Gross Energy

The gross energy content or total energy contained in dry feed (IE) varies depending on the carbohydrate, protein, lipid, and mineral content, but is of little value in assessing the value of a diet or dietary component as a source of energy for the animal. For example, starch and cellulose, both polymers of glucose, have similar heats of combustion. Starch is readily digested by most mammalian species, eventually to glucose. In contrast, cellulose is not attacked by mammalian digestive enzymes, and thus does not provide energy substrates to animals such as humans. In contrast, starch or cellulose may be digested in animals having pregastric microbial fermentation (e.g., ruminants) by amylolytic and cellulolytic microbes, resulting in the production of volatile fatty acids (VFA), CH4, water, and CO2, but the patterns of VFA and

Intake Energy (IE)

Digestion and Absorption (HdE)

Fermentation (HjE)

Waste Formation, Excretion (HwE)

Basal Metabolism (HeE) Voluntary Activity (RE) Thermal Regulation (HcE) Product Formation (HrE)

Heat Production (HE)

Intake Energy (IE)

Waste Formation, Excretion (HwE)

Fecal Energy (FE)

Undigested Food (F¡E) Metabolic (F E)

Methane Energy (GE)

Urinary Energy (UE)

Metabolizable Energy (ME)

Fecal Energy (FE)

Undigested Food (F¡E) Metabolic (F E)

Methane Energy (GE)

Urinary Energy (UE)

Metabolizable Energy (ME)

Recovered Energy (RE)

Tissue Energy (TE) Milk Energy (LE) " * Conceptas (YE), Hair, Wool (VE), Ovum (egg, OE)

Fig. 1 Schematic partition of energy in the animal.

quantities produced differ.[2] Starch and cellulose may also be digested in animals having a postgastric fermentation (e.g., horse, rabbit), such that much of the starch is digested and absorbed in the stomach and small intestine. Some starch, as well as the cellulose, is digested by microbial fermentation in the cecum and large intestine, but availability of the VFAs produced is reduced compared to the ruminant.

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