease." Feed mills pass feed ingredients over powerful magnets during processing so that stray bits of metal are removed from mixes. Nonmagnetic metals like aluminum will not be removed by magnets, though these metals are not in widespread use in feed manufacturing.

[FIGURE 2-5 OMITTED]

The exit point for feed material leaving the reticulum and rumen is located low on the wall of the reticulum where there is a connection with the omasum (Figure 2-5). The opening from the reticulum to the omasum is small, and thus larger particles in the rumen-reticulum are retained for regurgitation, chewing, and more complete fermentation.

Rumen or Paunch

Ingested feed arriving at the rumen is mixed to establish good contact with the microorganisms inhabiting the rumen and reticulum. These microorganisms are primarily bacteria, some protozoa, and a relatively small number of fungi. The bacteria colonize particles of feed and produce digestive enzymes. These enzymes contribute to fermentation of feed material, including cellulose, thereby releasing nutrients on which the microbes grow and reproduce. Fermentation also results in the synthesis of nutrients utilized by the ruminant. Bacterial enzymes break down the complex carbohydrates (such as cellulose and starch) by fermenting them into short-chain fatty acids. Acetic, propionic, and butyric acid are the primary short-chain fatty acids produced, and as a group, these are referred to as volatile fatty acids. The volatile fatty acids are absorbed directly from the rumen and reticulum and used as energy sources and for milk fat synthesis.

Similarly, protein in feed is broken down into peptides, amino acids, ammonia, and amines. The microorganisms use these substances as building blocks for their own cells. Eventually, the microorganisms are passed down the intestinal tract, digested, and used as a protein source by the ruminant.

Other end products of rumen fermentation are B vitamins and vitamin K. Thanks to the activities of the microbes inhabiting the rumen, ruminant animals do not require these nutrients in their diets.

The gases carbon dioxide (C[O.sub.2]) and methane (CH4) are also produced during rumen fermentation. Gas production accounts for 13 to 15 percent of the carbon consumed. The motility of the reticulum and rumen ensures that the gases produced make their way into the thoracic esophagus so that they can be belched or eructated out the mouth. Interference with the process of eructation results in a serious buildup of pressure in the reticulo-rumen and is called bloat.

The exit of undigested feed residue from the reticulo-rumen leads to the omasum; this location is termed the ostium. Feed particles too large to pass through the ostium are repeatedly returned to the mouth during regurgitation for rechewing to reduce particle size and make them more accessible to the microbes. Feed material that is fed and ingested as small, dense particles may leave the rumen after only a few minutes, whereas larger, less dense particles may spend several days in the rumen.

Omasum or Manyplies

The omasum of ruminant animals is a many-walled organ where part of the water is removed from the mass leaving the rumen and reticulum. By removing water and passing on a drier feed material, the omasum allows the secretions of the following chamber, the abomasum or true stomach, to be more effective in digestion.

The Abomasum, Proventriculus, or True Stomach

The true stomach of ruminant animals is called the abomasum. In poultry, the true stomach is called the proventriculus. Channel catfish and rainbow trout have a true stomach, but this is not the case for all commercially important fish species.

The walls of the true stomach contain glands made up of cells that manufacture and secrete substances that aid in the digestion of the different feed components. The walls of the true stomach contain parietal cells that secrete hydrochloric acid. The chief cells of the stomach wall secrete at least two enzymes that function in digestion. In young mammals, the chief cells secrete gastric lipase, which functions in fat digestion. The production of gastric lipase is less important in the adult. The chief cells also secrete enzymes to aid protein digestion. Pepsinogen is secreted by the chief cells of the true stomach. Pepsinogen is the zymogen (precursor) of the protein-digesting enzyme, pepsin. Pepsin is not secreted directly (to avoid the enzyme from digesting the protein tissues of the true stomach itself). The conversion of pepsinogen to pepsin is initiated by the presence of acid and active pepsin in the stomach contents. In young ruminants, stomach glands are responsible for the secretion of another protein-digesting enzyme, rennin (converted from the zymogen, prorennin). The semifluid mixture of digestive secretions and partially digested feed material in the true stomach is called chyme.

The Ventricular Groove

In ruminant animals, the wall of the reticulum participates in a shunt that permits liquids to bypass the rumen and reticulum. This shunt is described as the ventricular groove (formerly esophageal groove). It appears to be most important in preweaned ruminants, allowing milk to avoid the immature reticulum and rumen and move directly to the omasum and on to the abomasum (Figure 2-5). Glands in the wall of the abomasum are responsible for the production of enzymes (rennin and pepsin) and acid (hydrochloric acid) that cause the coagulation of milk proteins. This coagulation results in the formation of a curd in the abomasum. The curd contains protein and embedded fat. The curd is retained in the abomasum and digested over a 12- to 18-hour period (Merchen, 1988). Most ruminant nutritionists believe that curd formation in the abomasum is necessary for optimal digestion of liquid feed consumed by the preweaned calf.

The Gizzard

The poultry stomach is actually made up of two parts: the glandular proventriculus and the muscular ventriculus or gizzard. The gizzard connects the proventriculus and the small intestine. Its function is to grind coarse feed. This process may be aided by the presence of grit consumed in the diet. Grit is unnecessary if poultry are fed uniformly ground diets.

The Pancreas

The beginning of the small intestine is folded into a loop called the duodenum. Inside this loop lies the pancreas. The pancreas secretes digestive juices into the duodenum. These juices contain buffers to neutralize the hydrochloric acid added to the chyme in the true stomach. Pancreatic secretions also contain enzymes that degrade proteins (proteases), starches (amylases), and fats (lipases). Note that the pancreas does not produce enzymes capable of degrading and digesting cellulose.

The Liver

The liver produces bile, which contains salts that aid in the preparation of fats for absorption in the small intestine. In most animals, the bile made in the liver is stored in the gall bladder. The presence of feed in the duodenum causes the gall bladder to discharge its bile into the duodenum. Because horses lack a gall bladder, the bile is discharged directly from the liver into the duodenum.

The Small Intestine and Midgut

The pylorus is a sphincter muscle that prevents premature movement of the feed out of the stomach and into the small intestine. In fish, the structure downstream from the stomach is termed the midgut. In the midgut, trout have numerous out-pocketings called pyloric ceca. The role of pyloric ceca in fish digestion is unclear, though histologically, they are similar to the intestine. Pyloric ceca are absent in channel catfish.

The small intestine is normally considered to have two distinct parts: the uppermost part, called the duodenum, and the lower small intestine, which includes the jejunum and ileum. In swine nutrition, amino acid bioavailability is expressed as true ileal digestibility. Determination of true ileal digestibility for a given amino acid in a given feedstuff involves measuring the proportion of that feedstuff's amino acid that has disappeared from the gut when digesta reach the terminal ileum. This proportion is the amino acid's apparent ileal digestibility for that feedstuff. Two corrections are then applied to get true ileal digestibility. The first correction involves accounting for the amino acids that are absorbed in a form that cannot be fully utilized by the pig. The second correction involves accounting for endogenous amino acid losses. The companion application to this text uses true ileal digestibility in expressing swine amino acid requirements and in expressing feedstuff amino acid content.

Peristalsis moves food material through the small intestine and, in fish, the pyloric ceca. Enzymes secreted into the duodenum by the pancreas and by glands in the intestinal wall, along with the fish's pyloric ceca, continue the digestive process by breaking down the fragments of proteins and carbohydrates produced earlier into absorbable amino acids and monosaccharides. Bile from the liver also enters the duodenum via the bile duct to assist in fat digestion and absorption. The population of bacteria residing in the small intestine may contribute to overall carbohydrate digestion. In the dog, fermentation by these bacteria may contribute to overall starch digestion.

The tissue of the small intestine facing the lumen is lined with small, fingerlike projections called villi. These villi greatly increase the absorptive area of the intestine. This tremendous surface area makes possible rapid absorption of nutrients. Most of the nutrients needed by an animal must be absorbed at the small intestine.

Absorption

Absorption of most nutrients takes place across the wall of the small intestine (Figures 2-1 and 2-2). In general, there are two types of chemical actions that are applied to each of the three categories of organic compounds to make absorption possible. The first chemical action involves breaking the large organic molecule into smaller, manageable fragments. The second involves clipping off small, absorbable pieces from each fragment. For example, in the case of starch, pancreatic amylase splits starch into smaller fragments, and alpha-dextrinase from glands of the small intestine clips off individual monosaccharide molecules from these fragments. In the case of proteins, the pancreatic enzymes of trypsin and chymotrypsin continue the work of pepsin (at work in the true stomach) to break proteins down into smaller fragments of peptides. Carboxypeptidase cleaves off individual amino acids from the peptides. In the case of fats, bile made in the liver (and in most animals, secreted from the gall bladder) is used to emulsify the fat in chyme. Bile is not an enzyme, but emulsification is prerequisite for effective enzyme action on fat. Pancreatic lipase acts to cleave two of the three fatty acids on a triglyceride, leaving absorbable fatty acids and monoglyceride. For additional information regarding nutrient absorption, see Chapters 6, 7, and 8.

The Large Intestine

The large intestine is generally considered to include the cecum, the colon, and the rectum. In fish, there is little to distinguish the different portions of the intestine, and the term hindgut corresponds to the large intestine. As with the esophagus and small intestine, peristalsis moves food material through the large intestine. There is more variation in appearance of the large intestine from one species to another than there is of the small intestine.

The Cecum

Ceca is plural for cecum. While poultry have two ceca, most other livestock have just one cecum. The cecum/ceca are pouches that lie at the juncture of the lower small intestine and the remaining parts of the large intestine. Whereas the chicken's ceca are approximately 5 inches in length, the horse's cecum is 4 feet long. Because these structures are blind sacks, there is little flow through them and microbial populations are able to become established in the ceca of animals. These microbes ferment the feed material to produce high-energy volatile fatty acids and B vitamins, both potentially valuable nutrients for their host. In the horse, rabbit, and other nonruminant herbivores, the cecum plays a significant role in the digestion of fiber in feed. Although some absorption of the products of fermentation undoubtedly occurs across the wall of the large intestine, the primary site of nutrient absorption is the small intestine (Figure 2-1 and Figure 2-2). This means that much of the products of fermentation will pass in the manure. Through the practice of coprophagy, the chicken--and especially the rabbit--may ingest a portion of their fecal material and thereby pass the microbial products manufactured in the cecum through the small intestine. Without coprophagy, it is unclear how much the chicken and nonruminant herbivores benefit from the microbial activity in the cecum.

The Colon and Rectum

Although the small intestine is the primary site of nutrient absorption, some nutrient absorption undoubtedly occurs across the wall of the large intestine.

The colon of the horse starts out as a large-diameter structure (the large colon), but narrows toward the rectum (the small colon). Colic in horses refers to abdominal pain, and it has various causes. One cause is a blockage at this location where the colon diameter narrows.

The large intestine does not exist as such in chickens and fish. Chickens do not have a colon and in fish, there is no large intestine distinct from the small intestine.

The rectum is a relatively straight tube and is readily dilated for storage of feces.

The Anus, Cloaca, and Vent

The anus is the junction of the terminal part of the digestive tract and the skin. The cloaca of poultry is a chamber common to the digestive, urinary, and reproductive passages. The cloaca opens externally at the vent.

SUMMARY

Chapter 2 describes the mechanisms and organs of digestion and absorption. The comparative digestive anatomy of the livestock species is described. Whereas herbivores possess long and complex digestive tracts, carnivores possess relatively short and simple digestive tracts. The ruminant animal has a specialized compartment in its digestive system to accommodate a fermenting population of microbes that assist in the extraction of nutrients from ingested feeds. It is apparent that livestock have evolved to use different strategies to achieve the same goal: extraction and absorption of the nutrients in ingested feedstuffs.

END-OF-CHAPTER QUESTIONS

1. Compare the ratio of digestive-tract length to body length in herbivorous animals to that of carnivorous animals. Compare this ratio in ruminants and nonruminant herbivores. Explain your findings.

2. Describe the difference between digestion and absorption.

3. What nutrients are produced by the microbial inhabitants of the digestive tract in ruminant animals?

4. What are the activities and functions of the three digestive chambers upstream from the abomasum in ruminant animals? Describe the function of the ventricular groove in ruminants.

5. What are the activities and functions of the true stomach? What is chyme?

6. What are the activities and functions of the crop? What are the activities and functions of the gizzard?

7. What are the activities and functions of the small intestine? What are the activities and functions of the large intestine?

8. Give three products of the pancreas and describe their functions.

9. Where is bile made? What role does bile play in digestion?

10. Use the following terms in a description of absorption of most nonfat nutrients: villus, blood capillary, hepatic portal system, liver, and blood circulation. Use the following terms in a description of fat absorption: fatty acid, villus, lacteal, lymphatic system, and blood circulation.

REFERENCES

Calhoun, M. L. (1954). Microscopic anatomy of the digestive system of the chicken. Ames, IA: Iowa State University Press. Retrieved 12/1/2002 from http://www.extension. iastate.edu/publications/PM1696.pdf

Church, D. C. (Editor). (1988). The ruminant animal, digestive physiology and nutrition, D. C. Church, p. 120. Englewood Cliffs, NJ: Prentice-Hall.

Gillespie, J. R. (1998). Animal science. Clifton Park, NY: Thomson Delmar Learning.

Ishler, V., Heinrichs, J., & Varga, G. (1996). From feed to milk: Understanding rumen function. Extension Circular 422. http://www.das.psu.edu/dcn/catnut/PDF/rumen.pdf

McDougall, E. I. (1948). Studies on ruminant saliva. Biochemical Journal. 43, 99-109.

Merchen, N. R. (1988). Digestion, absorption and excretion in ruminants. In The ruminant animal, digestive physiology and nutrition, D. C. Church, editor. P. 174. Englewood Cliffs, NJ: Prentice-Hall.

Morrison, F. B. (1949). Feeds and Feeding, 21st edition. Ithaca, NY: The Morrison Publishing Company.

Murray, S. M., Flickinger, E. A., Patil, A. R., Merchen, N. R., Brent, J. L., Jr., & Fahey, G. C., Jr. (2001). In vitro fermentation characteristics of native and processed cereal grains and potato starch using ileal chime from dogs. Journal of Animal Science 79, 435-444.

Pond, W. G., Church, D. C., & Pond, K. R. (1995). Basic animal nutrition and feeding. NY: John Wiley & Sons.
Table 2-1
Saliva composition of sheep

Constituent   Sheep Saliva (mg/100 ml)

N             20
Na            370-462
K             16-46
Ca            1.6-3.0
Mg            0.6-1.0
P             37-72
Cl            25-43
pH            8.4-8.7

From McDougall, E. I. (1948). Studies on ruminant saliva. Biochemical
Journal. 43, 99.
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Author:Tisch, David A.
Publication:Animal Feeds, Feeding and Nutrition, and Ration Evaluation
Geographic Code:1USA
Date:Jan 1, 2006
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