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Animal Digestion System Essay

This essay focuses on the digestive system of ruminant animals like cattle, sheep, and goats. Ruminant animals are hoofed mammals that have a digestive system which enables them to utilize energy from fibrous plant material better than other herbivores. Unlike monogastric animals such as pigs and poultry, the digestive system or ruminants is designed to ferment feedstuffs and provide precursors for energy for the animal to use.

Ruminant Digestive system

The digestive system of ruminants like cattle enables them to efficiently use high roughage feedstuffs, including forages. Their digestive system is composed of the mouth, tongue, gall bladder, pancreas, the four compartment stomach (rumen, reticulum, omasum, and abomasum), salivary glands, the small intestine (duodenum, jejunum, and ileum), esophagus and the large intestine (cecum, colon, and rectum) (Brooker, 5).

Mouth and Teeth

A ruminant animal uses its mouth and tongue to harvest forages and consume feedstuffs during grazing. Cattle pick up feeds during grazing by grasping and gathering the plants with their tongues and pulling them to tear for consumption. On average, cattle take from 25,000 to over 40,000 prehensile bites to harvest forage while grazing each day. They typically spend over 30 percent of their time grazing, another 30 percent of their time chewing cud, and the rest of their time idling where they are not grazing or chewing cud (Hall, 9). The roof of their mouth is a hard dental pad without incisors. The incisors on the lower jaw work against this hard dental pad. The incisors of roughage selectors are wide with a shovel-shaped crown while those of concentrate selectors are narrower and chisel-shaped.

They have the same number of molars and premolars both on the upper and lower jaws. The ruminants use their teeth to crush and grind feeds during chewing and rumination (Hall, 9). Saliva helps in moistening the feeds hence making easy when chewing and swallowing. Saliva contains enzymes which breaks down the starch (salivary amylase) and fat (salivary lipase) and is also involved in recycling of nitrogen to the rumen. Saliva mainly reduces pH levels in the reticulum and rumen.

A mature cow produces up to 50 quarts of saliva per day, but this depends on the amount of time they spent in chewing feeds, which stimulates saliva production. The forage and feed mixes with saliva which contains sodium, potassium, phosphate, bicarbonate, and urea when consumed, to form a bolus. The bolus then moves from the mouth to the reticulum through the esophagus by the help of muscle contractions and pressure differences. Ruminants eat rapidly hence swallowing much of their feedstuffs without chewing it properly (Brooker, 8).


Their esophagus moves in both directions, hence allowing them to bring back their cud into the mouth for rumination (chewing of cud), if necessary. Rumination (chewing the cud) is a process where forage and other feedstuffs are pushed back to the mouth for further chewing and mixing with saliva. The chewed cud is then swallowed again and passed into the reticulum where the solid portion slowly moves to the rumen for storage, soaking and fermentation while most of the liquid portion moves from the reticulorumen into the omasum and then abomasums (Church, 13).

The Stomach

The ruminant stomach occupies almost 75 percent of the abdominen on the left side and also extending to the right side. The rumen and reticulum occupy 84 percent of the volume of the total stomach, the omasum occupies 12 percent, and the abomasum occupies 4 percent. The rumen is the largest stomach compartment, holding up to 40 gallons in a mature cow while the reticulum holds approximately 5 gallons in the mature cow. The rumen and reticulum are considered one organ known as reticulorumen because they have similar functions and are separated only by a small muscular fold of tissue. The omasum holds almost 15 gallons and the abomasum holds almost 7 gallons in a mature cow (Hall, 10). Microbes that include bacteria, protozoa, and fungi are found in the recticulorumen.

These microbes ferment and break down plant cell walls into carbohydrate fractions hence producing volatile fatty acids, such as acetate for fat synthesis, priopionate for glucose synthesis and butyrate. The volatile fatty acids are used for energy (Hall, 10). The reticulum is also known as the “honeycomb” because of the appearance of its lining and it is located next to the heart. There is no enzyme secretion in the reticulum. It collects the smaller digesta particles and moves them to the omasum while the larger particles remain in the rumen for further digestion. The reticulum, also known as the “hardware stomach”, catches and collects metal, hardware, heavy and dense objects like a nail, wire, or any other sharp object that the animal consumes.

During the normal digestive tract contractions, this object can penetrate the reticulum wall and make its way to the heart causing a hardware disease (Church, 15). The rumen, also known as the “paunch”, has a papillae lining for nutrient absorption. The rumen is filled with chewed and half chewed materials that the cow has ingested, swallowed, regurgitated and swallowed again. It acts as a fermentation section by hosting microbial fermentation. A large volume of starch and soluble sugar consumed is digested in the rumen. Rumen microbes, which are very sensitive to the drastic change of feeds, digest cellulose from plant cell walls, digest complex starch and synthesize proteins and some vitamins. The normal rumen pH ranges from 6.5 to 6.8 and when it decreases, the rumen stops working and the animal becomes sick. The environment of a rumen is anaerobic (has no oxygen) but has gases like carbon dioxide, methane, and hydrogen sulfide (Brooker, 12).

The omasum has a spherical shape and is connected to the reticulum by a short tunnel. It is also known as the “many piles” or the “butcher’s bible” due to the many leaf-like folds that resemble pages of a book. It has no enzyme walls. These folds increase the surface area, which increases the area that absorbs nutrients from feed and water. The omasum is the gateway to the abomasums and its major role is to reduce the particle size of the feeds and absorb water. Cattle have a highly developed, large omasum (Church, 16). The abomasum is the “true stomach” of a ruminant and functions like a stomach of a non ruminant. The abomasum produces hydrochloric acid and digestive enzymes, like pepsin, to start protein digestion and receives digestive enzymes from the pancreas, such as pancreatic lipase which breaks down fats. These secretions break the proteins into smaller sub-units, ready for digestion and absorption in the small intestine.

The main cells in the abomasum secrete mucous to protect the abomasal wall from acid damage. The ruminants eat large amounts of feeds thus there is a continuous flow of food through the abomasum (Djikstra, 15). The small intestine is a long tube running from the abomasum to the large intestine. It is normally 20 times longer than the length of the animal. Digesta entering the small intestine mix with secretions from the pancreas and liver, which increases the pH from 2.5 to 8 which enable the enzymes in the small intestine to work properly. The bile from the gall bladder is secreted into the duodenum, to help in the digestion of fats. In the small intestine, active nutrient absorption occurs throughout, including rumen bypass protein absorption.

The intestinal wall contains numerous “finger-like” projections known as the villi which increase intestinal surface area to help in the absorption of nutrients. Muscular contractions help in mixing digesta and moving it to the next section (Brooker, 18). The large intestine is mostly made up of colon, which is shorter in length and larger in diameter than the small intestine. The colon deals with the active movement of sodium and absorption of water by osmosis from the digested material that passes through it. It then removes the remaining undigested and unabsorbed material as feces via the rectum and anus. The cecum is a large blind pouch connected to the beginning large intestine and the illeum. In herbivores, it serves as a storage that allows bacteria and other microbes time to digest the cellulose further, unlike in horses (Church, 19).

The Development of the Ruminant Digestive System

Immature ruminants, like calves, from birth to about 3 months of age have a digestion system like that of non ruminants. Their reticular (esophageal) groove is formed by muscular folds of the reticulum. It takes milk directly to the omasum and then abomasum, bypassing the reticulorumen. The rumen of these animals must be protected with rumen microorganisms, including bacteria, fungi, and protozoa. This is accomplished through mature ruminants licking calves and environmental contact with these microorganisms (Church, 23). They must undergo reticulorumenomasal growth, including increases in size and muscle. In a calf at birth, the abomasum is the largest compartment of the stomach, making up more than half of the total stomach area.

The reticulorumen and omasum occupy 35 percent and 14 percent of the total stomach area in the newborn calf. As they grow, the reticulorumen and omasum grow faster hence increasing proportions of the total stomach area. In mature cattle, the abomasum encompasses only 21 percent of the total stomach capacity, whereas the reticulorumen and omasum make up 62 and 24 percent, respectively, of the total stomach area. Rumen papillae lengthen and decrease in numbers as part of rumen development (Church, 24). Due to the immature ruminants not having a functional rumen, they are not allowed to access feeds that contain non-protein nitrogen such as urea. They are also more sensitive to gossypol and dietary fat levels than mature ruminants hence the feeding of developing ruminants should be different from the mature ruminants. The feeding programs should be arranged according to the age of the animals (Church, 25).

This is a figure showing the relative size of stomach compartments in cattle and sheep at various ages (Church, 25).

The Feeding Types of Ruminants

The ruminants can be classified based on the feeds they prefer into concentrate selectors, roughage eaters, and intermediate types. The digestive system organs in ruminants differ according to feeding type hence creating differences in feeding patterns. Knowledge of grazing patterns among the ruminants helps in planning grazing systems for each individual species and also for multiple species grazed together or on the same acreage (Brooker, 22). Concentrate selectors, like deers and giraffes, have a small reticulorumen in relation to body size and selectively browse trees and shrubs. These animals select plants and plant parts that are easily digestible and nutrient dense substances such as plant starch, protein, and fat. Concentrate selectors have limited ability to digest the fibers and cellulose in plant cell walls (Church, 27).

Roughage (grass or bulk) eaters, like cattle and sheep, depend on feeds of grasses and other fibrous plant material. They prefer fresh grasses over legumes but can adequately manage rapidly fermenting feedstuffs. They have longer intestines in relation to the body length and a shorter proportion of large intestine to small intestine as compared with concentrate selectors (Church, 24). Intermediate types include goats and they prefer forbs, woody and shrubby plants. They have adaptations of both concentrate selectors and Roughage eaters. They have a fair though limited capacity to digest cellulose in plant cell walls (Church, 28).

Importance of Ruminant Digestive System

The digestive system of ruminants maximizes the use of rumen microbe fermentation products. This enables them to use resources like high-fiber forage that cannot be used by other animals. Ruminants are able to use such resources that are not demanded by humans but provide man with a vital food source. Ruminants also convert renewable resources from pasture into other products for human use such as hides, fertilizer, and other inedible products like horns and bones (Oltjen, 16)


In summary, a proper grazing system helps to improve agricultural sustainability. A land which is poor or erodible for cultivation can become productive when used for ruminant grazing. Ruminant livestock production also aids crop production whereby ruminants can use the byproducts of these crop systems that are not in demand for human use or consumption. Knowledge of the ruminant’s digestive system can help livestock producers plan appropriate feeding and nutritional programs and properly manage ruminant animals in various production systems. The gradual introduction of the ruminant to a specific feeding system is the best way of ensuring that the rumen functions properly.

Works Cited
Dijkstra J. 2005: Quantitative Aspects of Ruminant Digestion and Metabolism (2nd Edition).
CABI Publishing. Wallingford.
Brooker, R.J., Widmaier, E.P., Graham L.E. & Stiling P.D. 2008: Biology. McGraw-Hill. New
Church, D. C. ed. 1993. The Ruminant Animal Digestive Physiology and Nutrition. Waveland
Press, Inc. Prospect Heights, IL.
Oltjen, J. W., and J. L. Beckett. 1996. Role of ruminant livestock in sustainable agricultural
systems. J. Anim. Sci. 74:1406-1409. Hall, J.B., and Silver, S. 2009: Nutrition and Feeding of the Cow-Calf Herd: Digestive System of the Cow. Communications and Marketing, College of Agriculture and Life Sciences,
Virginia Polytechnic Institute and State University.

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