A generalized flow of energy in the animal is shown in Fig. 1. Briefly, heat of combustion of food consumed is termed intake energy (IE). A substantial 20 80% of the food energy consumed is voided from the animal as fecal energy (FE) and the difference (IE — FE) is termed apparently digested energy (DE). Portions of IE are also lost as combustible gaseous energy (GE) and as urinary energy (UE). The remainder (IE — FE — GE — UE) is termed metabolizable energy (ME). ME may be recovered (RE) as product such as tissue (TE), milk (LE), conceptus (YE), egg or ovum (OE), wool or hair (VE), etc., or may be transferred to the environment as heat (HE). Heat energy may be the result of a variety of functions including digestion and absorption (HdE), fermentation (HfE), waste formation and excretion (HwE), basal metabolism (HeE), activity (HjE), thermal regulation (HcE), and product formation (HrE). An increase in heat production following feeding is termed heat increment (HjE) and includes increases in HdE, HfE, HwE, and HrE. This scheme may be summarized by the equation: IE = FE + UE + GE + HE + RE. This identity partitions the food energy into the major components associated with animal energetics. It may be expanded to include a few or many of the intermediate steps involved, and each component can be divided into component parts. The inclusion or exclusion of intermediate transformations does not prejudice the balance of the equation. All energy balance techniques and all systems of expressing relationships between the animal's requirements and the usefulness of a food to supply those needs are related to this classical energy balance identity (see Ref. ).
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