Bioavailability of digestible lysine in heat-damaged soybean meal for chick growth

Previous experiments indicated that true digestible Lys values for soybean and cottonseed meals obtained with the precision-fed cecectomized rooster assay were accurate estimators of Lys bioavailability for chick growth. This study was conducted to determine whether the digestible Lys in overheated soybean meal (SBM) was totally bioavailable for protein synthesis. Soybean meal alone or mixed with dextrose (SBM:dextrose ratio = 3:1) was autoclaved at 114 C and 97 kPa for 0 to 60 min. True digestibility of Lys in SBM was determined using the precision-fed cecectomized rooster assay. Bioavailability was then assessed by chick growth assay using purified crystalline amino acid diets deficient in Lys that were supplemented with varying levels of crystalline L-Lys.Cl or a SBM. Multiple regression slope-ratio methodology indicated that the digestible Lys in unautoclaved SBM was 100% bioavailable. For SBM autoclaved for 40 or 60 min and SBM-dextrose autoclaved for 20 min, bioavailability of digestible Lys based on weight gain or carcass Lys gain was 81 to 87% and significantly less (P < 0.05) than 100%, whereas bioavailability values based on feed efficiency and carcass N gain did not differ (P > 0.05) from 100%. Bioavailability values for digestible Lys in SBM:dextrose autoclaved for 30 min ranged from 55 to 73% for all performance criteria. Partitioning the response criteria, so that only the response to the supplemental Lys source (not the basal diet) was being evaluated, increased the bioavailability values for all treatments except SBM:dextrose autoclaved for 30 min. The results of this study indicated that digestible Lys in severely heat-damaged SBM measured by the precision-fed cecectomized rooster assay was not totally bioavailable for protein synthesis.