CATTLE'S ABILITY TO DIGEST FORAGE CRITICAL TO NUTRITION

by: Stephen B. Blezinger
Ph.D., PAS

Part 1

For cattle producers, particularly those producing cattle on pasture and forages, the animal's ability to digest fiber is critical to its access to the nutrients necessary for life and normal performance. Monogastrics such as pigs and chickens do not have the capability to digest fiber particles to the extent that cattle, ruminant animals do. Other species have this same capability. These include sheep, goats, horses, a variety of wildlife such as deer, elk and antelope, big game such as elephants, camels, rhinos, and even small animals such as rabbits. These various species do not all share the same type of digestive system but their ability to extract nutrients from a very high fiber diet is similar.

Beef and dairy cattle belong to a class of animals known as ruminants. Ruminants are characterized by having cloven hooves and a four compartment stomach system. It is the configuration of this digestive system that allows cattle to consume, breakdown and digest a wide variety of plant materials that are either very poorly digestible or not digestible at all to other species. For this reason cattle can graze or be fed a wide variety of materials that would otherwise be unusable as a feed source. Additionally, understanding fiber digestion in cattle also helps determine how to best manage these sources of nutrition to optimize animal performance. It also helps us to develop specific tools that are also helpful to enhance cattle performance on forages or high fiber diets.

This article and the parts to follow will work to illustrate the cow's ability to digest and utilize fiber in its diet. We'll start with some animal physiology and later get into the plan materials themselves and how these various components work together. Hopefully this will provide insight that can be used to improve overall animal and forage management.

Mouth and Teeth Structure

In addition to a four compartment stomach system and cloven hooves, ruminants have an unusual configuration of teeth. The mouths of cattle are very different from most non-ruminant animals. Cattle have 32 teeth. They have six incisors and two canines in the front on the bottom. The canine teeth are not pointed and look like incisors. There are no incisors on the top. Instead cattle have what is known as a dental pad. Cattle also have 6 premolars and 6 molars on both top and bottom jaws for a total of 24 molars. Additionally, there is a large gap between the incisors and molars. This configuration allows cattle to harvest and chew a large amount of high fiber feed material.

With the teeth structured for grinding, cattle use their tongues to grasp or gather grass and then pinch it off between their incisors and dental pad. Without incisors, cattle cannot bite off grass very well, and they are inefficient at grazing closely. So in situations where pasture grasses are very short, such as in over-grazing situations or during drought, cattle struggle to consume adequate amounts of forage. Additionally, the inside of the cheeks and palate are rough which helps hold feed in while cattle chew with a side to side motion.

In addition to reducing the size of feed particles, the mouth aids in digestion by adding saliva to the feed. Cows will produce 20-35 gallons (75 to 140 liters) of saliva a day. The saliva helps moisten the feed. Saliva contains sodium bicarbonate to keep the rumen at the proper neutral pH (6.5-7.2) as well as plays a role in recycling nitrogen in the body. Both of these factors play a role in promoting good microbial growth in the first stomach compartment (rumen) which we will discuss more in a moment.

Stomach System

The four compartments of the cattle stomach are the rumen, reticulum, omasum, and abomasum. The rumen is the first and largest compartment, containing billions of bacteria, protozoa, fungi, and yeasts. These microorganisms live in a symbiotic manner with the cow, and they are the reason cattle can eat and digest large amounts of roughage. The rumen microorganisms are adaptable enough that cattle can digest a large variety of feeds from grass, hay, and corn to brewer's grains, corn stalks, silage, and even urea.

The bacteria and protozoa do most of the digestion of forages and feeds for the cow. The rumen is essentially a large fermentation vat. There are 25 to 50 billion bacteria and 200 to 500 thousand protozoa in every milliliter of rumen fluid (equal to about 0.06 oz.). These microorganisms digest the plant fiber and produce compounds known as volatile fatty acids (VFAs). VFAs are absorbed directly through the rumen wall and supply 60 to 80 percent of the energy needed by the cow. The wall of the rumen is covered with millions of tiny, finger-like projections known as villi. The presence of these villi give the rumen wall the appearance of a shag carpet. In addition to energy, the microorganisms produce protein which includes essential amino acids from the protein and nitrogen the cow ingests. Because the microbes can use nitrogen to make protein, cows can utilize urea and other sources of non-protein nitrogen. The microbes also synthesize Vitamins B and C (known as water-soluble vitamins).

As part of the digestive process, cattle ruminate, of which the most visible part of the process is chewing cud, which commonly occurs while resting. Cattle consume large particles of grass of hay. Later they will eructate (similar to burping or regurgitating) a bolus or clump of partially chewed feed which she will re-chew (chewing cud). In order for the microbes to digest fiber rapidly and efficiently it must be in small pieces, so cattle re-chew their food sev¬eral times. This rumination process also results in the production of extra saliva which helps buffer the rumen more, helping maintain the pH and maintaining a healthy environment for the microbial population and the animal itself. As the level of fiber decreases in the cow's diet the amount of time spent ruminating also decreases.

The process of normal rumen digestion and ruminating produces large amounts of gas (carbon dioxide, methane, hydrogen, etc.) When normal eructation occurs, these gasses are released thus relieving the pressure in the rumen. When cows experience difficulty eructating a condition known as bloat can develop. Bloating can be caused by a variety of conditions including grazing lush, succulent pastures in the spring, pure stands of clover or alfalfa (frothy bloat) or by a rapid change in feed or overeating grain (gaseous bloat). In some cases it can happen in some cattle that have a faulty release valve between the rumen and esophagus. This can occur by injury or exist as a congenital condition. Gaseous bloat is a result of improper digestion or fermentation of grain. It is treated by passing a tube into the rumen or using a trocar to make an external opening in the rumen to release the gas pressure. Frothy bloat is a result of proteins and carbohydrates in grasses and legumes causing gas to be trapped in a bubbly foam matrix. Different products can be used to reduce this surface tension including mineral or vegetable oil or even a dry laundry detergent. Prevention is a matter of proper management or use of products such as an ionophore (Rumensin® or Bovatec®) Poloxolene® or certain enzyme preparations such as Bovazyme® which current research is showing to have an effect on reducing bloating effects. The incidence of bloat in cattle grazing legumes can be reduced by maintaining at least 50 percent of the stand as grass. Also, cattle should not be turned out onto a pasture with a high percentage of legumes when cattle are hungry or the pasture is wet. Once cattle are adapted to legume/grass pastures, they can graze it even when wet.

Although rumen microbes can digest a great variety of different feeds, they are very sensitive to drastic changes in feeds. Some groups of microbes are better at digesting fiber (forages), whereas others are better at digesting starch (grains). Changing rapidly from a forage-based diet to a grain-based (high starch) diet causes millions of fiber-digesting microbes to die-off as they cannot digest the starch, and there are too few starch-digesting microbes to use the grain so the grain sours in the rumen. As a result, rumen pH decreases rapidly, the rumen stops working, and the animal becomes ill. In severe cases, cattle can develop acidosis, founder or in extreme situations, die.

The second stomach, known as the reticulum, with its honeycomb like lining, is a compartment of the stomach that is involved with rumination. It also acts as a trap for foreign objects ingested by the cow. It is not unusual to find rocks, nails, and pieces of wire and metal in the reticulum of cattle. Occasionally, if ingested, these small pieces of wire or metal can puncture the side of the reticulum (or the rumen) causing a condition known as “hardware disease.” As an aside, hardware disease is an irritation or infection of the diaphragm, heart or lungs. The best treatment is prevention, keeping metal trash out of pastures.

The third compartment, the omasum is characterized by many leaf-like folds. It functions as the entry to the abomasum, filtering large particles back to the reticulorumen and allowing fine particles and fluid to be passed to the abomasum. Though the complete function of this compartment remains unknown, it does aid in water resorption and recycling of buffers for the saliva. The omasum may also absorb some VFAs.

The fourth compartment or abomasum is also known as the “true stomach.” It functions much like the human stomach producing acid and some enzymes to start protein digestion. The enzymes and acids in the abomasums help break open or “denature” the protein molecules allowing for ready absorption in the lower gut.

The Lower Digestive Tract

The rest of digestion is performed in the small intestine and large intestine much as it is in humans and other mammals. Digesta that leaves the rumen and enters the lower digestive tract includes some microbes and undigested fiber, as well as protein and some sugars produced by the microbes. By-pass protein, fat, and carbohydrates also enter the lower digestive tract. By-pass protein, fat, and carbohydrates are nutrients that cannot be digested in the rumen but may be digested in the abomasum and small intestine.

Specific enzymes to digest proteins, sugars, and starches flow into the small intestine from the pancreas, while the gall bladder produces bile to help digest fats. The small intestine also produces some enzymes to aid in digestion, but its major function is absorption of digested nutrients. Except for the VFAs, as mentioned earlier, most of the nutrients are absorbed in the small intestine including protein, starch, fats, minerals and vitamins.

Water is primarily absorbed in the large intestine. Undigested feed, some excess water, and some metabolic wastes leave the large intestine as fecal material. The consistency of manure is an indicator of animal health and is dependent on water, fiber, and protein content of the feed. For example, cattle on lush spring forage will have profuse watery, greenish colored manure, whereas animals on a hay diet will have firm manure that is dark in color. Animals should produce manure that is indicative of the diet they are receiving. If not, it may indicate a digestive upset or disease. Light colored manure, manure tinged with blood, and watery manure (when on a dry diet) are not normal situations. Producers should learn to recognize changes in manure that indicate problems.

Rumen Management

Based on what has just been discussed, here are some guidelines that are useful in feeding management, especially taking care of the ruminal microbe population which is critical to this process.

1. Provide a diet that meets the energy, protein and mineral requirements of the animal.

2. Make sure water is clean and available.

3. Pay attention to fiber levels in diets; a fiber level between 30 percent and 70 percent is preferred.

4. Switch from high fiber diets to high grain diets slowly; change should occur gradually over days or weeks.

5. Diets should contain 5% or less fat. High fat levels will interfere with fiber digestion.

6. Do not move hungry or newly received cattle into pastures containing a high percentage (>30 percent) of legumes especially when these pastures are wet with dew.

7. Supply cattle grazing pasture containing >50 percent legumes with an ionophore, poloxolene or an appropriate enzyme supplement.

8. Monitor rumination and fecal output to aid in early detection of digestive problems.

Conclusion

This has been a crash-course in ruminant anatomy and physiology and hopefully provides some level of insight into the function of the system and why the digestive system of the cow works as it does. In the parts to follow we will discuss the differences in forages and fibers and how this works within this complex system.

Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at sblez@verizon.net or at (903) 352-3475. For more information please visit us on at www.facebook/reveillelivestockconcepts.







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