lecture 2 (carbhydrates).pdf

# Energy sources in livestock rations Carbohydrates and lipids are the principal sources of energy in livestock diets, with protein also contributing some energy. ### 1.1 Primary energy sources * **Carbohydrates:** These are the most significant energy providers in livestock feed due to their abundance, ready availability, and high digestibility in most feedstuffs [2](#page=2). * **Lipids:** Fats and oils represent the second most important source of energy for livestock [2](#page=2). * **Protein:** While not a primary energy source, protein can also supply energy to livestock rations [2](#page=2). --- # Carbohydrate classification and composition Carbohydrates are fundamental organic compounds primarily composed of carbon (C), hydrogen (H), and oxygen (O), serving as major constituents within plant tissues. They are a diverse group of compounds that can be classified based on their molecular size and structure, ranging from simple sugars to complex polymers [3](#page=3) [4](#page=4). ### 2.1 Chemical composition and occurrence Carbohydrates are defined by their elemental makeup, consisting of carbon, hydrogen, and oxygen atoms. In plant tissues, they are particularly abundant, often constituting up to 70% or more of the dry matter, especially in forages. The specific ratio of hydrogen to oxygen is frequently 2:1, mirroring that of water, which is a characteristic seen in many simple sugars [3](#page=3). ### 2.2 Classification of carbohydrates Carbohydrates are systematically classified into several main categories based on the number of sugar units they contain: #### 2.2.1 Monosaccharides Monosaccharides are the simplest forms of carbohydrates, often referred to as simple sugars. They cannot be hydrolyzed into simpler units. The general formula for monosaccharides is often represented as $C_n H_{2n} O_n$. They are further categorized by the number of carbon atoms they possess [4](#page=4): * **Pentoses (5-carbon sugars):** These have the molecular formula $C_5 H_{10} O_5$. Common examples include [4](#page=4): * Arabinose [4](#page=4). * Xylose [4](#page=4). * Ribose [4](#page=4). * Occurrence: Found in corn cobs and wood. Ribose is a component of nucleic acids [4](#page=4). * **Hexoses (6-carbon sugars):** These have the molecular formula $C_6 H_{12} O_6$. Common examples include [4](#page=4): * Glucose [4](#page=4). * Fructose [4](#page=4). * Galactose [4](#page=4). * Mannose [4](#page=4). * Occurrence: Glucose and fructose are found in sugar cane. Glucose is also a primary component of starch [4](#page=4). #### 2.2.2 Disaccharides Disaccharides are formed when two monosaccharide units are linked together by a glycosidic bond, with the release of one molecule of water. Their general formula is $C_{12} H_{22} O_{11}$. Common disaccharides include [4](#page=4): * Sucrose (Glucose + Fructose) [4](#page=4). * Maltose (Glucose + Glucose) [4](#page=4). * Lactose (Glucose + Galactose) [4](#page=4). * Cellobiose (Glucose + Glucose) [4](#page=4). * Occurrence: Lactose is found in milk [4](#page=4). > **Tip:** Disaccharides are the simplest carbohydrates that can be hydrolyzed into their constituent monosaccharides. #### 2.2.3 Trisaccharides Trisaccharides are composed of three monosaccharide units linked together. Their general formula is $C_{18} H_{32} O_{16}$ [4](#page=4). * Raffinose (Glucose + Fructose + Galactose) [4](#page=4). #### 2.2.4 Polysaccharides Polysaccharides are complex carbohydrates formed by the polymerization of many monosaccharide units. They can be further classified based on the type of monosaccharide unit they contain and their structure: * **Pentosanes:** These are polysaccharides made up of pentose units (5-carbon sugars). Their general formula is $(C_5 H_8 O_4)_n$ [4](#page=4). * Araban [4](#page=4). * Xylan [4](#page=4). * Occurrence: Found in corn cobs and wood [4](#page=4). * **Hexosans:** These are polysaccharides made up of hexose units (6-carbon sugars). Their general formula is $(C_6 H_{10} O_5)_n$ [4](#page=4). * Starch [4](#page=4). * Dextrin [4](#page=4). * Cellulose [4](#page=4). * Glycogen [4](#page=4). * Occurrence: Starch is found in starchy plants. Cellulose is a primary component of the cell wall in plants and is found in fibrous plants. Glycogen is found in the liver and muscles [4](#page=4). * **Mixed polysaccharides:** These polysaccharides are composed of a mixture of pentose and hexose units, or they can be composed of pentoses and hexoses along with salts of complex acids [4](#page=4). * Hemicellulose [4](#page=4). * Pectin [4](#page=4). * Gums [4](#page=4). * Occurrence: Hemicellulose and pectin are found in fibrous plants. Gums, such as those from Acacia trees, are also included in this category. Pectin is also referred to as a type of polysaccharide [4](#page=4). ### 2.3 Carbohydrate compounds found in plants The document lists several specific carbohydrate compounds that are present in plants, categorized by their classification: * **Monosaccharides:** Glucose, fructose, galactose, mannose, arabinose, xylose [4](#page=4). * **Disaccharides:** Sucrose, maltose, lactose, cellobiose [4](#page=4). * **Trisaccharides:** Raffinose [4](#page=4). * **Polysaccharides:** Starch, dextrin, cellulose, glycogen, pentosanes (araban, xylan), hemicellulose, pectin, gums. Lignin is also mentioned as a carbohydrate compound found in plants, though not explicitly classified within the sugar unit structure [3](#page=3) [4](#page=4). > **Example:** Cellulose, a major structural component of plant cell walls, is a polysaccharide made up of repeating glucose units. Starch, a storage carbohydrate in plants, is also a polymer of glucose [4](#page=4). --- # Nutritional partitioning and functions of carbohydrates Carbohydrates are nutritionally partitioned into fibrous and readily available components, serving crucial roles in energy provision, regulation, and synthesis within the body [5](#page=5) [6](#page=6) [7](#page=7). ### 3.1 Partitioning of carbohydrates for nutritional purposes For nutritional assessment, carbohydrates are broadly categorized into two main fractions: crude fiber and nitrogen-free extract [5](#page=5) [6](#page=6). #### 3.1.1 Fibrous carbohydrates or crude fibre (CF) Crude fibre represents the insoluble portion of carbohydrates in feedstuffs. It primarily consists of [5](#page=5): * **Cellulose:** A fundamental structural component of plant cell walls [5](#page=5). * **Hemicelluloses:** A variable proportion of these polysaccharides are included in CF [5](#page=5). * **Lignin:** A significant amount of lignin is present, forming lignocelluloses [5](#page=5). #### 3.1.2 Readily available carbohydrates or nitrogen-free extract (NFE) Nitrogen-free extract encompasses the soluble and more digestible carbohydrate fractions, including: * **Starches:** The primary storage form of carbohydrates in plants [6](#page=6). * **Sugars:** All simple sugars are part of this fraction [6](#page=6). * **Cellulose:** A portion of cellulose, depending on its solubility and digestibility, can be classified under NFE [6](#page=6). * **Soluble minerals and vitamins:** These are also included in the NFE fraction [6](#page=6). ### 3.2 Functions of carbohydrates Carbohydrates perform several vital functions within the organism [7](#page=7) [8](#page=8) [9](#page=9). #### 3.2.1 Source of energy * The primary role of carbohydrates is to provide energy for basic life processes [7](#page=7). * They are an economical, common, and readily accessible energy source [7](#page=7). * The energy derived from carbohydrates is utilized for maintaining body temperature (heat production), performing physical activities (mechanical work), and for the storage of energy in the form of fat [7](#page=7). #### 3.2.2 Formation of body regulators * Metabolic byproducts of carbohydrates can act as catalysts or promoters of oxidation, aiding in the breakdown of various feed components [8](#page=8). #### 3.2.3 Synthesis of other compounds * Carbohydrates serve as precursors for the biological synthesis of other essential molecules in the body, including: * Fatty acids [8](#page=8). * Certain amino acids [8](#page=8). #### 3.2.4 Synthesis of structural materials * Fat derived from carbohydrate metabolism contributes to cellular growth and structure [9](#page=9). * Carbohydrates are integral to the structure of several biologically significant compounds: * Glycolipids [9](#page=9). * Glycoproteins [9](#page=9). * Heparin [9](#page=9). * Nucleic acids [9](#page=9). --- # Crude fiber in animal nutrition Crude fiber plays a multifaceted and crucial role in animal nutrition, contributing to digestive health, function, and energy provision [10](#page=10) [11](#page=11) [12](#page=12). ### 4.1 Roles of crude fiber in animal diets #### 4.1.1 Providing bulk and meeting dry matter requirements Crude fiber content is a significant determinant of the bulk of an animal's ration. This bulk is essential for ensuring that the animal meets its dry matter requirements, which is fundamental for overall nutritional intake and health [10](#page=10). #### 4.1.2 Preventing digestive disturbances A high crude fiber content helps prevent the formation of a dough-like mass within the stomach. Such a mass can resist the action of digestive juices, leading to significant digestive disturbances and related health issues [10](#page=10). #### 4.1.3 Increasing digesta surface area Crude fiber contributes to the formation of channels or spaces within the digesta mass. This structural alteration significantly increases the surface area of the digesta that is exposed to both microbial and enzymatic digestion. A larger exposed surface area enhances the efficiency of nutrient breakdown and absorption [11](#page=11). #### 4.1.4 Maintaining digestive tract function The presence of crude fiber is vital for providing normal distention of the digestive tract. This distention is a key factor for the most effective functioning of the tract. Regular peristaltic movements, the muscular contractions that propel food through the intestines, depend heavily on adequate internal distention provided by the digesta, including its fiber component. Furthermore, feeds high in crude fiber are recognized for their laxative effect, which facilitates the passage of fecal matter through the large intestine [11](#page=11) [12](#page=12). #### 4.1.5 Serving as an energy source through microbial fermentation Crude fiber, particularly cellulose, acts as an energy source for certain animal digestive systems, primarily through microbial fermentation [12](#page=12). * **Ruminants:** Rumen microorganisms are capable of breaking down cellulose to meet their own energy needs. This process yields volatile fatty acids (VFAs) and gases as byproducts [12](#page=12). * **Hindgut Fermenters:** Microbial digestion of cellulose also occurs in the cecum of animals like horses and rabbits, similarly producing VFAs and gases [12](#page=12). > **Tip:** While crude fiber is crucial, the optimal level varies significantly by species and physiological state. Understanding these differences is key to formulating balanced diets. > > **Example:** In ruminants, a certain level of forage (high in crude fiber) is essential for maintaining rumen health and function, whereas for monogastric animals like pigs, excessive crude fiber can dilute the energy density of the diet. --- # Carbohydrate digestion and fermentation in ruminants Carbohydrate digestion in ruminants relies heavily on microbial fermentation in the reticulo-rumen, a process that significantly differs from that in non-ruminants [17](#page=17). ### 5.1 Carbohydrate digestion in non-ruminants In simple-stomached animals (non-ruminants), carbohydrate digestion begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase. Disaccharides like maltose, sucrose, and lactose are then broken down into monosaccharides by specific enzymes: maltase, sucrase, and lactase, respectively. Maltose yields glucose and glucose, sucrose yields glucose and fructose, and lactose yields glucose and galactose. The digestibility of starch is influenced by its source and the animal species [13](#page=13) [16](#page=16). ### 5.2 Carbohydrate digestion in ruminants The primary distinction in ruminant digestion is the extensive fermentative or microbial digestion occurring in the reticulo-rumen, where ingesta resides for an extended period. This environment, characterized by moisture, warmth, and a specific pH, is conducive to microbial survival and activity. Rumen microorganisms, comprising bacteria, fungi, and protozoa, play a crucial role in breaking down carbohydrates [17](#page=17) [19](#page=19) [20](#page=20). #### 5.2.1 Microbial fermentation of carbohydrates All plant carbohydrates, including fibrous components, are subjected to digestion by rumen microbes. Ruminants possess a superior ability to utilize fibrous carbohydrates compared to other herbivores like horses or rabbits, which rely more on microbial digestion in the cecum and large intestine [23](#page=23). ##### 5.2.1.1 End products of carbohydrate fermentation The fermentation of carbohydrates by rumen microbes yields two main categories of products: volatile fatty acids (VFAs) and gases [24](#page=24). **Volatile Fatty Acids (VFAs):** The major VFAs produced are: * Acetic acid: Constituting approximately 65% of the VFAs [24](#page=24). * Propionic acid: Making up 15-20% of the VFAs [24](#page=24). * Butyric acid: Comprising 10-15% of the VFAs [24](#page=24). * Isobutyric and valeric acids: Produced in small quantities [24](#page=24). The concentration of these primary VFAs in the rumen is influenced by several factors: * **Ration composition:** The ratio of roughages to concentrates [25](#page=25). * **Microbial population:** The types of organisms present [25](#page=25). * **Feed intake level:** The amount of food consumed [25](#page=25). * **Feeding frequency:** How often the animal is fed [25](#page=25). Mature fibrous forages tend to increase VFA production, with a higher proportion of acetic acid (around 70%). Conversely, increasing the concentrate proportion in the diet leads to a higher proportion of propionic acid [25](#page=25). **Gases:** Significant amounts of gases are also produced during rumen fermentation, primarily: * Carbon dioxide: Accounts for approximately 40% [24](#page=24) [28](#page=28). * Methane: Constitutes 30-40% [24](#page=24) [28](#page=28). * Hydrogen: Makes up about 5% [24](#page=24) [28](#page=28). Other gases like nitrogen, oxygen, and hydrogen sulfide can also be present [28](#page=28). > **Tip:** The production of methane is a significant concern in ruminant agriculture due to its contribution to greenhouse gas emissions. ##### 5.2.1.2 Absorption and utilization of volatile fatty acids (VFAs) VFAs produced in the rumen are absorbed directly from the rumen, reticulum, omasum, and the large intestine [26](#page=26). * **Acetic acid:** Absorbed acetic acid enters the systemic circulation and is distributed to various organs and tissues. It serves as a primary source of energy and is utilized for fatty acid synthesis. Mammary glands specifically use acetate for the synthesis of milk fatty acids. Each molecule of acetic acid yields approximately 10 ATP [26](#page=26). * **Propionic acid:** In the liver, propionic acid is converted into glucose. This process of gluconeogenesis from propionic acid is critical for ruminants to prevent ketosis, a metabolic disorder. Each molecule of propionic acid provides around 17 ATP [26](#page=26) [27](#page=27). * **Butyric acid:** This VFA is metabolized within the rumen wall and then enters the portal blood circulation as $\beta$-hydroxybutyric acid (BHBA). Each molecule of butyric acid yields approximately 25 ATP [27](#page=27). #### 5.2.2 Fate of gases The gases produced during fermentation are primarily expelled from the rumen through eructation, commonly known as belching. If gas accumulates and cannot be expelled, it leads to a serious condition called bloat [28](#page=28). --- # Factors affecting crude fiber digestion and carbohydrate metabolism This section examines the various factors influencing the microbial digestion of crude fiber and outlines the metabolic fate of absorbed carbohydrates [38](#page=38) [39](#page=39) [40](#page=40) [41](#page=41) [42](#page=42). ### 6.1 Factors affecting crude fiber digestion The ability of an animal to digest crude fiber is influenced by several key factors [38](#page=38): #### 6.1.1 Species * **Ruminants and herbivorous animals** possess a high capacity for crude fiber digestion, able to break down at least 50% of the crude fiber in most feeds [38](#page=38). * **Omnivorous animals** have a limited ability to digest complex polysaccharides [38](#page=38). #### 6.1.2 Age of animal * Young animals generally exhibit a greater capacity for cellulose digestion compared to adult animals [38](#page=38). #### 6.1.3 Nutritional habits of the animal * The digestibility of crude fiber increases with the duration for which crude fiber has been a consistent component of the diet [39](#page=39). * This suggests the development of a specialized microflora within the digestive tract that is capable of breaking down cellulose and hemicellulose due to the sustained ingestion of these materials [39](#page=39). #### 6.1.4 Nutrient makeup of the diet * **Easily digestible carbohydrates:** The addition of readily digestible carbohydrates, such as starch, cane sugar, or molasses, to the diet of cattle can decrease the digestibility of fiber [40](#page=40). * **Protein-rich feeds:** Conversely, feeds high in protein tend to promote the microbiological breakdown of fiber [40](#page=40). * **Microflora composition:** The digestive tract microflora comprises various species, which explains the differential effects of diet composition [40](#page=40). * **Deficiencies:** Diets lacking essential minerals, vitamins, or nitrogen can lead to a reduction in the growth and multiplication of cellulose-splitting bacteria, thereby impairing cellulose digestion [40](#page=40). #### 6.1.5 Nature of fibre * **Maturity of plants:** The complex polysaccharides found in mature plants are less digestible than those in young, actively growing plants [41](#page=41). * **Forage type:** Crude fiber in growing pasture grass is more digestible than that found in hay [41](#page=41). * **Harvesting time:** Hay cut early in the growing season is more digestible than hay harvested during late bloom or seed formation stages [41](#page=41). ### 6.2 Carbohydrate metabolism Following absorption, carbohydrates primarily in the form of glucose, galactose, and fructose, undergo metabolism through three main pathways [42](#page=42): 1. **Energy source:** They serve as an intermediate source of energy for the body's metabolic processes [42](#page=42). 2. **Glycogen synthesis:** They are utilized as precursors for the synthesis of glycogen, which is stored in the liver and muscles [42](#page=42). 3. **Triglyceride synthesis:** They can also be converted into triglycerides, a form of stored fat [42](#page=42). --- # Bloat in cattle and sheep Bloat is a significant gastrointestinal issue in ruminants characterized by the excessive accumulation of gases within the rumen, hindering normal eructation [43](#page=43). ### 7.1 Definition and cause Bloat is defined as a problem of gas accumulation in the rumen of cattle and sheep. This condition most commonly arises when these animals graze on pastures rich in legumes such as alfalfa or clover. The fundamental cause is the formation of stable foam and excessive frothing during rumen fermentation, which obstructs the animal's ability to eliminate gases through belching (eructation) [43](#page=43). Several factors contribute to this foam formation: * Soluble proteins present in fresh legumes [44](#page=44). * Saponins, which are naturally occurring compounds found in some plants [44](#page=44). * Salivary mucoproteins, components of the animal's saliva [44](#page=44). * Specific slime-producing bacteria within the rumen environment [44](#page=44). Bloat can also be induced by: * Cattle grazing on young, lush legumes [44](#page=44). * Feeding large quantities of concentrates [44](#page=44). > **Tip:** Understanding the role of dietary components and rumen microbes is crucial for comprehending bloat pathogenesis. ### 7.2 Symptoms The primary symptom of bloat is a noticeable inflation and swelling of the rumen, typically observed on the animal's left flank. This distention occurs when gases accumulate in the rumen in excessive amounts, or when the animal's capacity to expel these gases is compromised [45](#page=45). ### 7.3 Prevention and control Management strategies for bloat focus on preventing gas accumulation and foam formation: * **Antifoaming agents:** The administration of antifoaming agents, such as mineral oils, has proven effective in preventing bloat [45](#page=45). * **Pasture management:** Spraying pastures with an antifoaming agent before grazing has also been utilized as a control measure [45](#page=45). * **Antibiotics:** Certain antibiotics can aid in controlling bloat, likely by modulating the rumen microbial population involved in fermentation and foam production [45](#page=45). > **Example:** Administering a drench of mineral oil to cattle before they are turned onto a high-risk legume pasture can help reduce the incidence of bloat by breaking down the foam in the rumen. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Carbohydrates | Organic compounds primarily composed of carbon, hydrogen, and oxygen, serving as essential energy sources in livestock diets and major components of plant tissues. | | Lipids | Fats and oils that are a secondary source of energy in livestock rations, contributing to energy supply but are generally less abundant and digestible than carbohydrates. | | Protein | A nutrient that can be utilized for energy, though its primary role is in building and repairing tissues; it contributes to the energy content of livestock rations. | | Monosaccharides | Simple sugars with the general formula $C_nH_{2n}O_n$, such as pentoses (5-carbon sugars like arabinose, xylose) and hexoses (6-carbon sugars like glucose, fructose). | | Disaccharides | Sugars formed from the combination of two monosaccharide units, including sucrose, maltose, and lactose, which are broken down into simple sugars during digestion. | | Polysaccharides | Complex carbohydrates made up of many monosaccharide units linked together, such as starch, cellulose, hemicellulose, and glycogen, serving as energy storage or structural components. | | Starch | A principal reserve carbohydrate in plants, composed of glucose units, which is readily digestible and a significant source of energy for livestock. | | Cellulose | A structural polysaccharide forming the plant cell wall, classified as a fibrous carbohydrate, which can be digested by microbial fermentation in the rumen and cecum. | | Hemicellulose | A component of plant cell walls, often found alongside cellulose, that is a fibrous carbohydrate and can be partially digested by microbial action. | | Lignin | A complex phenolic polymer found in plant cell walls, forming lignocelluloses with cellulose and hemicellulose; it is largely indigestible by most animals. | | Crude fibre (CF) | The insoluble portion of carbohydrates in feeds, primarily consisting of cellulose, a variable proportion of hemicelluloses, and a high proportion of lignin, playing a crucial role in digestive tract function. | | Nitrogen-free extract (NFE) | Readily available carbohydrates that include all starches and sugars, and a large part of cellulose, representing the more digestible carbohydrate fraction in feeds. | | Volatile fatty acids (VFA) | Short-chain fatty acids such as acetic acid, propionic acid, and butyric acid, produced by microbial fermentation of carbohydrates in the rumen; they are the primary energy source for ruminant animals. | | Acetic acid | A major volatile fatty acid produced during rumen fermentation (approx. 65%), absorbed and used systemically as an energy source and by mammary glands for milk fat synthesis. | | Propionic acid | A volatile fatty acid produced during rumen fermentation (15-20%), which is converted to glucose in the liver, crucial for preventing ketosis in ruminants. | | Butyric acid | A volatile fatty acid produced during rumen fermentation (10-15%), which is converted to B-hydroxy butyric acid in the rumen wall and absorbed as an energy source. | | Rumen | The largest compartment of the ruminant stomach, characterized by microbial fermentation of ingested feed, where most carbohydrate digestion occurs before enzymatic digestion. | | Microbial digestion | The breakdown of feed components, particularly carbohydrates, by microorganisms (bacteria, protozoa, fungi) residing in the digestive tract, especially prominent in ruminants and hindgut fermenters. | | Bloat | A serious condition in ruminants characterized by the accumulation of gases in the rumen, often caused by excessive frothing and stable foam formation that prevents normal gas elimination by belching. | | Eructation | The act of belching or expelling gas from the stomach, which is the normal process for eliminating gases produced during rumen fermentation. |