Variation in Amino Acid Standarized Ileal Digestability of Soybean Meal

Heat treatment of soybean meal (SBM) causes amino acid (AA) damage, particularly to lysine. Further, birds and pigs cannot use heat-damaged AA for growth. Lysine is the first- and second- limiting AA in corn-soybean meal diets for pigs and poultry, respectively. Currently, there is no reliable laboratory analysis for the determination of digestible lysine in soybean meal. The objective of this study was to compare SBM AA digestibility using in vivo and vitro procedures.

In vivo standardized ileal digestibility (SID, Stein et al. 2007) of AA in 24 SBM samples was determined in 28-d-old Cobb 500 broilers and using a 2-step in vitro digestion process (Wilfart et al. 2007). A semi-purified diet was fed from d 21 to 28 of age where SBM was the only source of protein. Then, birds were euthanized and dissected to collect ileal contents (Lemme et al. 2004). A Complete Randomized Design with 4 pens of 5 birds each per SBM was used and data were analyzed using proc mixed (SAS® Institute, Gary, NC)

In vivo Lys SID ranged from 69-93%. Amino acid content of SBM was not correlated (r = -0.16 to 0.21; P = 0.33 to 0.98) with in vivo SID. Increased Lys SID was associated with increased SID of Phe, Leu, Ile, Val, Tyr, Ala, Thr, Glx, Asx, Met, His, and Gly (r² = 0.63 to 0.93; P < 0.001). Correlations among SID for Lys Pro, Arg, and Ser were poor (r² = 0.14 to 0.43; P = 0.001 to 0.003). Lastly, results indicated no association (r² = 0.00 to 0.08; P = 0.17 to 0.99) between in vivo and in vitro SID for any of the AA tested.

It appears that Lys SID may be a good indicator of SID of most other essential AA. Individual AA content of SBM is not a good indicator of its digestibility.

The lack of correlation between AA level and in vivo SID in SBM supports the practical use an AA-specific digestibility coefficient for different SBM batches. Finally, current in vitro digestibility techniques seemed inadequate in identifying in vivo SID differences and further analytical improvements are needed.

Lemme A et al. 2004. World's. Poult. Sci. J. 60:423-437; Stein H et al. 2007. J. Anim. Sci. 85:172-180; Wilfart A et al. 2007. Anim. Feed Sci. Technol. 141:171-183

Keywords: Amino acid, digestibility, broiler.