ASSESSMENT OF POSTRUMINAL AMINO-ACID DIGESTIBILITY OF ROASTED AND EXTRUDED WHOLE SOYBEANS WITH THE PRECISION-FED ROOSTER ASSAY

The objectives of these studies were to predict the effects of roastin g and extrusion temperatures of whole soybeans (SB) on intestinal prot ein digestibility in cattle. Intestinal digestibility was assessed wit h a two-stage in vitro or in situ ruminal incubation/precision-fed cec ectomized rooster bioassay. In Exp. 1, whole SB (raw SB or SB roasted to 141, 149, or 157 degrees C exit temperature from a commercial roast er and steeped for 30 min) were incubated in strained ruminal fluid an d McDougall's buffer (50:50) at 39 degrees C for 16 h. In Exp. 2, SB ( ground raw SB or SB extruded at 116, 138,or 160 degrees C) were placed in polyester bags (20 x 30 cm) and suspended in the ventral rumen of steers for 16 h. Lyophilized residue of the in vitro or in situ incuba tions and samples of raw SB and most extensively heated SB (roasted SB at 157 degrees C or extruded SB at 160 degrees C) for each respective experiment were crop-intubated to cecectomized roosters. Total excret a were collected for 48 h after intubation and lyophilized, and amino acid (AA) concentrations were determined. In Exp. 1, total AA digestib ility was 61.6 and 84.5% for unincubated whole raw SB and 157 degrees C roasted SB, respectively, and 66.2, 88.9, 91.3, and 91.6% for in vit ro residues of whole raw SB and SB roasted at 141, 149, and 157 degree s C, respectively. Trypsin inhibitor (TI) activity was 20.09, 1.69, 1. 54, and 1.84 mg/g fat-free DM for unincubated whole raw SB and 141, 14 9, and, 157 degrees C roasted SB, respectively, and 30.84, 1.01, .90, and .26 mg/g fat-free DM for in vitro residues of whole raw SB, 141, 1 49, and 157 degrees C roasted SB, respectively. In Exp. 2, total AA di gestibility was 68.5 and 87.7% for unincubated ground raw SB and 160 d egrees C extruded SB, respectively, and 81.9, 91.3, 89.7, and 89.4 for in situ residues of ground raw SB and 116, 138, and 160 degrees C ext ruded SB, respectively. Trypsin inhibitor activity was 17.61, 4.89, 4. 08, and 1.56 mg/g fat-free DM for unincubated ground raw SB,116, 138, and 160 degrees C extruded SB, respectively, and 3.62, .59, .55, and . 21 mg/g fat-free DM for incubated ground raw SB, 116, 138,and 160 degr ees C extruded SB, respectively. Heat treatment by roasting and extrus ion improved AA digestibilities of SB, but there were no differences d etected among the roasting or extrusion temperatures. Ruminal fermenta tion did not eliminate the negative effects of TI activity on intestin al digestibility of AA in whole SB but did reduce TI activity in groun d SB.