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Determination of TMEn and standardized amino acid digestibility in new lines of soybeans with high protein and high sulfur amino acids fed to chickens

Cristobal, M., P. Utterback, A. Ortiz, L. Torres-Mendoza, W. Kim, H. Krishnan and C. Parsons
2024

Two experiments were conducted to test the hypothesis that nitrogen-corrected true metabolizable energy (TMEn) and standardized amino acid (AA) digestibility are not different in 3 new lines of soybeans with increased crude protein (CP) and increased sulfur AA (SAA) compared with conventional soybeans fed to chickens. The 3 new cross lines of soybeans were developed by crossing a high SAA (i.e. CS) with a high CP (i.e. Lee 5) soybean lines. Six soybeans (the 2 parent CS and Lee 5 lines, the 3 cross lines, and Maverick conventional) were evaluated. The 6 soybeans were autoclaved for 25 min to reduce antinutritional factors. The Maverick conventional contained 35.5% CP (1.11% SAA) and 20.1% crude fat. The CS contained similar CP and fat levels, but greater SAA content (1.42% SAA) compared with conventional soybeans. The Lee 5 contained 44.7% CP (1.28% SAA) and 13.3% fat. Thus, as expected, the new cross lines of soybeans had greater CP and SAA content, but lower crude fat levels compared with conventional soybeans. All AA levels were increased in the 3 cross lines, with the largest increase being for Cys which increased from 0.58% in the conventional to 0.82-0.99% in the 3 cross lines. In Experiments 1 and 2, two precision-fed rooster assays were conducted to determine TMEn and standardized AA digestibility in the 6 soybeans using conventional and cecectomized roosters, respectively. For each experiment, 30 individually caged White Leghorn roosters (1 year old) were fasted for 26 h and then crop intubated with 25 g of 1 of the 6 soybeans with a total of 5 roosters per soybean sample. Excreta were then collected quantitatively for 48 h post-feeding. Excreta samples were analyzed for gross energy, nitrogen, and AA. Standardized AA digestibility values were obtained from apparent AA digestibility then corrected for endogenous AA losses. Data were analyzed as a one-way ANOVA for a completely randomized design. Differences among treatments were considered significant at P < 0.05. Results from Experiment 1 yielded TMEn values between 2.921 and 3.651 kcal/g DM with the values being greater (P < 0.05) for conventional than the Lee 5 and 3 cross soybeans. These results may be explained by the higher fat content in the conventional compared with the other soybean lines. Results from Experiment 2 showed no differences (P > 0.05) in standardized digestibility coefficients for AA among soybeans.

Overall, the results obtained of this study indicated that it was possible to increase protein and SAA content in soybeans without affecting digestibility of AA, but reducing crude fat had a negative impact on TMEn. Digestible AA concentrations, particularly SAA, can be increased by crossing soybean lines.