An experiment was conducted to test the hypothesis that values for standardized ileal digestibility (SID) of amino acids (AA) in cereal grains and fiber rich ingredients obtained using the direct method are not different from values obtained using the difference method.

Sixteen ileal-cannulated barrows (69.45 ± 5.01 kg) were allotted to an 8 diets × 4 period Youden square design. Each period consisted of 5 d of adaptation and 2 d of ileal digesta collection. Four diets were based on soybean meal (SBM), corn, wheat, or wheat middlings as the only AA-containing ingredients. Three additional diets were based on a mixture of SBM and corn, wheat, or wheat middlings and a N-free diet was also used. The SID of AA in the 4 diets containing SBM, corn, wheat, or wheat middlings as the sole source of AA were calculated using the direct method. The SID of AA in corn, wheat, and wheat middlings were also calculated by difference from the 3 diets containing SBM and corn, wheat, or wheat middlings by subtracting the contribution from SBM of each AA from the SID values for the mixed diet.

Data were analyzed using a model that included the method of determination of AA digestibility as the fixed effect, and pig and period as random effects. There was no difference between the direct and the difference method for the SID of most AA in corn, wheat, and wheat middlings, although the SID of Trp was lower (P < 0.05) for the difference method than values derived by the direct method for both corn and wheat middlings.