The digestive system of beef cattle is essential to their ability to acquire, digest, and absorb nutrients. One important aspect of the digestive system that is unique to ruminants, such as beef cattle, compared to nonruminants, is that four major digestive organs exist in place of the stomach. These organs are the rumen, reticulum, omasum, and abomasum. Their functions are often oversimplified, and they are commonly referred to as the “four stomachs.” These unique features enable cattle and other ruminant animals to survive and be productive on diets and feed resources that cannot support many nonruminant animals. Understanding the components of the ruminant digestive system and the roles they play is helpful when designing nutritional management programs or identifying the cause of nutrition-related digestive disorders.
The Mouth in Beef Cattle
As with any other animal species, feed enters the digestive system of beef cattle through the mouth. Cattle use the tongue, lower incisor and canine teeth, and an upper dental pad to graze or otherwise acquire feed. The dental pad is a coarse, fibrous tissue that replaces the upper incisor and canine teeth; it is a feature unique to ruminants.
The ruminant mouth also contains salivary glands that secrete saliva, which serves several purposes. Saliva moistens feed and helps to ensure that digestive secretions mix and interact with the feed, while also acting as a lubricant to facilitate swallowing. Saliva also contains two major components that are important for digestion in beef cattle: bicarbonate, which acts as a buffer, and digestive enzymes, which initiate the chemical digestion of feed in the mouth.
Feed is generally chewed by cattle only briefly before being swallowed for the first time. Initially, chewing decreases the particle size by only a small amount, but it plays a greater role in mixing and swallowing.
The Rumen in Beef Cattle
Once swallowed, feed enters the rumen, which acts as a large fermentation vat. The rumen encompasses the entire left side of the animal’s abdominal cavity. Microorganisms, including bacteria, protozoa, and fungi, digest or convert feed into its nutrient components through fermentation. These microorganisms use chemical and physical means of breaking down feed particles into more simple nutrients, thereby producing nutrient forms that both they and the animal need.
One major product of carbohydrate fermentation is volatile fatty acids, which provide the animal with most of its energy. The wall of the rumen is lined with small, fingerlike projections that are the primary site of absorption for volatile fatty acids. Rumen microorganisms also break down the portion of protein that is susceptible to fermentation and use it as building blocks to synthesize their own amino acids, which are incorporated into the microorganism. These microorganisms are also capable of synthesizing amino acids from sources of nitrogen that are not true protein, which are more commonly referred to as nonprotein nitrogen.
In addition to yielding nutrients that are beneficial to the animal, fermentation produces gaseous by-products, such as carbon dioxide, methane, ammonia, and hydrogen sulfide, that must be expelled by the animal. These gases are expelled through a process known as eructation. The prolonged inability to release these gases through eructation results in a condition known as bloat.
The Reticulum in Beef Cattle
The reticulum, a small outcropping of the rumen, is also a site of fermentation; however, it has the specific role of accumulating and regurgitating large feed particles. Once accumulated, small boluses of feed particles are transported up the esophagus to the mouth, where they are chewed more thoroughly. This process is known as rumination or, more commonly, as “chewing cud.”
Rumination serves two main purposes. The first is to decrease particle size, thereby increasing surface area and thus the interaction with microorganisms and digestive secretions. The second purpose is to increase the production and secretion of saliva.
Saliva is crucial to ruminal fermentation because the bicarbonate it provides helps to buffer the acid that is produced during fermentation and maintain a rumen pH in which the rumen microorganisms can thrive.
The Omasum and Abomasum in Beef Cattle
After a bolus of large particles has been rechewed, it is swallowed and further subjected to ruminal fermentation. Eventually, ruminal contents are digested to the extent that they become small enough to flow to the next compartment, the omasum. The omasum is a dense, bowling ball–shaped organ whose main function is absorption of water and water-soluble nutrients.
After a substantial portion of the water has been removed, the digesta then moves on to the abomasum, the compartment that acts as a true stomach. In the abomasum, the digesta undergoes acid and enzymatic digestion. After being mixed with acid and other digestive secretions in the abomasum, the digesta moves on to the small intestine.
The Small Intestine in Beef Cattle
In the small intestine the digesta is further broken down and emulsified by digestive secretions from the pancreas, liver, and gallbladder. The small intestine is also responsible for most of the remaining nutrient absorption. From there, the remaining digesta moves on to the large intestine, where it undergoes additional fermentation by microorganisms in the cecum and colon, as well as further absorption of water and water-soluble nutrients in the colon, before the remainder is excreted as feces.
The Liver in Beef Cattle
Once absorbed, nutrients are transported by blood or lymph to the liver, where they are filtered and further processed before being transported throughout the body. Nutrients are then either metabolized or converted into other forms and stored in the body to be used at another time. Once the remaining metabolites are metabolized, the kidneys remove them from the blood and excrete them in urine or recirculate them throughout the body.
The Digestive System of Calves
Newborn and suckling calves are somewhat different from other cattle. During the nursing process, the rumen, reticulum, omasum, and abomasum align to form what is known as the esophageal groove. This passageway allows milk to bypass the rumen, reticulum, and omasum, and enter the abomasum directly.
During the first 24 hours after birth, the calf’s gut wall is porous, allowing the absorption of antibodies found in colostrum (the first milk produced by the dam). These colostral antibodies are essential to populating the calf’s immune system with antibodies, because calves are born with a naive immune system that does not initially contain circulating antibodies. The pores in the gut wall begin to close soon after birth, and closure is complete within 24–48 hours after birth. For this reason, the consumption of colostrum as soon as possible after birth is critical for the development of a fully functional immune system, and therefore imperative to calf health.
Over time, as the calf begins to consume other feedstuffs and its rumen becomes populated with microorganisms, the rumen develops to become a fully functional component of the digestive system. Before this time, however, the lack of a fully functional rumen makes the calf’s digestive system very similar to that of a nonruminant. Therefore, calves are often much more sensitive to antinutritive factors such as trypsin inhibitor or gossypol, toxins, or other compounds than are cattle with a fully functional rumen.
