FIBER DIGESTION IMPORTANT IN GRAZING CATTLE

by: Stephen B. Blezinger
Ph.D, PAS

Part 2

In Part 1 of this series we began a discussion of the importance of fiber digestion by cattle. The cow's ability to consume and digest high fiber materials that are fundamentally indigestible to other animal species (except for other ruminants such as sheep and goats and some hind gut fermenters such as equines) is highly important to agricultural production in general. Part 1 discussed, in brief, the digestive tract physiology and what structures are in place to make this process work in the cow. In this part we will examine the fiber itself and the mechanics of the animal's ability to extract valuable nutrients from this material.

With a discussion of the fiber component of forages comes assessment of forage “quality.” Forages with good or at least acceptable quality are the main asset of any livestock operation. They are also the foundation of most rations in a forage-based diet. In these situations, forages and forage quality affect individual and group animal performance. The available nutrients that a forage carries affects individual animal production (i.e. weight gain) while the amount of forage produced affects production per acre. Forages possess a mixture of chemical, physical, and structural characteristics that determine the quality of that pasture or the accessibility of nutrients to that animal.

When considering forage quality, different terms come to mind. These may include nutrients such as energy, protein, digestibility, fiber, mineral and vitamins. In this context we also have to consider animal production. In the most practical of terms forage quality is correlated with:

• Milk production

• Weight gain

• Reproduction

• Animal health and well being

In defining forage quality, a distinction will be made between forage quality and its nutritive value. In many cases, these terms are used interchangeably but in this case a distinction is made between the two terms. Forage nutritive value usually refers to concentration of available energy, historically based on total digestible nutrients or TDN and concentration of crude protein. Forage quality is a broader term that not only includes nutritive value but also digestibility and subsequent forage intake.

As discussed, forages are made up, predominantly, of different types of fiber, with certain types of fiber being more digestible than others. These fibers are found mainly in the cell walls of the plant and are what give it structure. The percentages of these various fibers are dictated by a combination of the plant species, maturity, moisture availability and soil fertility. All these variables play a role in what the fiber composition is in the plant material at any given point in time. Secondly, within the fiber matrix and within the plant cells are contained other nutrients such as proteins, fat, minerals and vitamins. In order to access these nutrients, the fiber component has to be broken down. The more digestible or degradable the fiber component is, the more accessible these nutrients will be to the animal for digestion. So let's talk about fiber.

What is Fiber?

As pointed out, fiber is the predominant fraction of the plant cell wall and primarily comprised of specific combinations of certain carbohydrates. The primary components of fiber are cellulose, hemicellulose, and lignin. There are other components as well but these are of the most important. The fibers are composed of differing base sugars or carbohydrates. On a chemical basis, cellulose is comprised of linear chains of glucose molecules. Comparatively, starch (amylose), the carbohydrate source in grains, is also comprised of glucose molecules arranged somewhat differently. In cellulose, the glucose molecules are connected in what is referred to as a (Beta) 1,4 link whereas in starch, it is an (alpha)1,4 link. This simply refers to how the various glucose molecules are connected. The 1,4 link is more easily broken by microbial enzymatic action in the rumen which is why starch is more digestible than fiber and is also higher in energy. This same linkage is also easily broken by other digestive system chemicals.

Only microbial enzymes can digest the 1,4 linked glucose in cellulose. Hemicellulose is also dependent on microbial enzymes for digestion because it has a complex structure made primarily of the sugar xylose that is also in 1,4 links. Hemicellulose is closely associated with lignin that has a strong negative influence on fiber digestion.

Hemicellulose is more complex and is made up of xylose, mannose, galactose, rhamnose, and arabinose. As mentioned, xylose is commonly present in the largest quantity but this depends on the plant species

Lignin is even more complex and fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components. It is strongly linked to hemicellulose and, therefore, cross links different plant polysaccharides (combinations of basic sugar molecules), conferring mechanical strength to the cell wall and to the plant as a whole. While lignin is for the most part indigestible it plays a crucial part in conducting water in plant stems. One of the features that increases the indigestibility of lignin is the fact that it is repellant to water (hydrophobic). This feature makes it possible for the plant's vascular tissue to conduct water efficiently.

During the early stage of plant growth the amount of cellulose is found in higher concentrations. As the plant matures the hemicellulose and lignin contents increase subsequently reducing the digestibility of the plant material.

Fiber Digestion in the Rumen

The rumen is an environment with a diverse population of microorganisms. Bacteria and protozoa dominate the fermentation both in terms of numbers and metabolic processes. Of the various bacteria present in the rumen, several general types of bacteria can be described based on their metabolic functions and the materials they act on. Fibrolytic bacteria ferment fiber while amylolytic bacteria specialize in fermenting starch from grains and concentrates. Different populations of bacteria will predominate rumen fermentation related to the type of diet being fed. Cattle fed diets solely of forage with high fiber, such as those on pasture will have a ruminal bacterial population that is high in fibrolytic bacteria. Basically, the fermentation of fiber (cellulose and hemicellulose) results in the production of acetic acid (a volatile fatty acid or VFA) that, when absorbed through the rumen wall, is used by the cow for energy and is the primary building block of fat in milk. In contrast, digestion of sugars and starches yields propionic acid (another VFA) that is converted to glucose in the liver of the cow and used for energy.

The amount and size of fiber particles in the diets of grazing cattle is important to maintaining optimal rumen function. Long fiber in the rumen forms a “mat” in the rumen. The mat is where fibers are entangled because they are too long to pass to the lower gut. Fiber from the mat is regurgitated and chewed (cud chewing) producing large amounts of saliva that naturally buffers the rumen. When large forage particles (pasture grass, hay, silage/haylage) are chewed, the surface area is increased giving greater access to rumen microbes. The bacteria secrete a variety of enzymes which continually work to breakdown the forage and feed particles to ever decreasing particle sizes. In order for forage and feed particles to pass out of the rumen and into the lower ruminal system compartments, it must attain a size of about one millimeter or less. Passage of particles from the rumen is important because without digestion and passage, food would fill the rumen and depress intake. There must also be a balance between time in the rumen for microbial digestion and passage. For example, grinding fiber to extremely small particles may assist in passage from the rumen but ruminal digestion of that fiber particle may actually be decreased if it passes too quickly. The normal process of particle size reduction in the rumen leads to increased surface area for microbial attachment and digestion.     

Digestion of fiber in the rumen varies greatly and is dependent on a number of different factors. For example, as indicated previously, lignin acts as a sort of intracellular cement that gives plants rigidity. Unfortunately, it also has negative effects on rumen microbe's ability to breakdown forage particles. Lignification or the natural increase in lignin content of the plant cell wall generally increases with increased plant maturity. Within a given forage species, increased lignification is associated with reduced digestion. Conversely, immature forages generally have more digestible fiber than mature forages. This helps us understand why harvesting bermudagrass in rapidly growing vegetative phases is more beneficial than harvesting it in the “headed out” stage of maturity. This works for other plant species as well. However, harvesting alfalfa even earlier than in the late bud stage would increase digestibility but total dry matter yield would be reduced. Thus, harvesting forages at optimum stages of maturity is often a compromise between yield and high digestibility. Location also has an effect on the fiber of plants. Generally speaking, forages grown in hotter climates closer to the equator (e.g., south Texas) have more lignin and are less digestible than forages grown in more temperate regions (e.g., Minnesota). In attempts to improve forage quality, forages have been developed to contain lowered levels of lignin. Management which focuses on harvest and preservation at more optimal stages of plant maturity are highly desirable in efforts to improve forage digestibility and animal performance.

pH Effects, Concentrate Feeding

The pH of the rumen has profound effects on the growth of rumen microbes and the digestion that takes place. A pH of 7 is considered neutral, where the amount of acid and base are equal. When pH drops below 7, we consider the rumen environment to be acidic. A number of different factors can affect ruminal pH. Lack of sufficient fiber or fiber that is chopped too finely, reduces chewing times and thus, reduces saliva production causing a decrease in ruminal pH. Cows fed a diet with long fiber particles such as pasture grasses and or hay chew for more than 10 hours, ruminate for about 6 hours, and can produce as much as 50 gallons of saliva per day. The ratio of forage:concentrate in the diet of lactating cows also affects ruminal pH. When concentrate feeding increases, such as in situations of high energy supplementation, total acid production in the rumen increases, causing a decrease in pH. The type of concentrate can also affect ruminal pH as the starch in corn is more readily fermented to acids in the rumen than the starch from oats. Besides requiring carbon skeletons (from fiber or starch), ammonia-Nitrogen and energy for growth, fibrolytic bacteria in the rumen grow best when the pH of the rumen is between 6.2 and 6.8. If rumen pH falls below 6.0-6.2, fiber digestion in the rumen begins to decline. As rumen pH decreases, fibrolytic bacteria in the rumen become less active and fiber digestion is decreased. When ruminal pH falls below 5.8-5.9, the rumen is mildly acidic and fiber digestion in the rumen ceases completely. When ruminal pH drops below 5.2 to 5.5, animals can succumb to acidosis. Common signs of acidosis include laminitis (cows with swollen feet and hocks, soreness), low milk fat, variable feed and forage intakes, a high incidence of problems like a displaced abomasum, loose manure and reduced cud chewing. This said supplements for grazing cattle should be chosen carefully to insure the rumen environment remains healthy and productive.

Conclusion

Fiber digestion in ruminants is a complex, highly variable system that is both sensitive to upset and amazingly resilient. Understanding at least the basics of this system can dramatically help the producer understand how to best manage cattle on pastures and forage based diets. This can go a long way toward improving efficiency and reducing production costs for the cow herd.

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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