Apparent and standardized ileal digestibilities of amino acids for pigs fed corn- and soybean meal-based diets at varying crude protein levels.

The objective of this study was to determine the effect of CP level in corn- and soybean meal-based diets on apparent (AID) and standardized ileal digestibility (SID) of AA. Six pigs (initial BW, 47.1 ± 1.0 kg) fitted with T-cannula at the distal ileum were fed 6 diets for 6 periods in a 6 × 6 Latin square design. The 6 diets consisted of a nitrogen-free diet and 5 corn- and soybean meal-based diets that contained CP of 68, 105, 141, 177, and 214 g/kg. Each period consisted of a 5-d adjustment period and 2 d of ileal digesta collection for 10 h on each of d 6 and 7. The ratio of corn:soybean meal was fixed at 3 to 2 by weight and cornstarch was added to dilute the CP concentration. Chromic oxide was added at 5 g/kg as an indigestible marker. The results showed basal endogenous loss ranged from 65 mg/kg of DMI for Met to 3,104 mg/kg of DMI for Pro. Proline and Gly (1,053 mg/kg of DMI) were the 2 most abundant AA in endogenous flow and together accounted for approximately 43% of the total endogenous AA flow. Of the basal ileal endogenous CP, total AA accounted for 82%. The AID were 80.9 to 84.7%, 85.1 to 87.4%, 72.9 to 79.5%, and 86.5 to 87.9% for Lys, Met, Thr, and Trp, respectively, with corresponding SID being 86.6 to 89.0%, 87.5 to 90.5%, 82.7 to 88.2%, and 90.2 to 94.6%, respectively, as dietary CP increased from 68 to 214 g/kg. There were linear increases in AID of N, Arg, Gly, Ile, Lys, Ser, Thr, Tyr, and Val (P ≤ 0.05) as CP increased and linear decreases in SID of N and all AA measured in this study except Lys, Met, and Pro (P ≤ 0.05). Both linear and quadratic effects were observed in AID for Pro (P < 0.05). In conclusion, the protein content of corn-soybean meal diets evaluated in the current study affected SID of most indispensable and dispensable AA, excluding Lys, Met, and Pro.