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Apparent ileal digestibility (AID) of amino acids (AA) can be affected by the AA content in the feed. This is because endogenous AA accounts for a greater proportion of the total AA in ileal digesta when feeding pigs a low-protein diet versus a high-protein diet. Standardized ileal digestibility (SID) of AA in protein-containing ingredients could be used to predict the digestibility coefficients of mixed diets more accurately when the diet contains a low-protein ingredient because SID of AA are less influenced by CP and, thus, are more additive than AID of AA. Considering the aforementioned factors that might challenge the relationship between CP level and AID and SID values of AA, this study was designed to test the hypothesis that the amount of dietary protein does not affect the SID of AA in corn- and soybean meal-based diets. The objective of the study was to determine the AID and SID of AA responses to dietary CP level. Basal endogenous ileal AA losses for AID of AA correction were determined using a nitrogen-free diet for growing-finishing pigs. 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. Prol 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.
H Zhai and O Adeola, 2011. Apparent and standardized ileal digestibilities of amino acids for pigs fed corn- and soybean meal-based diets at varying crude protein levels. Journal of Animal Science, 89: 3626-3633. http://dx.doi.org/10.2527/jas.2010-3732