Principles of Animal Nutrition - Chemical Evaluation of Feedstuffs - PDF

**Principles of Animal Nutrition 1** Lecturer: Henaku A. Addo **CHEMICAL EVALUATION OF FEEDSTUFFS** **Weende System of Feed Analysis (Proximate Analysis)** Proximate analysis is a chemical analysis method used to determine the composition of animal feed. The proximate analysis system was develop...

**Principles of Animal Nutrition 1** Lecturer: Henaku A. Addo **CHEMICAL EVALUATION OF FEEDSTUFFS** **Weende System of Feed Analysis (Proximate Analysis)** Proximate analysis is a chemical analysis method used to determine the composition of animal feed. The proximate analysis system was developed at the Weende Experimental Station at the University of Gottingen in Germany by Henneberg and Stohmann in 1865. Although this system is very old and has some limitations, it still forms the basis of feed evaluation and provides a quantitative approach to determining various constituents of food. Based on common bio-chemical properties, the system evaluates and analyses six categories of micronutrients found in food and feedstuffs. These categories are moisture (crude water), crude ash (CA), crude protein (CP), ether extracts (fats or lipids), crude fibre (CF), and nitrogen-free extract (NFE). **Water & Dry Matter (DM)** Accurate determination of dry matter (DM) in feeds is critical since reliable measurements of the remainder proximate constituents of feed are dependent on this accuracy. The water and dry matter of a feed sample is determined by evaporating all the moisture from the feedstuff. The weight of a representative sample of the feedstuff to be analysed is recorded before being initially dried in an oven to remove moisture. Most test material is usually dried at a temperature of 100-135ºC until all the water has evaporated. Oven-drying methods approved by the Association of Official Analytic Chemists (AOAC) include drying at 100ºC under vacuum of 1.3 × 10^4^ Pa for five (5) hours, drying samples in a forced-air oven at 135ºC for two (2) hours, or drying at 105ºC for six (6) hours (Cherney, 2000). However, when ascertaining DM and moisture content of fermentable feeds which contain volatile compounds, laboratories in some countries often employ a two-step procedure that normally involves firstly determining a partial DM value by drying samples in a forced-air oven at 60ºC overnight, then secondly applying AOAC methods to ground samples of the first step to get a total DM value. Alternative methods to accurately determine moisture in a feed sample include toluene distillation (best with fermentable feeds), saponification, Karl Fischer method, gas chromatography, and Near-infrared reflectance spectroscopy (NIRS). **Crude Protein (CP)** Crude Protein (CP) of a sample is determined using the Kjeldhal laboratory procedure which measures the total nitrogen content of the feedstuff. This procedure involves three (3) steps (digestion, neutralization, and titration) and can be summarized as follows: Firstly, the material to be analyzed is digested with sulphuric acid to convert all forms of nitrogen (except nitrates & nitrites) to ammonia. In this step it should be noted that ammonia gas is not liberated since the nitrogen in the digested material is in the form of ammonium sulphate salt. Secondly, ammonia gas is liberated when the ammonium sulphate precipitate is neutralized with an alkaline such as sodium hydroxide (NaOH). Further, the ammonia is combined with boric acid to convert the gas to ammonium ions (ammonium borate). Lastly, the nitrogen concentration of the feed is estimated by titrating ammonium borate with either sulphuric or hydrochloric acid. The concentration of hydrogen ions required to reach the titration end-point is equivalent to the concentration of nitrogen in the original sample. Percentage CP is obtained by multiplying the derived nitrogen figure by a factor of 6.25. In the calculations it is assumed that proteins contain approximately 16% nitrogen. The CP value encompasses both 'true protein' and non-protein nitrogen (NPN) compounds (free amino acids, ammonium salts, and urea). **Crude Ash** The ash content is estimated by burning a pre-determined weight of the dry matter portion of the test material in a muffled oven at 550 to 600ºC to remove all organic matter. Leftover residue after the burn represents the inorganic constituents and total mineral content of the sample. Similar to dry matter analysis of fermentable feeds, trace elements such as selenium (Se), lead (Pb), and cadmium may be volatilized during the ashing process. Proximate ash analysis is often a preliminary step to specific mineral analysis using specialized equipment. **Crude Fibre (CF)** Proximal analysis of crude fibre estimates the less soluble, fibrous fractions (lignin, cellulose, and hemi-cellulose) associated with feed carbohydrates. The procedure is performed by boiling ether extract fat-free residue in acid solution before rinsing, then boiling in sodium hydroxide and rinsing again. Lastly, the residue is dried, weighed, ashed, and re-weighed. The crude fibre content is calculated as the difference between the pre-ash and post-ash weight. The detergent system of feed analysis was developed by Peter Van Soest at the United States Department of Agriculture in the 1960s and is today one of the most important sets of feed assays in ruminant nutrition, but also, increasingly, in non-ruminant research. The concept behind detergent fibre analysis is that plant cells can be divided into less digestible cell walls (comprising hemicellulose, cellulose and lignin) and mostly digestible cell contents (comprising starch and sugars). These two components can be separated by using two detergents: a neutral detergent and an acid detergent. Neutral Detergent Fibre (NDF) is a good indicator of bulk and thus feed intake. Acid Detergent Fibre (ADF) is a good indicator of digestibility and thus energy intake. **Ether Extract (EE)/Crude Fat** Ether extract/crude fat analysis evaluates the amount of lipids in feeds. Oven dried samples are ground and extracted with an organic solvent such as diethyl ether, and the remaining residue is dried and weighed. Ether extract is determined to be the difference between the original dried sample and the ether extract residue. **Nitrogen-Free Extract (NFE)** Nitrogen-Free Extract represents a mixture of constituents not determined in the previous proximate analysis fractions. The NFE fraction mostly contains starches, sugars, pectins, and hemi-cellulose. The percentage of NFE is calculated by subtracting the sum of moisture, crude protein, crude fibre, ash and ether extract (expressed as a percentage) from one hundred (100). C:\\Users\\jc277072\\Desktop\\proximate%20analysis.jpg **[Proximate Analysis Limitations]** Like all systems, the proximate analysis is not perfect and has several inherent limitations. Aspects of the system where shortcomings are most evident include the analyses of the ash, crude fibre (CF), crude protein (CP), and nitrogen-free extract (NFE) proximate constituents. A brief overview of some of the failings of some components of the proximate system and improved and/or modified procedures adequately address these shortcomings will be examined. **Ash** The remaining residual ash after ignition and burning of the dry matter sample is not truly reflective of inorganic material in food. Major drawbacks in the ashing process include loss of some inorganic material through volatilization, and presence of contaminants in the final product such as silica and carbon. Moreover, the analysis is rendered less than meaningful because it fails to provide quantitative information on specific minerals in feed. Alternative methods used to correct this analytical deficiency include 'wet ashing' (used to analyse volatile trace minerals) and spectrophotometric analysis. **Crude Fibre (CF)/Nitrogen-Free Extract** The primary source of error in analysis of CF/EE is that the procedure assumes that all substances soluble in organic solvents such as diethyl ether are lipids. For example, ether extracts also contain fat-soluble vitamins, waxes, and pigments due to solubilisation of plant cell-wall components. Inadequacies in the crude fibre and nitrogen-free extract fractions of the proximate analysis have been addressed by the development and utilization of the detergent-based analytical system. The detergent system was devised by V.J. Van Soest and divides feeds into two parts: (1) a high digestible fraction (plant cell contents) made up of sugar, starches, soluble protein, pectin and lipids; and (2) a variable digestible fraction (plant cell wall contents) comprised of insoluble protein, hemi-cellulose, cellulose, lignin and bound nitrogen. The final products of this system are neutral detergent solubles (cell contents) and neutral detergent fibre (cell wall components). These two fractions are a more accurate representation of the carbohydrate constituent of feedstuff. Use of the detergent system either independently or in conjunction with the proximate analysis system sufficiently corrects the deficiencies of the CF/NFE system. **Crude Protein (CP)** In general, crude protein analysis using the proximate analysis is a fairly good indicator of the protein content of the test material. However, crude protein analysis using the proximate system is based on the faulty premise that the average nitrogen content of crude protein in forages and feed is always 16%. Modern procedures such as the enhanced Dumas method (combustion elemental analysis) and UV-visible spectroscopy have vastly improved the efficiency and accuracy of measuring total nitrogen and protein in various feeds. **[Detergent Method of Forage Analysis (Van Soest Method)]** The generally unsatisfactory nature of the Weende method for estimating the crude fiber content of foods and feeds has been recognized for many years. Boiling with dilute acid and with dilute alkali, which is taken to simulate gastric and intestinal digestion, frequently bears little relationship to the avail ability of complex carbohydrate determined by digestibility studies with ani mals. Although many attempts have been made to modify the crude fiber method or to devise new methods, until recently few approaches have proven satisfactory. The **Van Soest method of fiber analysis** is a widely used system for determining the fiber content of plant materials, especially for animal nutrition studies. It was developed by **Peter J. Van Soest** and provides a more detailed breakdown of fiber components compared to earlier methods like crude fiber analysis. The concept behind the detergent fiber analysis is that plant cells can be divided into less digestible cell walls (contains hemicellulose, cellulose and lignin) and mostly digestible cell contents (contains starch and sugars). Van Soest separated these two components successfully by use of two detergents: a neutral detergent (Na-lauryl sulfate, EDTA, pH =7.0) and an acid detergent (cetyl trimethyl ammonium bromide in 1 N H2SO4).\ Hemicellulose, cellulose and lignin are indigestible in non-ruminants, while Hemicellulose and Cellulose are partially digestible in ruminants. NDF = Hemicellulose + Cellulose + Lignin. ADF = Cellulose + Lignin\ Neutral Detergent Fiber is a good indicator of \"bulk\" and thus feed intake. Acid detergent fiber is a good indicator of digestibility and thus energy intake **The Van Soest carbohydrate classification system** -- -- -- -- -- -- **Approximate digestibilities of foods fed to cattle** -- -- -- -- -- -- **PRINCIPLES OF ENERGY PARTIONING** Energy is the capacity to do work and it exists in many forms including chemical energy, mechanical energy, and heat energy. The energy contained in the food consumed by animals is not available to the animal until the feed has been broken down into its nutrient components. The energy absorbed by an animal is in the nutrients (VFAs, amino acids, glucose, and lipids). Not all the energy contained in feed is available to the animal. **The unit of energy that is used is the joule (J). There are 4.184 joules in a calorie. 10^6^J = 1 megajoule (MJ).** **Partition of feed energy** #### Gross energy (GE) #### Gross energy is the total amount of energy in food. Not all of the energy in food is available for use by animals as some is excreted in urine and faeces, or lost as heat or gases as the feed is digested and metabolized. #### Digestible energy (DE) #### #### Digestible energy (DE) refers to the amount of energy in a food source that is actually absorbed by an animal after digestion**.** The energy in the food that is not digested by the animal (i.e. ends up in the faeces) is not available to the animal. Thus, DE represents the difference between the energy contained in the food and that lost in the faeces. Although digestibility is not a direct measure of energy, it can be used to indicate feed quality. Generally, the greater the digestibility of a feed, the greater the energy available to the animal for metabolic activities (metabolizable energy). Digestibility is generally expressed as a percentage of the feed dry matter (%DM). #### Metabolizable energy (ME) Not all the energy released from digestion is available to animals. A small proportion will be belched as gas (methane and carbon dioxide), excreted in the urine, or lost as heat during rumen fermentation. Metabolizable energy is the energy remaining for the cow to use for metabolic processes (e.g. maintenance, milk production, body condition gain, activity, pregnancy, and growth). The loss of energy in urine and methane is approximately 19% of DE. Therefore, ME of a diet has a constant relationship with DE content and DMD of the diet. This relationship can be expressed in the formula: **ME = 0.81 DE** ME values as an expression of the energy content of feedstuffs are a key measure in determining energy intake and formulating rations or supplements for ruminants. #### Heat increment (HI) #### When ME is converted to net energy (NE), heat is generated by a number of processes. This heat must be debited against the ME content of a food before the quantity of NE that is available to an animal can be calculated. Where do these losses occur? - Ingesting food requires muscular activity by the jaws and by the muscles of the gastro-intestinal tract. - Microbial activity generates heat and this heat is supplied by the chemical energy in the food. - Absorbing some digested nutrients from the gut also involves chemical reactions that require energy to drive them. The quantity of energy required in the utilization of circulating nutrients depends to some extent on what the nutrients are going to be used for. Metabolizing glucose for ATP to use for maintenance is a simpler chemical reaction that requires less energy input than using the same glucose molecule to synthesis fat. The HI is sometimes referred to as the specific dynamic action of food. The efficiency of converting ME to NE is given the symbol k. **K = NE/ME** The two important points about efficiency of converting ME to NE are that it varies depending on the: - ME content of the diet; and - The purpose for which the NE is used **Efficiency of converting ME to NE for various foodstuffs** **Feed** **ME** **k~m~** **k~l~** **k~g~** -------------- -------- ---------- ---------- ---------- Young grass 11.0 0.72 0.62 0.39 Mature grass 8.0 0.66 0.56 0.20 Dry grass 6.5 0.63 0.53 0.10 Maize 13.0 0.76 0.66 0.51 #### Net energy (NE) Energy required at the tissue level for maintenance, growth, milk synthesis, etc. is referred to as net energy. In the ME system, these energy requirements are expressed in units of ME, that is the NE values are converted to ME values by using appropriate k values. Animals can also obtain NE for maintenance, lactation, or gestation by mobilizing net energy that has already been stored in the body in the form of protein and fat (i.e. they lose weight).