Comparative Aspects of Carbohydrate Digestion in Cattle

and Horse

Institutionen för husdjurens Individual project utfodring och vård Comparative Nutrition

Swedish University of Agricultural Sciences Uppsala January 2012Department of Animal Nutrition and Management

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Table of contents

Abstract …………………………………………..31. Introduction …………………………………………..3 2. Carbohydrate …………………………………………..3 2.1. Definition ..........................................................3 2.2. Classification ................................................4 2.2.1. Non structural carbohydrate ........................................4 2.2.2. Structural carbohydrate..............................................53. Sources of carbohydrate .....................................................

5 3.1. Forages ............................................................5 3.2. Grasses ............................................................5 3.3. Legumes ............................................................7 3.4. Cereal grains ............................................................74. Digestive tract ............................................................8 4.1. Horses ............................................................8 4.2. Cattle ............................................................85. Discussion ............................................................9 5.1. Saliva in cattle and horse..................................................9 5.2. Stomach ..........................................................10 5.2.1. Horse ..........................................................10 5.2.2. Cattle ..........................................................10 5.3. Small intestine ..........................................................12 5.3.1. Horse ..........................................................12 5.3.2. Cattle ..........................................................13 5.4. Large intestine ..........................................................13 5.4.1 Horse ..........................................................13 5.4.2. Cattle ..........................................................146. Conclusion ..........................................................157. Reference ..........................................................15

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AbstractThis paper investigates about comparative aspects of carbohydrate utilization and digestion in cattle and horse. Herbivores consume different feedstuffs such as grasses, cereal grains and clovers in order to meet their nutritional requirements. Nutritional value of different forages will be variable during the growth. Protein content of plant which mostly occurs in leafy parts gradually decreases while a cell wall content (fiber content) which occurs in stem will slowly increase. Structural carbohydrates such as cellulose and hemicellulose, due to lower digestibility than leafy parts must be fermented by different microorganisms in rumen and hindgut. Digestion in horses is not the same as in cattle. Cattle have large fore stomach and are called ruminant’s because they can ruminate. They store food in their rumen, and regurgitate and re-chew their food to gain more nutrients.  By contrast, horses have a small stomach, and have to graze little and often to maintain nutrient intake. Both cattle and horse are able to utilize and digest dietary fiber but the site of fiber digestion differs among the animals. Rumen is the main site of microbial fermentation in the cattle while large intestine (cecum, colon) is the site of microbial activities in the horse.All mammals are dependent on the gut micro flora for the digestion of the structural carbohydrates in forage. In both fermentation systems volatile fatty acid such as prop ionic, butyric and acetic acid are the main products.

1. Introduction Carbohydrates are important chemical compounds, which acts as the main source of energy for herbivores. They are essential components that are broken down by different enzymes during digestion. According to the structure, carbohydrates are divided into different types, which can be found in a variety of foods. Some forage contains indigestible carbohydrates, which can not be digested by non-ruminant animals. The horse and cattle are able to adapt to a wide variety of foods because of their unique digestive tract. Domesticated animals such as cattle and horse have different digestibility rate of carbohydrate in different part of the gastrointestinal (GI) tract. Fiber can be utilized by ruminant and non-ruminant after fermentation in the large intestine. The aim of this study is to investigate briefly about dietary carbohydrate, different source of carbohydrate, and finally different aspects of carbohydrate digestibility in GI tract of cattle and horse. You will also learn what makes the cattle and horse’s digestive system unique.

2. Carbohydrate

2.1. DefinitionA carbohydrate is a chemical compound composed of hydrogen (H), carbon (C), and oxygen (O) with (CH2O)n as a general formula where n varies from 3 or more (McDonald et al., 2002). Carbohydrates are synthesized in plants from carbon dioxide, water and solar energy (Cheeke and Dierenfield, 2010). This formula is not followed by

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all carbohydrate compounds. Deoxyribose (C5H10O4) is an example which oxygen and carbon are not in the same ratio (McDonald et al., 2002)

2.2. ClassificationCarbohydrates can be divided in four main groups as follows: Monosaccharides, Oligosaccharides, Polysaccharides and Complex carbohydrates (Mc Donald et al,. 2002) Monosaccharide, which is the simplest sugar according to carbon atom units divided to five sub-groups which includes: Triose (C3), Tetrose (C4), Pentose (C5), Hexose (C6) and Heptose (C7). Glucose and Fructose, which are the most important simple sugars, are classified as hexose (McDonald et al., 2002).Oligosaccharides are formed by combination of monosaccharides that according to the amount of monosaccharide units can be divided to di-, tri-, tetra saccharide. Sucrose (glucose + fructose) and lactose (glucose + galactose) are two examples of disaccharides which are abundantly found in plants e.g. sugar beet (McDonald et al., 2002). Polysaccharides are long carbohydrate molecules, which consist of frequent units of monosaccharides. Polysaccharides depending on the types of monosaccharide in the structure are divided to homo polysaccharides and hetero polysaccharides (McDonald et al., 2002).The most important homo polysaccharides in plant origin are starch and cellulose which are formed by glucose units. Starch is composed α-1, 4-bonds while cellulose composed β-1, 4-bonds of glucose units (Cheeke and Dierenfield, 2010). By contrast, hemicelluloses, which are classified as hetero polysaccharides, are composed mainly of different monosaccharide units e.g. glucose, galactose, mannose, xylose and arabinose. Complex carbohydrates are a combination of hydrocarbon and other biochemical compounds which are linked together with protein or lipids; e.g. glycolipids and glycoproteins (McDonald et al., 2002).Lignin is not classified as a carbohydrate but is strongly associated with these compounds. Lignin, which is one of the main components of cell walls, has specifically been interested in the ruminant nutrition. This can be due to a high resistance of lignin against gastrointestinal (GI) degradation. Lignin is bound to cell wall polysaccharides, but until recently little was known about the form of this cross-linkage (Jung, 1997).Lignin also reduces the accessibility of some essential minerals, such as iron, by providing organic ligands that confiscate the mineral ions and results in the prevention of mineral digestion (Clark et al., 2001; Philips, 2010).

2.2.1. Non structural carbohydrate (NSC)Non structural carbohydrates such as sugars and starch are in addition to organic acid the main sources of energy for ruminant and non ruminant animals. Due to high digestibility and energy content NSC must be included in diet to meet the energy requirements of the animal. Fermentability of NSC is varying according to different factors such as conservation, processing methods and type of food (Clark et al., 2001).Starch, which considered as a storage carbohydrate in many plants, is abundant in fruits, tubers, seeds and roots. Starches are insoluble in water and thus can serve as storage depot of glucose. Plants convert excess glucose into starch for storage. Starch consists of numerous glucose units joined together by glycosidic bonds. There are two structural forms of starch which depending on glucose linkages are divided into

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amylose and amylopectin (McDonald et al., 2002). Linear and branched in structure for amylose and amylopectine, respectively, led to different characteristics such as solubility and digestibility rate in animal’s digestive tract. Furthermore, amylopectin is the main component in the most starches with 70-80% abundance. Difference in structure between amylose and amylopectin is associated with glucose linkages in which amylose is formed by carbon atom 1 of a molecule and carbon atom 4 of adjacent molecule α-(1→4) linkages while in amylopectin additionally, α-(1→6) linkages are present in the structure (Englyst and Hudson, 1996; McDonald et al., 2002).

2.2.2. Structural carbohydrate In contrast to NSC, structural carbohydrates occur in the cell wall section of the plant and must be fermented by bacteria living in gastrointestinal tract of animal. Structural carbohydrates can be analyzed and quantified as crude fiber, acid detergent fiber (ADF), and neutral detergent fiber (NDF). Neutral detergent fiber consists most of the structural components in plant cells e.g. cellulose, hemicellulose and lignin whilst acid detergent fiber does not include hemicellulose (Clark et al., 2001). The major component of cell wall includes cellulose, hemicellulose, which is slowly degradable in animals and lignin, which is unavailable in plant cell walls and silica (McDonald et al., 2002; Philips, 2010). The strength of cell wall is due to the presence of cellulose and lignin. The nutritional value of forage varies depending on the relative amount of cell wall components and lignification level of cell wall (McDonald et al., 2002; Cheeke and Dierenfield, 2010).

3. Sources of carbohydrate

3.1. ForagesForages are represented by the areal parts of pasture grasses, legumes, and forbs (Lawrence et al., 2007). Different part of forages, which includes stem, leaf and sheath differ in chemical composition and nutritional value. The proportions may change during a season. Leafy young plants contain more minerals, protein, and water and less fiber and lignin compared with older and less leafy plants (Lawrence et al., 2007).With plant growth, plant content gradually changes. These variations have direct effects upon the chemical composition of forage. The most important aspect regarding plant maturity, which has substantial effect on digestibility rate in ruminant and non ruminant gastro intestinal tract, is the cell content: cell wall ratio. In other words, when leaf growth slows, stem is concurrently elongated and digestible parts of cell content consequently will reduce. Cell wall contents occupy dominantly plant cells resulting to low digestibility of plant (Lawrence et al., 2007). Forage quality is affected by plant species, maturity, and environment (Temperature, moisture, and etc). For example, high temperature and strong light will reduce digestibility through elevating cell wall proportion. Consequently, warm-season forages have lower quality and digestibility than cold-season forages. Generally, factors that decrease growth rate and postpone the maturity of plant can increase forage digestibility in animal, via raising cell content to cell wall ratios (Table 1) (Lawrence et al,. 2007).

3.2. Grasses

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The composition of pasture grasses is variable. The fiber content has an inverse relationship to the crude protein (CP) content. The ADF content may vary between 200 g/kg in young plants up till 450 g/kg in very mature species of some area e.g. moorlands (McDonald et al., 2002). The moisture content of grasses is variable from 750-850 g/kg in very young grasses until 650 g/kg of full-grown plants (McDonald et al., 2002; Vasconcelos and Galyean, 2008). Waite and Boyd (2006) have reported that the content of hexoses which is a water-soluble carbohydrate, are very low at all times in grasses, but the concentration of sucrose differs among the stage of growth and the effect of the stage of growth on the amount of sucrose was similar in both leaf and stem. The xylans are more abundant in grass walls than in legumes (Jung and Allen, 1995).

Table.1. Classification of carbohydrates which are non-digestible by non-ruminant endogenous enzymes Polysaccharides (dietary fiber) Non-starch polysaccharides (NSP)

Resistant starch Cell wall NSPCategory Monomeric

residuesSources Category Monomeric

residuesSources

Physical inaccessible

starch Glucose

Partly milled

grains and seeds

Cellulose Glucose Most cereals and legumes

Resistant starch granules Glucose

Raw potato, banana

Mixed linked β-glucans Glucose

Barley, oats, rye

Retrograded starch Glucose

Heat-treated starch

productsArabinoxylans

Xylose, arabinose

Rye, wheat, barley

Non-cell wall NSP ArabinogalactansGalactose, arabinose

Cereal co-products

Fructans Fructose Rye XyloglucansGlucose, xylose Cereal flours

Mannans MannoseCoconut

cake, palm cake

RhamnogalacturansUronic acids,

rhamnoseHulls of pea

PectinsUronic acids,

rhamnose

Apple, sugar-beet

pulpGalactans Galactose

Soya bean meal, sugar-

beet pulp

GalactomannansGalactose, mannose

Guar gum

Montagne et al., 2003

Table2.A and B shows the composition of water-soluble carbohydrate of Italian ryegrass in young leafy stage and variation of fiber content in different stages of growth. Fiber content is gradually increasing over the time while the protein content is reducing. It can be due to the occurrence of fiber in stem fraction which has higher growth rate compared with leafy parts (McDonald et al., 2002; Watson, 2007).

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Table 2.A. Composition of the dry matter of Italian ryegrass (g/kg) 2. B. The variation of fiber content in different stages of growth in Italian ryegrass 2

2.A. carbohydrates Proximate compositions

2.B. Constituent 2 weeks 6 weeks 10 weeks

Glucose 16 Fiber 20.45 21.62 25.33Fructose 13 Ether extract 3.75 2.42 2,1Sucrose 45 Crude protein 18.45 12.12 6.90Fructans 70 True protein 13.32 7.80 5.47Galactan 9Araban 29Xylan 63

Cellulose 202(Other)Lignin 52

McDonald et al., 2002 2Watson, S.J., 2007

3.3. LegumesThe common legumes in pasture are clovers (Trifolium spp) (McDonald et al., 2002). Red (T. pretense) and white clover (T. repens) are the main families of the clovers, which are used in ruminant and non ruminant animal diets, due to higher protein and mineral content and also less nutritional variability with age than grasses. Cattle, given white clover as forage diet, consumes 20% more dry matter than grasses at the same metabolisable energy content. Sugars such as sucrose in clover have similar proportion to grasses. Although, starch is present in clovers with approximately 50g/kg DM, clovers have deficiency in fructan (McDonald et al., 2002).Lucerne or alfalfa (medicago sativa) as another legume source is mostly grown in tropical regions with high temperature and similar to other legumes such as red clover is high in protein content (McDonald et al., 2002).Legumes contain large amounts of pectic substances, whereas grasses have relatively lower amount (Jung and Allen, 1995). Fonnesbeck (1968) has reported the red clover and alfalfa contain more digestible organic matter and digestible energy than any of the grass forages. Compared to the legumes, the grass forages contained substantially larger amounts of the cell wall content (Fonnesbeck, 1968).

3.4. Cereal grainsA large variety of grains because of high energy and protein content are consumed by cattle and horse. Barley, maize, oats, and wheat are commonest sources of cereals having different nutritional value and can be utilized in different range. Oats and rice, due to presence of husk and hulls in the structure, contain higher crude fiber content than ‘naked’ grains such as wheat and maize. They can be used in the diet to increase the amount of fiber (McDonald et al., 2002; Lawrence et al., 2007).Starch in the form of granules in grains consists of 25 per cent amylose and 75 percent amylopectin (McDonald et al., 2002). Normally, the composition of grains is less variable than forages, but nutrient compositions of grains can change depending on soil fertility and growth situation (Lawrence et al., 2007).

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Cereals usually constitute a lower fraction of ruminant’s diet compared to non-ruminant, although they are the main constituent of the concentrate ration. In many parts of the world, barley is the main concentrate in the diets of ruminants (McDonald et al., 2002).Table 3 shows some ingredients of ruminant and non ruminant feedstuffs. A remarkable point concerning of data is variable ranges of starch in different forage groups and also converse relationship with NDF and ADF contents.

Table3. Carbohydrate composition of selected feeds ingredients Feed2 %NDF %ADF %NFC3 %Starch

Grass pasture 58.8 35.6 19.6 3.5Alfalfa cubes 43.3 33.6 26.6 2.0Legume hay 38.5 30.0 30.8 2.4

Fresh Bermuda grass 66.7 38.4 14.0 2.6Bermuda grass hay 67.7 35.6 16.5 6.1

Barley 19.6 7.7 63.9 53.9Oats 27.9 13.5 50.9 44.3

Wheat midds 37.1 12.9 37.9 26.0Molasses 0.7 0.3 76.7 1.1Beet pulp 41.9 25.6 44.4 1.3

Soybean meal 13.1 8.4 28.3 2.0Soy hulls 61.7 44.0 19.7 1.7

Corn gluten feed 36.0 11.1 33.4 16.8Lawrence et al., 20072The amounts of data are a mean of variable ranges and can be varied in different sources. 3NFC = 100- (%CP-%NDF-%EE+%ASH)

4. Digestive tract

4.1. HorseHorses are classified as a non ruminant herbivores or hindgut fermenters (Cheeke and Dierenfield, 2010). From the feeding strategy point of view, horses like cattle are classified as terrestrial herbivores that utilize bulk and roughage to meet their nutritional requirements (Cheeke and Dierenfield, 2010). In horses it takes about 65-75 hours for ingested chow to pass completely through the gut (Brown et al, 2003). The horse’s digestive system is divided to two main parts: foregut and hindgut. The foregut of horse consists of the mouth, esophagus, stomach, and small intestine. The hindgut, which includes cecum, small and large colon and rectum, is the main site of microbial fermentation (Brown et al, 2003). Horse has no gallbladder consequently, can not store bile. But the attendance of HCL in small intestine can stimulate bile secretion from the liver (Cheeke and Dierenfield, 2010). The variation in the size of different segment is another point in which the hindgut (caecum and colon) forms the largest part in the gastrointestinal (GI) tract of the horse, approximately 64 per cent of the empty weight of the system Whilst the small intestine occupies 27 per cent of GIT (Brown et al, 2003). However, large quantities of fiber-rich forages are utilized by the horse and adequate amounts of fiber in diet assist for normal function of the hindgut (Cheeke and Dierenfield, 2010).

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Small Small

4.2. CattleSimilar to the horse, cattle is also classified as terrestrial herbivores that can utilize roughage to meet their nutritional requirements. Unlike horses, cattles are foregut fermenters, where rumen is the microbial activity site (Cheeke and Dierenfield, 2010). Ruminant digestive system include the mouth, esophagus, stomach, pancreas, gall bladder, small intestine, and large intestine (cecum, colon, and rectum) (Oltjen and Beckett, 1996). Unlike horse stomach, which comprises about 8-10 per cent of the total digestive tract and has a simple form like other non ruminants, cattle has a compartmentalized stomach which consists of rumen, reticulum, omasum, and abomasums. Rumen is the largest compartment of stomach in cattle and functions as a fermentation vat (Oltjen and Beckett, 1996; Brown et al, 2003). The reticulum, which is honeycomb form, prevents entering foreign materials such as nail or wire to protect the digestive tract (Cheeke and Dierenfield, 2010). The abomasums, which is called true stomach, is most similar to a stomach in non ruminant animals. In general, the fiber digestibility in cattle (foregut fermenters) is higher than in horse (hindgut fermenters) due to slower passage rate and more optimal environment for microbial growth (Cheeke and Dierenfield, 2010).

5. Discussion Digestion is a process in which the organic compounds of food with many insoluble macro molecules split into micro molecules in order to be prepared for absorption (McDonald et al., 2002). Digestive process includes three different activities as follows: mechanical, in which animal by mastication breaks down the food components to smaller particles; chemical, in which the secreted enzymes by animal interfere to decomposition of food components; microbial, in which the microorganisms such bacteria, protozoa, and fungi accomplish the digestive process (McDonald et al., 2002). Carbohydrates can be classified according to degradation rate: Fraction A, which has fast degradability such as sugars; Fraction B1, which has intermediate degradability such as starch; Fraction B2, which has slow degradability such as celluloses and hemicelluloses (available cell wall); and Fraction C is unavailable cell wall (lignin) (Sniffen et al., 1992). Although fiber digestibility of forages is not constant for all animals and feeding conditions, much of the variation is due to composition and structural differences of the forage, harvest date and height at harvest. ). Carbohydrates may be divided into two main groups: those Carbohydrates with α-1, 4 linked molecules such as starch hydrolyzed to simple sugars in the small intestine, and those carbohydrate with β-1, 4 linked molecules such as cellulose that undergo bacterial fermentation to volatile fatty acids in the hindgut (Hoffman et al, 2003).The hindgut’s micro flora such as bacteria, protozoa and fungi take place the microbial fermentation. Small animals require relatively higher digestible energy intake than large animals in accordance with the metabolic size. In other hand, gastrointestinal tract and fermentation content increases with body-weight. At the same level of intake, the retention time of digesta has direct relationship with body-weight. Therefore, small animals have lower retention time compared with large animals. This fact may clarify why small animals have a lower fiber digestibility than large ones (Uden and vansoest, 1981).

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5.1. Saliva in cattle and horseDigestion starts in the mouth, which is also called the oral cavity. Horse alike to the cattle has four different salivary glands. The glands consist of the large parotid salivary gland, the mandibular salivary gland, the sublingual salivary gland and two buccal salivary glands (Seymour, 1917; GiffIn et al, 2008). Salivary glands secrete saliva to keep the mouth moist. Saliva, which has lubricities property, is mixed up with food particles by mastication and consequently, facilitates the food movement through alimentary canal. Seymour states unlike in human, which contains abundantly salivary α-amylase* (ptyalin), horses saliva contains partial quantity of ptyalin, as a result has no massive effect on carbohydrate digestion. In other words, saliva has no enzymatic activity but containing bicarbonate which has buffering capacity (McDonald et al., 2002). Although the parotid saliva of the horse does not contain ptyalin but contain zymogens which is transformed into ptyalin during mastication. The horse’s saliva has no digestibility effect on cellulose and sucrose (Seymour, R.J 1917; McDonald et al., 2002). Fiber and starch goes through alimentary canal into other digestive tracts to further digestion and absorption.

5.2. Stomach

5.2.1. Horse The horse’s stomach can not contain large mass of food every time due to the small volume, resulting in eating frequent, small portions of feed. Limited enzymatic digestion and some fermentative digestion from a small microbial population occur in the stomach. This limited capacity and any excess gas products in the stomach can cause the rupture of the stomach, other digestive upsets and death (Jones, 2009). The stomach has different rates of passage of the food mass, relative to the fiber content. Highly fibrous feeds, such a poor quality hay or chaff, are passed more quickly through the stomach as compared to denser grains, which accumulate within the lower glandular section of the stomach (Kohnke, 2008). The equine stomach contains microbes capable of fermenting carbohydrates. It’s also probable both the hydrolysis of fructan by gastric acid and the fermentation in stomach as well (Lawrence et al., 2007).There is limited microbial fermentation in the esophageal and fundic regions of the stomach, with the production of lactic acid. The PH of digesta falls down when hydrochloric acid is added in pyloric region (McDonald et al., 2002). The major role of stomach is the regulation of entrance digesta to small intestine to efficient digestion.

5.2.2. Cattle Unlike a horse, which has a simple stomach with low microbial activity, cattle have large stomach with four compartments and high microbial fermentation ratio. The most significant microbial activities take place in rumen, which contains different microorganisms such as Bactria, protozoa, fungi, and yeasts (McDonald et al., 2002; Cheeke and Dierenfield, 2010). This special system of digestion enable the animal to utilize forage, and fibrous roughage, consists largely of cell wall contents, which other mammalian are not able to breaks entirely down. Generally, decomposition of ingesta is mainly brought about by physical and chemical actions. * α-Amylase hydrolyses number of polysaccharides such as starch and glycogen to glucose and maltose.

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One of the main dissimilarities between horse and cattle is the capability of cattle to regurgitate the food into the esophagus and subsequently to the mouth by a wave of esophageal contractions (Rumination). In reality, the animal will be able to re-chew the coarser materials, resulting in preparation the chow to better digestion prior goes into the rumen (McDonald et al., 2002). Chewing activity and thus saliva production, consider as two significant factors for GI health and productivity in dairy cows (Lawrence et al., 2007). Food and water goes into the rumen and the food in some measure fermented to volatile fatty acids (VFAs) such propionic, butyric, and acetic acids, microbial cells, methane (CH4), and carbon dioxide (CO2). The digestive process of carbohydrate in the rumen can be divided into two phases. The first step comprises the decomposition of complex carbohydrate such as cellulose, and starch to simple sugars (Figure 1). As the picture shows, pyruvate is the end-product of first stage in the rumen. A group of digestive enzymes are contributed in different steps e.g. β-1, 4-glucosidase converts the cellulose to cellobiose; amylase, converts starch to maltose and isomaltose; maltase decompose maltose to glucose (McDonald et al., 2002).

Fig.1. Conversion of carbohydrates to pyrovat in the rumen (McDonald et al., 2002)

Sugars are fermented rapidly by ruminal microorganisms. Starches from ensiled and processed grains are rapidly digested in the rumen, but dried grains contain a significant amount of insoluble starch with low digestibility rate.In second step, produced pyrovate of complex carbohydrate, converts to VFAs, which are the main products of carbohydrate fermentation. Two different pathways for propionate formation are di-carboxylic acid and acrylate pathway (Figure 2). But the amount of VFA ratio is variable and has directly dependence on diet composition. Full-grown forages raise the proportion of acetic acid and conversely, immature or young forages increase

Cellulose Starch

Cellobiose Isomaltose

Fructose Fructan

Sucrose

Glucose

Maltose

Pentoses

Uronic acids

Hemicelluloses

Pentosanns

Pectin

Glucose-1-phosphate

Fructose-1, 6-phosphate

Fructose-6-phosphate

Glucose-6-phosphate

Pyruvate

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the propionic ratio (McDonald et al., 2002; Cheeke and Dierenfield, 2010). Soybean hulls increased VFA concentrations in the rumen of cows when substituted for corn and in the rumen of sheep compared with other fibrous by-products such as oat hulls, cottonseed hulls, or corn fiber. In contrast, increasing NFC concentration in diets fed to cows elevated total ruminal VFA concentration (Mansfield et al 1994). The VFA concentrations promoted by soybean hulls denote the excellent fermentability of the fiber component that constitutes a majority of soybean hull DM (Mansfield et al 1994).In addition, the rumen-reticular fermentation system is based on a "steady state" (pH near neutral point) depending on the balance between SCFA production, continuous buffer carbonate saliva influx and SCFA absorption in its acid form via papillary surface enlargements of the ruminal lining (no glandular mucosa) defined as "dilution rate (Hofmann, R.R., 1989).Bacteria that produce cellulase are called cellulytic bacteria. They attach to fiber particles and the cell walls of fibrous plant substance utilized by animal (Cheeke and Dierenfield, 2010). The extent of fiber digestion depends on the size of the indigestible fraction and the competition between the rates of degradation and passage out of the rumen. Ruminal fiber digestibility is affected by the passage rate of particulate matter out of the rumen. Rate of passage is affected primarily by intake. However, feed particle size, concentrations of dietary fiber and NFC, and rate of digestion of the potentially digestible fiber fraction may affect passage rate (Cheeke and Dierenfield, 2010).

5.3. Small intestine

5.3.1. HorseEfficiency of carbohydrate digestion in the small intestine appears to be important to increase the energy available to the horse and decrease the potential for colic or founder caused by excessive carbohydrates reaching the hindgut (Jones, 2009). Soluble carbohydrates are primarily digested and absorbed in the small intestine. Though, high intakes of starch allow some starch to bypass the small intestine and accumulate in the hindgut (Cymbaluk and Hintz, 1994). Unlike the ruminant, which has no enzymatic digestion before rumen microbial fermentation, the horse has enzymic activity in small intestine before microbial fermentation in the hind gut. Among these reactions soluble carbohydrates convert to simple sugars and absorbed for use as energy. In horses, the small intestine is the main site of non-fibrous carbohydrate digestion and microorganisms, which are located in large intestine, take place microbial fermentation of fibrous compounds (McDonald et al., 2002; Jones, 2009). Digestible dietary carbohydrates are hydrolyzed in the intestinal lumen, to the monosaccharides by several intestinal enzymes such as pancreatic Alfa-amylase and brush-border membrane disaccharides, sucrase, maltase and lactase (Dyer et al, 2002). Dyer et al (2002) have shown that sucrase, maltase and lactase are expressed in the equine intestinal brush-border membrane. They also reported sucrase activity is highest in the proximal small intestine of the horse and levels are similar to those reported in the intestine of other non ruminant species.Although starches and simple sugars are digested in the small intestine, amylase activity is lower compared to other mono gastric animals such pigs; consequently, some

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undigested starches get to large intestine when high amount of carbohydrate is given to animal (Resistant starch) (Mc Donald et al., 2002). If levels of these diets surpass the ability of small intestine to digest and absorb them, they reach the large intestine, cause variations in micro flora and the fermentation products, and prompt the horse to gastrointestinal disorders (Dyer et al, 2002). Carbohydrate quality and quantity varies among cereal grains. Furthermore, Grains starch have different digestibility in small intestine. For instance; oat starch generally is more digestible than corn starch and barley starch. Processing may influence the extent of prececal starch disappearance by decreasing particle size and increase g surface area (Lawrence et al., 2007).

Fig.2. Pathway of pyruvate metabolism in the rumen (Cheeke and Dierenfield, 2010)

5.3.2. CattleThe function of small intestine in cattle is approximately similar to other mammalian. Digesta that departs the rumen and enters to the small intestine contains some microbes and undigested fiber, as well as protein and some sugars produced by the microbes. These materials can be digested or absorbed in the small intestine.Similar to horses, the process of intestinal starch digestion in cattle starts in the lumen of the small intestine with the secretion and action of pancreatic Alfa-amylase. Alfa-amylase is synthesized in the pancreatic acinar cells (Harmon, 2009). Alfa-amylase is an endoglucosidase, which is able to hydrolyze internal alfa-1-4 glycoside bonds.Both pancreatic concentration of α-amylase and total content of α-amylase in the pancreas were decreased in calves consuming the 90% grain diet compared with those fed forage (Harmon, 2009).

5.4. Large intestine

5.4.1 HorseIn many non-ruminant herbivores, the large intestine has microbial population by which microbial fermentation is carried out similar to that of the rumen in ruminants. The

Acetyl CoALactate

Malate

Fumarate

Succinate

Propionate Acetate Butyrate

Acrylate

Methane

Oxaloacetic acid

pyruvate

Aceto-acetyl CoAFormate

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hindgut of horses has several unique characteristics rather than cattle’s. It is much larger than that of cattle due to being the main site of microbial activities and occupies the larger part of GI tract. The colon also has sacculations with slender and large sections differentiated by distinct bands of smooth muscle (Cymbaluk and Hintz, 1994). The sacculations possibly decrease the passage rate of digesta and consequently improve microbial fermentation and digestion due to longer exposure of digesta to the bacteria (Cymbaluk and Hintz, 1994). The proportion of fiber digestibility in hindgut fermentors such horses is normally lower than in ruminants due to higher passage rate and less proper environment for microbial growth (Cymbaluk and Hintz, 1994; Cheeke and Dierenfield, 2010). As in the rumen, the major productions of fermentation process, which occurs prominently in the hind gut, consists of short-chain or volatile fatty acids (VFAs), in particular acetic, propionic, and butyric, lactate and succinate, and various gases (CO2, H2, CH4,) (Harris, 1997; Montagne et al, 2003; Jones, 2009). Micro flora produces cellulase, which hydrolyzes β-1, 4 glucose linkages in hemicelluloses and cellulose. Ligno-cellulose may be degraded to cellulose by fungi present in the large intestine, while lignin remains undigested and is excreted in feces (Hofmann, 1989).Volatile fatty acids, which consider as energy source for the horse, can be different according to composition of the diet (Jones, 2009). For instance, if the amount of grain increases rather than the hay in the diet, the proportion of propionate will increasingly exceed than the acetate (Harris, 1997). The VFAs produced in the hindgut are quickly absorbed from the lumen particularly when the luminal pH is low or when there is a high concentration of SCFA in the lumen (Montagne et al, 2003).VFAs have different functions in the body soon after production in the body. Acetate and propionate transmit to the liver and then respectively acts as an energy substrate for muscle tissue and convert to glucose. The ratio of acetate to propionate is important because unlike the acetate, the propionate is a major source of glucose. Approximately 7% of total glucose production in the horse arises from propionate produced in the caecum (Cymbaluk and Hintz, 1994). Butyrate is used primarily by the colonocytes, and provides a major source of energy for its metabolic activities (Montagne et al, 2003).The rumen is located anterior to the small intestine, which is the primary site of absorption of many nutrients. Thus, bacterial end products can be digested in the small intestine of the ruminant. The hindgut of horses is posterior to the small intestine, and many nutrients produced by the equine micro flora are not effectively absorbed (Montagne et al, 2003).

5.4.2. CattleThe hindgut of ruminant has microbial activity such as that of the rumen. The cecum has little function in a ruminant, unlike its role in horses. The colon is the site of most of the water absorption in the large intestine. As a comparative aspect of hindgut and foregut fermenters, some deficiencies can be named for hindgut fermenters compared with ruminants. The nutrient components such as minerals, vitamins, sugars, etc are absent in the substances entering the hindgut because some of the materials have already digested in the small intestine. In other words, microbes, which are located in hindgut, receive poor quality nutrients compared with rumen microorganisms, consequently they can not reach to maximal growth as well as rumen microbes (Cheeke and Dierenfield, 2010). The

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hindgut is a less efficient area for nutrient absorption. Microbes as a product from microbial fermentation in rumen can be digested or absorbed in rumen or small intestine but in hindgut fermenters, microbes just by the consumption of feces (coprophagy) can be resumed in the body because there is not an area to reabsorb the microbes after fermentation. Finally, the passage rate in hindgut is faster than rumen results in lower fiber digestion rate than the rumen (Cheeke and Dierenfield, 2010).

6. Conclusion Carbohydrates can be divided into structural and non-structural forms. Structural form occurs in the cell wall and endogenous enzymes are not able to digest these compounds. Dietary fiber must be fermented by microorganisms present in gastrointestinal (GI) tract of the animal. Utilization and digestion of fiber by cattle and horse is due to anatomy of the gastrointestinal tract. There is similarity in the process of microbial fermentation in cattle and horse. Site of microbial fermentation differs between cattle and horse. Although, horse has a simpler stomach than that of the cattle, hind gut of the horse is massive and larger than cattle. Microbial fermentation in the cattles because of lower passage rate and more optimal environment for microbial growth seems to be more efficient than horses. Different microorganisms in rumen and hindgut takes place the microbial fermentation. Both rumen and hindgut fermentation enables animal to utilize plant cell walls (dietary fiber). Volatile fatty acids are the main end-products of microbial fermentation and according to the type of diet can be variable in quantity.

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