D Metabolizable Energy AME or TME

Metabolizable energy is the most costly nutrient in an ingredient or diet, yet unfortunately it is the most difficult to measure. As for digestible amino acids, estimates of AME or TME require a bioassay involving live animals. The only lab assay for energy is gross energy and this is merely a starting point used in AME or TME determinations. Gross energy is the total heat evolved when an ingredient or diet is burned in an atmosphere of oxygen. Wood and corn have approximately the same gross energy.

In an energy bioassay, birds are fed diets containing a given quantity of the ingredient, and feed intake and excreta output measured over a 3-5 d balance period. Gross energy is determined on feed and dried excreta and calculations made to determine the metabolizable energy derived from the ingredient under test. In the TME assay, the bird is force-fed only the ingredient under test, and so the estimate of ME is simplified. With all the laboratory and sample preparation necessary for the test, it is challenging to generate results within a 2-3 week period, at a cost approaching $1,000 USD per sample.

Because of the complexity and cost involved in measuring AME or TME, various chemical or in vitro systems have been developed. Essentially these methods attempt to correlate more easily measurable components, with available energy. One of the first such calculations was applied by Carpenter and Clegg (1956) and their equation is still as good as anything developed in the last 50 years.

ME (kcal/kg) = 53 + 38(%CP + 2.25 x % fat + 1.1 x % starch + % sugar)

This type of prediction equation is accurate to within ± 200 kcal/kg and so is useful for giving an estimate of AME for a novel ingredient. There have also been ME assays based on enzyme digestion. The most successful uses duodenal fluid taken from a pig, and measuring the gross energy of solubilized components after 1 - 2 hr of incubation. AME has also been predicted by NIRA (see next section).

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