The direct and difference procedures result in similar estimates for amino acid digestibility in feed ingredients fed to growing pigs

An experiment was conducted to test the hypothesis that values for standardized ileal digestibility (SID) of amino acids (AA) in corn, wheat, and wheat middlings obtained using the direct procedure are not different from values obtained using the difference procedure. Sixteen ileal-cannulated barrows (initial BW: 69.5 +/- 5.0 kg) were allotted to a replicated 8 x 4 Youden Square design with 8 diets and 4 periods. Each period consisted of 5 d of adaptation to the diet and 2 d of collection of ileal digesta. 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, SBM and wheat, or SBM and wheat middlings, and an N-free diet was also used. The apparent ileal digestibility (AID) and the SID of crude protein (CP) and AA for the 4 diets containing SBM, corn, wheat, or wheat middlings as the sole source of AA were calculated using the direct procedure. The AID and SID of CP and AA for the 3 mixed diets containing SBM and corn, wheat, or wheat middlings were also calculated and the contribution of digestible AA from SBM was subtracted from the AID or SID values for the diets. The AID or SID of AA in corn, wheat, and wheat middlings were subsequently calculated by difference. Results indicated that the AID values for a few AA were lower (P < 0.05) if the direct procedure was used instead of the difference procedure, regardless of ingredient. The AID of Trp was greater in corn and wheat middlings, and the SID of Trp in corn and wheat middlings tended to be greater, if the direct procedure rather than the difference procedure was used, but that was not the case for wheat (interaction, P < 0.05 and P < 0.10, respectively). However, for all other indispensable AA, and for most of the dispensable AA, the SID of AA in corn, wheat, and wheat middlings was not different between the difference procedure and the direct procedure. Therefore, values for SID of AA in cereal grains and fiber-rich ingredients may be determined using either the direct or the difference procedure.