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 DIGESTION


COLLECTION: GOAT HANDBOOK
ORIGIN: United States
DATE INCLUDED: June 1992

Extension Goat Handbook

This material was contributed from collections at the National Agricultural Library. However, users should direct all inquires about the contents to authors or originating agencies.

DOCN 000000048
NO F-5
DIGESTION
G. F. W. Haenlein R. Caccese; U. of Delaware, Newark
P. H. Sammelwitz; U. of Delaware, Newark
Anatomy and Physiology

1 Digestion in the goat is the process by which ingested feed materials are broken down and readied for absorption through the gastrointestinal (GI) tract in order to furnish energy and building blocks needed by its metabolism. All feed sources originally are the result of plants utilizing the energy of the sun and the minerals of the soil, without which the herbivorous goats and other farm animals could not exist.

2 In the food chain, the primary consumers are the herbivorous animals. They have specially developed fore-stomachs and GI tracts that enable them to utilize cellulose and other forms of plant polysaccharides. They are also maintaining large symbiotic populations of bacteria and protozoa in these fore-stomachs, which can break up the complex structures of plant celluloses into digestible sugars and a variety of useful organic acids. Some energy from this breakdown is used by the bacteria to synthesize their own body mass, proteins and vitamins, which are then later utilized by the herbivore when the bacteria pass on down the GItract and are themselves digested by their host, e.g. the goat.

3 Herbivores are classified into: pre-gastric fermentors, ruminants or pseudo-ruminants; and post-gastric fermentors, whose cecum has evolved to function similarly to a degree as the forestomach of ruminants. Cows, sheep, goats and deer are ruminants, horses and rabbits are members of the second group.

4 Pre-gastric fermentation enables the most efficient utilization of roughages, which are fermented and chemically degraded before passing into the small intestine, where most nutrient absorption takes place. In post-gastric fermentation, the bacterial break-down occurs in the cecum, after roughages have passed through the small intestine. Such animals derive benefit from cecal fermentation by resorting to the practice of coprophagy (feces eating).

5 Feedstuffs basically consist of six kinds of nutritive materials: carbohydrates, fats, proteins, vitamins, minerals and water. Through digestion, they become available to the goat by way of absorption in simple chemical forms such as sugars, fatty acids, glycerol, amino acids and salts.

6 Mouth The initial step in digestion is prehension, the conveyance of food into the mouth. In the goat, the lips, teeth and tongue are the primary prehensile organs. The lips of goats and sheep are most important in seizing feed, while the cow makes more use of the tongue for grasping and tearing of grasses and forbs. Thus, the goat is capable of greater selectivity in its grazing. Goats, like other ruminants, lack an upper set of incisor teeth. Instead they possess a hardened dental pad, against which they bite.

7 Mastication, or chewing, is necessary for the reduction of feed to smaller particle sizes. Goats, exhibit an accentuated lateral movement of the jaws while chewing, which greatly increases the grinding action of the teeth. These lateral movements result in the molars developing a sharp, pointed surface on the inner edge of the lower teeth and on the outer edge of the upper teeth. Due to the fact that the upper jaw is wider than the lower jaw, only one side of the mouth can be used at a time to grind the feed.

8 While grazing or browsing, the feed is only chewed enough to be mixed with saliva to form a bolus of suitable size to swallow. Saliva, which is a mixture of serous and mucous fluids, is released by the stimulation of chewing from five pairs of glands. The parotids, which are located behind the angle of the jaw; the submaxillaries, on the inner sides of the parotids; the sublinguals, on each side of the tongue base; and the inferior molar and buccal glands in the cheeks. Saliva functions primarily as an aid in mastication and swallowing, along with providing some enzymatic (amylase) breakdown of starches. Saliva contributes very importantly to the buffering of rumen fermentation, provides nutrients to rumen microorganisms and certain anti-frothing properties.

9 Pharynx and Esophagus When the bolus has been formed, it is passed through the pharynx into the esophagus, which is a musculo-membranous passageway connecting the pharynx to the rumen. The bolus is projected forcefully through the esophagus and into the rumen. The rapid movement is made possible by the presence of striated muscles in the esophagus of ruminants, not found in most other animals.

10 Compound Stomach Upon entering the rumen, the bolus is subject to microbial fermentation. In young ruminants, liquid feed will by-pass the rumen, entering into the omasum instead. This is accomplished by means of the esophageal groove, a fold of mucous membranes that can be formed in response to the mechanics of sucking or pursing. The rumen (paunch) is the largest compartment of the compound stomach in the adult ruminant consisting of four compartments (the rumen, reticulum, omasum and abomasum) or three fore-stomachs and the true stomach. The rumen has a capacity of 4-5 gallons in the adult goat. The pH in the rumen is kept within a range of 5-7 normally. The rumen serves as fermentation vat, organ of maceration and grinding, bacterial digestor, and organ of absorption. The bacterial populations may count 10 billion cells per gram. Ciliated protozoa usually are also present in the rumen, besides a number of flagellated microorganisms. Rumen muscular contractions churn the boluses with the other contents of the rumen. The rumen microbes break down plant fibers into sugars and organic acids. Soluble compounds are absorbed through the rumen walls which are covered with many papillae, while the larger food particles may require to be remasticated before they can pass out of the rumen. This involves also the reticulum, the second of the four stomach compartments.

11 The act of rumination is accomplished by a combination of factors. There is an increase in fluid pressure of the reticulorumen compartments accomplished by extra-reticular contraction; there is also, a contraction of the diaphragm, creating negative pressures in the trachea; and the sudden contraction of the diaphram tightens the esophagus which is normally in a collapsed, flacid state. This, results in the passage of reticulorumen material into the esophagus. Anti-peristaltic movements of the esophagus transfer the bolus to the mouth, where ''cud-chewing'' occurs. The bolus is rechewed at the goat's leisure, re-insalivated and swallowed again. The feed materials, because of smaller particle size, have a comparatively higher specific gravity and will eventually pass through the reticulum into the omasum. The total residence time of feed spent in the rumen is usually about 8 hours.

12 The predominant end products of fermentation in the rumen are the volatile fatty acids (VFA), which consists of 60-70acetic acid, 15-200001:0000ropionic acid, 10-15butyric acid and smaller amounts of longer chain VFA's. Some alcohol is also formed in small amounts. These fermentation products from the rumen are absorbed through the wall and pass into the bloodstream. About 900f the energy required by the ruminant is provided by these short chain fatty acids.

13 Rumen fermentation results in large quantities of gas, principally methane and carbon dioxide. Amount and rate of gas evolution is directly related to type of feed. For this reason, ruminants are prominent belchers, and if the belching process is inhibited, e.g. by foaming, the animal will bloat. The mechanics of eructation (belching) are complex, and are associated with secondary rumen and specific reticulum contractions. Rumen gas is forced out by contraction of the dorsal blind sac of the rumen. It is allowed to enter the esophagus by the relaxation of its lower sphincter diaphragm. The esophagus fills with gas, the lower sphincter closes, and the upper pharyngo-esophageal sphincter relaxes. The gas is released into the nasopharynx, while the nasopharyngeal sphincter is closed. The glottis remains open while the lips are closed, so that much of the gas enters into the lungs and respiratory system.

14 There is a free flow of material between the rumen and reticulum and vice versa. The reticulum is the second and smallest of the four compartments of the goat compound stomach, making up only about 50f total volume. It is lined with a mucous membrane that is folded into hexagon like patterns (honeycomb). The reticulum receives the heavier substances from the rumen and serves as a muscular transit reservoir. Contraction of the reticulum aids in the rumination process, but is not vital. Rumination will occur even if the reticulum is removed.

15 Feed passes then into the 3rd compartment, the omasum, which consists of many folds (maniply) and comprises about 70f the total volume of the compound stomach of goats. Here, excess moisture is removed from the ingested feed, and finer grinding occurs here also, even after rumen-recticular fermentation. The omasum is not involved in the rumination process.

16 Feed then passes into the abomasum, or true stomach. Glandular secretions of gastric enzymes and hydrochloric acid are produced, and its functions are similar to the stomach of monagastric species. It makes up about 80f the total volume of the compound stomach and is essential to life. The abomasum consists of 3 parts, the fundic, body and pyloric regions. The pyloric region joins the small intestine, separated by the pyloric sphincter muscle. Glands of the fundic area, near the omasoabomasal orifice, secrete a mucus which protects the walls of the abomasum from protein-degrading enzymes and hydrochloric acid, secreted by the fundic and pyloric glands. The fundic glands are the most important for digestion. A specialized secretion from the fundus combines with vitamin B12 to stimulate the production of red blood cells.

17 Gastric juice, consists of water, inorganic salts, organic substances and hydrochloric acid, and between four to six liters per 24 hrs are secreted in goats. The enzymes rennin, pepsin and gastric lipase are part of the organic constituents of gastric juice. Rennin is the enzyme for milk coagulation, acting on milk casein by producing a gel-like mass, in preparation for the protein digestion by pepsin.

18 Pepsin, from its precursor pepsinogen, converts proteins into peptides, intermediary products of digestion. Further breakdown of proteins does not occur in the abomasum. Gastric lipase degrades fats eventually to fatty acids and glycerol, operating in conjunction with bile (an emulsifying agent) after the chyme (the liquid feed) leaves the stomach and reaches the small intestine. Feed chyme is moved through the compound stomach and abomasum in peristaltic waves of contraction. Some movement is also attributed to reticular actions that lift the body of the abomasum by virtue of its thin muscular attachment. The acidity of the abomasal contents usually remains close to pH of 3.

19 Small Intestine Feed passes from the abomasum into the duodenum, the first segment of the small intestine, in a series of spurts of 30-40 ml each, as the pyloric sphincter opens. Such spurts may last for 10-15 minutes, after which little feed passes. Some feed may be returned to the abomasum from the duodenum by antiperistaltic contractions; in goats, about 40 of the chyme reaching the duodenum, may be returned to the abomasum for a short time. The rate and volume of discharge from the abomasum is dependent upon the amount of feed that is present in the duodenum.

20 While digestion in the abomasum occurs in an acid environment, duodenal digestion depends on an alkaline medium. By the time the feed chyme has reached the lower end of the duodenum, the pH must have risen from 3 to about 8 normally. Pancreas, liver and glands of the duodenum contribute secretions that make the medium alkaline.

21 The pancreas is a diffuse dual-purpose gland located along the duodenum and secretes enzymes for starch, fat and protein digestion via the pancreatic juice into the intestinal tract, when stimulated by the hormones pancreozymin and secretin which are produced in the small intestine. The pancreas also produces the hormones insulin and glucagon.

22 Another glandular secretion, the bile, is produced by the liver and stored in its gall bladder (horses and rats have no gall bladder, but goats do). Cholecystokinin, a hormone that comes from the small intestine, controls release of bile from the gall bladder. The purpose of bile is to emulsify fats in the chyme, thus enabling lipase enzymes to work effectively. Bile also aids in alkalinization of the chyme in the duodenal tract.

23 There are four general types of glands located in the small intestine. Brunner's glands are situated in the submucosa of the duodenum, producing an amylase enzyme which is needed for the reduction of sugars. The crypts of Lieberkuhn are deep folds in the intestinal membranes, and although they are not true glands, they contain enzyme producing cells. Peyer's patches are congregations of lymphoid tissue in the walls of the small intestine. They control populations of local bacteria, function in antibody production and aid in fluid filtration. Specialized secreting cells of Paneth, the crypts of Lieberkuhn, and goblet cells of the intestinal tract secrete an intestinal mucus.

24 Movements of the intestines churn the contents, and mix in the digestive secretions. Intestinal motility also brings the chyme into contact with intestinal membranes for enhanced absorption of nutrients. Peristalic contractions propel the chyme through the intestine, and assist the flow of blood and lymph in the walls of the intestine.

25 There are three main aspects of intestinal movement. One is of a propulsive nature, including peristaltic and antiperistaltic motions. Pendular movements and rhythmic actions are related to intestinal absorption. Third, there are control movements which initiate or stop peristalsis by the creation of waves.

26 After the duodenum, feed chyme passes through the section of the small intestine, called ileum and jejunum to the large intestine.

27 Large Intestine The large intestine mainly functions to remove water from the intestinal contents, and to concentrate the feed residues for excretion. In order to maximize water removal efficiency, the large intestine causes delay in the rate of passage of intestinal contents. While feed may transverse through the small intestine in about 3 hours, it takes approximately 18 hours to move through the large intestines. This is especially impressive considering that the large intestine of goats is only about 61/2 feet long, while the small intestine is almost 36 feet long. The extent of water resorption from the large intestines in goats is relatively high, as evidenced by their dry faces, and is an important water conservation mechanism in goats.

28 Material passes from the cecum, a blind pouch located at the juncture of the small and large intestine, to the first part of the large intestine by peristaltic movements, which undergo periods of high activity and almost inactivity. During high activity, pressure waves of 6-36 seconds can occur, with a movement occurring about once each minute. The extent of digestion and absorption of nutrients in the large intestine has not been studied closely.

29 The last foot of the large intestine, the rectum, is a modified storage organ for the fecal material. Upon accumulation of sufficient amounts, nervous stimulation triggers defecation. The anus is located at the termination of the digestive tract. It is made up of two sphincter and a retractor muscle, all of which are normally closed, except during defecation.

30 Studies of goats have indicated that it takes about 11-15 hours for feed material to pass through the digestive system. Maximum excretion is achieved in about 30 hours but does not reach completion until 6-7 days later. In goats kids, the rate of passage increases to a constant level at and after weaning.

31 The compound stomach accounts for 670f the total digestive capacity of the goat, while the small intestine holds 21 The cecum, of little significance in ruminants, has a capacity of only 20f the total. The large intestine and rectum account for the final 10

32 The ruminant digestive system of the goat, by virtue of its large rumen ''holding tank,'' is working non-stop throughout the adult life of the animal. Other animals have digestive organs and secretions that alternate between periods of stress and inactivity, while the goat must continually manufacture digestive juices and enzymes day and night, 24 hours a day. A breakdown in this complex process for even a brief period can result in acute and potentially deadly situations for the ruminant.

DIGESTION
COLLECTION;GOAT HANDBOOK
ORIGIN;United States
DATE_INCLUDED;June 1992


 
 


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