Predicting the energy value of foods

An alternative approach to chemical analysis is to estimate digestibility by in vitro fermentation (see p. 241) and predict the energy value from the digestible organic matter content of the food. For forages given to ruminants, the following equation is often used  

The introduction of new methods of analysis, such as near-infrared spectrometry (NIRS), has speeded up the prediction of digestibility and energy values for a range of foods (see Chapter 10). However, it must be remembered that all laboratory methods of predicting the energy value of foods are dependent on the availability of reliable data obtained from properly conducted metabolism trials with animals. It is the systematic evaluation of many food samples in such trials that provides the regression equations that are subsequently used to predict energy values from laboratory measurements. 

The four equations differ in principle. The equation for poultry assigns a logically derived factor for each constituent. For example, assuming the energy value of fat to be 0.0393 MJ/g, the factor for acid ether extract indicates that 0.03431/0.0393 = 

87 of the gross energy of the lipid in poultry food is metabolisable. The equations for horses and pigs start with a constant value and adjust it upwards, depending on the protein and acid ether extract content, and downwards, depending on the ash and fibre content. The equation for ruminants is based on a biological rather than a chemical evaluation of the food but includes an adjustment for acid ether extract. 

Earlier equations were based on the original components of the proximate analysis. An example of an earlier equation for pig foods, based on a compilation of European studies made by the European Association for Animal Production, is  

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As a consequence of their differing digestive systems and related metabolism, different systems are used for expressing the energy value of foods according to the type of animal concerned. Chapter 12 describes these systems and the methods used to predict the energy value of foods. 

The energy value of some foods is variable. Consequently, most systems include equations for predicting the energy value of foods from their chemical composition determined by laboratory analysis. 

Why Do We Need to Know This Material The laws of thermodynamics govern chemistry and life. They explain why reactions take place and let us predict how much heat reactions release and how much work they can do. Thermodynamics plays a role in every part of our lives. For example, the energy released as heat can be used to compare fuels, and the energy resources of food lets us assess its nutritional value. The material in this chapter provides a foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions. 

For the Cornell net carbohydrate and protein system for predicting the requirements of cattle, the intake of lactating cows is predicted from the net energy content of the food and the milk yield and milk fat content of the animal. Predicted values are then adjusted for six other factors age, breed and body fat content of the cattle, the inclusion in the diet of any additives, the environmental temperature, and another component of the environment called mud . 

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