Abstract
SUMMARY Databases compiling the amino acid (AA) digestibility values of 20 common poultry feed ingredients from 2 commercial companies, Evonik Degussa (Burr Ridge, IL; chick assays) and Ajinomoto Heartland (Chicago, IL; rooster assays), were used to investigate the influence of AA concentration on observed digestibility values. Casual observation of the rooster assay database suggested that AA digestibility may be related to AA concentration. Ideally, digestibility should be a property of the feed ingredient and should not be related to the concentration at which an ingredient is fed. If the AA concentration during an assay has a large effect on its digestibility, then the results may not be valid at different levels of feed ingredients or when the AA concentration in the test situation is different from actual feeding levels in a diet. Differences in digestibility values in the Evonik Degussa and Ajinomoto Heartland databases were compared. Multiple regression techniques were then used to test the hypothesis that the results of the AA digestibility assays were related to the AA concentration in the feed ingredient being tested. Total AA concentration and digestible AA averaged 6 and 14% higher in the rooster database than in the chick database, respectively. Differences in assay type were quantitative because slopes of digestible AA = f(total AA concentration) were positive for 19 of 20 ingredients in each database. However, when all data were included in 1 model, the influence of concentration on digestibility was very unlikely to have been due to chance (P < 0.0006), as evidenced by the interaction of concentration × assay type. Removing the lowest (Trp) or highest (Ile) AA was not critical to the conclusions. The effect of AA concentration on its digestibility was quantitatively the same for each AA (P < 0.50) except Cys (P < 0.05). We confirmed, based on data from the 3 selected articles, that this relationship was not due only to the different samples in the different databases. Therefore, to best predict the digestibility of any AA in any ingredient, its intended concentration in the diet should be known.
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