Studies were carried out to investigate the effect of dietary amino acid level on apparent ileal amino acid digestibility, Six barrows, average initial BW 35 kg, were fitted with a simple T-cannula at the distal ileum and fed six diets according to a 6 x 6 Latin square design. Six cornstarch-based diets containing six levels of CP from SBM (4, 8, 12, 16, 20, and 24% CP, respectively) were formulated. Chromic oxide was included as a digestibility marker. Each experimental period consisted of 8 d. After a 6-d adaptation period, ileal digesta were collected for 24 h during d 7 and 9 at 2-h intervals. The pigs were fed twice daily, equal amounts, at 0800 and 2000. The dietary allowance was 1,600 g/d during the first period and increased by 100 g each following period. There was a quadratic increase (P < .05) in apparent ileal amino acid digestibility as the dietary CP content was increased from 4 to 24%. Initially, the apparent ileal amino acid digestibilities increased sharply then gradually reached their plateaus, after which there were no further increases and the digestibility values became independent of the dietary amino acid levels. The lower end points of 95% confidence intervals of the plateau ileal digestibility values were defined to be the initial plateau digestibilities. The dietary CP and amino acid contents, corresponding to the initial plateau digestibility values, represent the dietary threshold levels. For CP and the indispensable amino acids, the dietary threshold levels (percentage of diet, DM basis) and the corresponding initial plateau digestibility values (percentage, in parentheses) in SBM were as follows: CP, 17.1 (80.9); arginine, 1.15 (90.6); histidine, .43 (87.2); isoleucine, .83 (86.2); leucine, 1.33 (85.6); lysine, .94 (86.0); methionine, .24 (89.0); phenylalanine, .82 (87.9); threonine, .63 (77.0); and valine, .84 (83.7). In conclusion, these studies show that the dietary amino acid levels quadratically affect apparent ileal amino acid digestibilities. The plateau apparent ileal amino acid digestibilities, which are independent of the dietary amino acid contents, should be determined. To estimate the plateau apparent ileal amino acid digestibilities, the amino acid contents in the assay diet should exceed the corresponding threshold levels.
