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In order to formulate, produce and market animal diets it is necessary to gain information about substance classes (i. e. that contribute to the energy content) and special compounds (i. e. that influence digestibility). Different chemical, physical and biological methods have been developed to obtain the information needed.
The most important precondition when analyzing feedstuffs and diets is the sampling procedure. Thereby the person taking the samples has to keep the following principles in mind:
The proximate or Weende analysis of feed is a quantitative method to determine different macronutrients in feed. Basically it is the partition of feed compounds into six categories by means of common chemical properties. The categories are moisture (crude water), crude ash (CA), crude protein (CP), ether extracts (fats or lipids; EE), crude fiber (CF) and nitrogenfree extractives (NFE).
The feed sample is initially dried at 103 °C for 4 hours. The weight loss of the sample is determined and the crude water fraction is calculated. Ashing the sample at 550 °C for 4 hours removes the carbon from the sample, viz. all organic compounds are removed. Again calculating the weight loss of the feed sample from the dry matter to crude ash (CA) content mathematically determines the organic matter fraction. The nitrogen content of the food is the basis for calculating the crude protein (CP) content of the feed. The method established by Kjeldahl converts the nitrogen present in the sample to ammonia which is determined by titration. Assuming that the average nitrogen content of proteins is 16 percent multiplying the nitrogen content in % obtained via Kjeldahl analysis with 6.25 gives an approximate protein content of the sample. Alternatively the Dumas method can be applied to measure CP. Fats and lipids are extracted continuously with ether, after evaporation of the solvent the residue remaining is the ether extract (EE) fraction. The carbohydrates in a feed sample are retrieved in two fractions (CF, NFE) of the proximate analysis. The fraction, which is not soluble in a defined concentration of alkalis and acids, is defined as crude fiber (CF). This fraction contains cellulose, hemicellulose and lignin. Sugars, starch, pectins and hemicellulose etc. are defined as nitrogen-fee extractives (NFE). This fraction again is not determined chemically it is rather calculated by substracting CP, EE and CF from organic matter.
In recent years the over 100 year old proximate system has been advanced and improved. Especially the imprecision of CA, CF and NFE as well as CP had been criticized. Modern methods to determine the exact composition of the CA fraction via atomic absorption spectroscopy and the CP fraction via amino acid analyzers, near infrared spectroscopy (NIRS) etc. have been established. Improving the information gained from analysis of feedstuffs and diets also involves the determination of sugars and starch (polarimetric methods) contained in the NFE fraction of the proximate analysis. Van Soest developed a procedure to detect the different components of the cell wall. This helps specify the CF and NFE fraction. Thereby the complete amount of cell wall components is obtained by digesting (boiling) the feed sample in a so called neutral detergent solution and results in the neutral detergent fiber fraction (NDF). The residue after digestion in a solution with sulfuric acid is called the acid detergent fiber (ADF) and contains mainly cellulose and lignin. Finally the remaining sample is treated with a sulfuric acid with an even higher concentration resulting in a decomposition of cellulose leaving mainly lignin. This fraction is called acid detergent lignin (ADL).
The combination of the proximate analysis with these modern methods allows for a detailed feedstuff analysis.