ASSESSMENT OF PROTEIN-QUALITY IN HEAT-TREATED SOYBEAN PRODUCTS USING THE GROWTH-RESPONSES OF LAMBS AND CALVES AND A NYLON BAG-ROOSTER ASSAY

Three studies were conducted to evaluate N and amino acid (AA) availab ility from processed soybean (SB) products. In Exp. 1 and 2, treatment s consisted of com-cottonseed hull basal diets plus SB products includ ing SB meal (SBM); ground, raw SB (RSB); or extruded SB (ESB). In Exp. 1, 15 Suffolk (28.6 kg) and 15 Louisiana native breed (16.3 kg) ewe l ambs were used in a 35-d growth trial. Although breed affected (P < .1 0) DMI and ADG of lambs, the diets did not (P > . 1 0). In Exp. 2, 12 Suffolk wether lambs (34.0 kg) were used in a 3 x 3 multiple-square La tin square design N metabolism trial. Lambs fed RSB tended (P = .13) t o retain more N than those fed ESB, but N retained by lambs fed RSB an d ESB was similar (P > .10) to that of lambs fed SBM. In Exp. 3, four crossbred steer calves (240 kg) were used in a 4 x 4 Latin square desi gn N metabolism trial with the above treatments plus heat-damaged SB ( HDSB). Calves fed RSB and ESB retained similar (P > .10) amounts of N. Compared with calves fed RSB and ESB, calves fed SBM retained more (P < .10) N. Calves fed HDSB retained less (P < .10) N than those fed ot her diets. The individual SB products and the total mixed diets used i n Exp. 3 were incubated intraruminally in nylon bags and the residual DM recovered and precision-fed to cecectomized roosters to estimate it s intestinal AA digestibility. Digestibility of residual AA seemed to be relatively uniform across AA. The nylon bag-rooster assay seemed to provide biologically meaningful estimates of the protein quality of t he SB products.