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Chapter 6 Carbohydrates.

In the biosphere there is probably more carbohydrate than all other organic matter combined, thanks largely to the abundance in the plant world of two polymers of D-glucose, starch and cellulose.



In most healthful and profitable herbivore diets, carbohydrates will be the most important source of energy. Carbohydrate nutrition has become very complex as it has become clear that there are many different compounds that make up the carbohydrates. Management of carbohydrate nutrition is one of the greatest challenges facing farmers and ranchers feeding herbivore livestock.


Starch is the form of carbohydrate in which most plants store their reserve energy. Cellulose in a plant is structural; it gives a plant the rigidity it needs to grow against gravity. Together, starch and cellulose are the most abundant constituents of herbivore diets. When these carbohydrates are consumed, digested, and absorbed by livestock, the energy in them is released and captured during animal metabolism.

Carbohydrates are the major source of energy in herbivore diets. Because of this, and because energy is the nutrient required in greatest amounts, the physical characteristics of the carbohydrate sources in the ration will largely determine the physical characteristics of the ration.

With the possible exception of some fish, starch is easily digested by all domestic animals because domestic animals are capable of making the enzymes necessary to hydrolyze starch into absorbable monosaccharides. However, livestock do not make the enzymes necessary to hydrolyze cellulose. Livestock that are fed and utilize large amounts of cellulose-containing feedstuffs such as pasture, green chop, hay, and silage depend on a population of microbes living in their digestive tracts to produce the enzymes necessary to digest or ferment hemicellulose and cellulose.

Through the action of microbes living in the digestive tract, herbivores ferment both starch and cellulose carbohydrates into volatile fatty acids (VFA), the most important of which are acetic acid, propionic acid, and butyric acid. Ruminants, horses, and rabbits absorb these VFA and use them to meet at least a portion of their energy requirements.


Carbohydrates, or saccharides, are compounds of carbon, hydrogen, and oxygen. The hydrogen and oxygen in carbohydrates are almost always in the same mutual proportion as in water ([H.sub.2]O). The chemical shorthand for monosaccharides is (C[H.sub.2]O)n, where n is greater than or equal to three.

All carbohydrates are built from simple sugars or monosaccharides. There are two types of monosaccharides: those made with five carbons (pentoses) and those made with six carbons (hexoses). The important pentoses in feed are arabinose and xylose. Ribose is another pentose found in nucleic acids but ribose is generally not considered to be an important compound in animal nutrition. The important hexoses are glucose, fructose, galactose, and mannose. Nutritionally, the hexoses are more important than the pentoses.

Monosaccharides are built into disaccharides (two monosaccharides) and polysaccharides (many monosaccharides). The term oligosaccharide is used to denote a compound containing a small number of monosaccharides. A disaccharide is a type of oligosaccharide. In all of these compounds, the monosaccharides are joined together by glycosidic linkages.

Figure 6-1a shows the open chain form of the monosaccharide, glucose. In nature, the #1 carbon is usually found bonded to the hydroxyl group (--OH) at the #5 carbon to create a ring or cyclic form (Figure 6-1b). This ring formation means that the glucose molecule could have two different conformations, depending on the orientation of the bonds associated with the #1 carbon (Figures 6-2a and 6-2b).

The two conformations of glucose are called anomers. There is the alpha ([alpha]) anomer and the beta ([beta]) anomer, and the difference between them has tremendous significance in animal nutrition. Whereas a glycosidic bond made between [alpha] anomers of glucose is relatively easily hydrolyzed by mammalian enzymes, a glycosidic bond made between [beta] anomers of glucose can only be hydrolyzed by the enzymes made by microbes such as those inhabiting the digestive tract of livestock.

A disaccharide is built when two monosaccharides are bonded together in a glycosidic linkage. Other oligosaccharides and polysaccharides are compounds created with repeated glycosidic linkages. These linkages may be made between [alpha] glucose anomers or [beta] glucose anomers. Though it varies with the type of carbohydrate, the carbons most likely to be involved in this type of linkage are the #1 and #4 carbons (Figure 6-3a and 6-3b).


There are also some nutritionally important mixtures in feedstuffs that contain carbohydrates. Hemicelluloses are mixtures of pentoses and hexoses. Pectins and gums are mixtures of pentoses, hexoses, and salts of complex acids. Figure 6-4 displays the variety and complexity of carbohydrates and carbohydrate mixtures.








Hydrolysis of the glycosidic bonds in oligosaccharides and polysaccharides is important because only the monosaccharide ([alpha] or [beta]) may be absorbed from the intestine into the blood. This hydrolysis occurs through the action of enzymes produced either by the animal itself or by the microbes inhabiting the digestive tract.

The hydrolysis of oligosaccharides and polysaccharides with [alpha] linkages is accomplished by enzymes produced primarily in the pancreas (Figure 6-5a). Starch is the most important example of this type of carbohydrate.

The hydrolysis of oligosaccharides and polysaccharides with [beta] linkages is accomplished by the enzymes produced by the microbes inhabiting the digestive tracts of livestock (Figure 6-5b). Cellulose is the most important example of this type of carbohydrate. Note that the enzymes working on the [alpha] and [beta] linkages must have different structures.

The Sodium Pump

Most monosaccharides are absorbed via the sodium pump. Sodium is the main cation of extracellular fluid. Potassium is the main cation of intracellular fluid. All cell membranes use sodium pumps (also called sodium-potassium pumps) to pump sodium cations out of the cell. Potassium cations are moved into the cell to maintain the cell's characteristic resting potential. But sodium cations are drawn down the concentration gradient back into the cell and the cycle continues. Most monosaccharides are absorbed into cells (from the intestinal lumen into the intestinal epithelium, and from the interstitial fluid into other cells of the body) by means of the sodium pump. The co-transport of sodium and monosaccharide by a proposed carrier molecule is illustrated in Chapter 9, Figure 9-3.




Whereas the various carbohydrates are widely distributed in plant tissues, relatively little carbohydrate is found in animal tissues. Some feed sources of the various carbohydrates are listed in Table 6-1.


Although carbohydrate will be the primary source of energy in most economical and healthful herbivore diets, any organic compound can be used as a source of energy. Because of this, specific requirements for carbohydrate have not been established for livestock. In spite of the fact that specific dietary requirements for carbohydrate levels do not exist, carbohydrate nutrition has become very complex. As the primary source of energy for herbivores, carbohydrates represent the primary component of the ration. The form of carbohydrate basically dictates the form of the ration, and diet form will have an impact on animal health.

As described in Chapter 4, feed carbohydrates are broadly classified as fiber and nonfiber carbohydrates. The fiber carbohydrates are included in the NDF (neutral detergent fiber) fraction of the feed and the nonfiber carbohydrates are included in the NFC fraction of the feed. The acid detergent fiber (ADF) carbohydrate fraction contains the more difficult-to-digest components of NDF. ADF is useful in evaluating the overall digestibility of feedstuffs.

The beef National Research Council (NRC) (2000) level II and the Cornell Net Carbohydrate and Protein System (CNCPS 4.0) (Fox et al., 2000) break down each feedstuff's carbohydrate into four fractions (Table 6-2). These fractions differ in their rates of digestibility. Carbohydrate fraction A represents a feedstuff's most rapidly digested carbohydrate. Fraction Aincludes sugars and organic acids. Carbohydrate fraction B1 represents a feedstuff's content of intermediate rate of digestion carbohydrate. Fraction B1 includes starch and pectins. Carbohydrate fraction B2 represents a feedstuff's slowest digested carbohydrate. Fraction B2 includes digestible NDF. Carbohydrate fraction C includes the feedstuff's lignin content, which is indigestible. Describing a feedstuff's carbohydrate in this way gives ruminant nutritionists a great deal of information with which to evaluate carbohydrate nutrition.
Figure 6-6
Calculation of total digestible

Total digestible NFC = 0.98 x (100 - [% CP + (% NDF - % NDFIP) +
fat + % ash)] x processing adjustment factor.

Figure 6-7
Calculation of the NDF
digestibility coefficient

NDF digestibility coefficient - (0.75 x (% NDF - % NDFIP - % lignin) x
[1 - [(% lignin / (% NDF - % NDFIP)).sup.0667])] / NDF

In the dairy NRC (2001) and in the companion application for dairy cattle, NFC is assumed to be 98 percent digestible, but feedstuffs are assigned a processing adjustment factor that may increase or decrease NFC digestibility. Additionally, the NDFIP value is considered in the calculation of total digestible NFC for a given feedstuff (Figure 6-6).

The dairy NRC (2001) handles NDF by calculating an NDF digestibility coefficient for each feedstuff (Figure 6-7). Digestible NDF is calculated by multiplying the feedstuff's NDF contribution by its NDF digestibility coefficient.


Carbohydrates are the largest component of herbivore diets. This carbohydrate may be in the form of cellulose reported in neutral detergent fiber, or it may be in the form of starch reported in NFC. Within each category, however, are dozens of compounds with more or less similar nutritional characteristics. In the dairy NRC (2001), all the NFC is assumed to be 98 percent digestible but each feedstuff is assigned an NDF digestibility coefficient. The CNCPS and beef NRC (2000) level II isolate and classify feed carbohydrate into four components, each with different digestion characteristics.


1. Why is the form of dietary sources of carbohydrate important in herbivore diets?

2. Domestic herbivores do not make the enzymes necessary to digest cellulose, yet the rations formulated for these animals contain pasture, green chop, silages, and/or hay, all of which contain significant amounts of cellulose. Explain why.

3. Use the following terms in a description of the structural and nutritional differences between starch and cellulose: monosaccharide, glycosidic bond, alpha anomer, beta anomer.

4. What is hydrolysis and what role does it play in carbohydrate digestion and absorption?

5. Explain how (CH2O)n is shorthand for carbohydrate.

6. Explain the relationship between the following terms: monosaccharide, disaccharide, polysaccharide, starch, cellulose.

7. Name three components in each of the following mixtures of carbohydrate: hemicellulose and pectin.

8. Why have the NRC committees not determined specific carbohydrate requirement values for domestic animals?

9. Name and describe the four carbohydrate fractions used by the beef NRC level II and the Cornell Net Carbohydrate and Protein System (CNCPS).

10. Give a feed source for each of the following types of carbohydrate: sucrose, starch, hemicellulose, cellulose, pectin.


Fox, D. G., Tylutki, T. P., Van Amburgh, M. E., Chase, L. E., Pell, A. N., Overton, T. R., Tedeschi, L. O., Rasmussen, C. N., & Durbal, V. M. (2000). The net carbohydrate and protein system for evaluating herd nutrition and nutrient excretion (CNCPS, version 4.0). Animal Science Department Mimeo 213, Cornell University, Ithaca, NY.

Lehninger, A. L. (1978). Biochemistry (2nd edition). New York: Worth Publishers, Inc.

National Research Council. (2000). Nutrient requirements of beef cattle (7th revised edition). Washington, DC: National Academy Press.

National Research Council. (2001). Nutrient requirements of dairy cattle (7th revised edition.). Washington, DC: National Academy Press.
Table 6-1
Carbohydrate components of
feed materials

Feed Source                      Carbohydrate Component

Sugar cane, sugar beets          Sucrose
Starchy plants and roots         Maltose
Grains, seeds, tubers            Starch
Potatoes, tubers, artichokes     Inulin
Citrus fruits, apples            Pectins
Cottonseed, sugar beets          Raffinose
Acacia trees                     Gums
Fibrous portion of plants        Cellobiose
Fibrous portion of plants        Hemicellulose
Corn cobs, wood                  Xylose
Plant cell walls                 Cellulose
Liver and muscle                 Glycogen

Table 6-2
The carbohydrate pools used
in the CNCPS and beef NRC
level II

Carbohydrate Fraction    Fraction Components

          A              Sugars, organic acids, oligosaccharides
          B1             Starch, pectins
          B2             Digestible NDF
          C              Lignin

Source: Fox et al., 2000 and NRC, 2000.
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Author:Tisch, David A.
Publication:Animal Feeds, Feeding and Nutrition, and Ration Evaluation
Geographic Code:1USA
Date:Jan 1, 2006
Previous Article:Chapter 5 Dietary energy.
Next Article:Chapter 7 Proteins.

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