Effect of roasting on site and extent of digestion of soybean meal by sheep: I. Digestion of nitrogen and amino acids.

Six mature wethers (BW 72 kg) with ruminal, duodenal, and ileal cannulas were fed a control (no added soybean meal [SBM]) diet or diets containing unheated SBM or SBM roasted at 165 degrees C for 75, 150, 180, or 210 min in a 6 x 6 Latin square experiment. Concentrations of indicators of heat exposure (ADIN, ADF, NDF) in SBM increased with increasing roasting time. Duodenal flows of total N, non-bacterial N, and SBM N increased (P < .05) linearly with increased roasting time. Small intestinal (SI) digestibility (percentage entering SI) of total N and SBM N was influenced quadratically (P < .05) by roasting time; SI digestibilities were modestly increased by heating SBM to 150 min, then declined dramatically when SBM was heated for 180 and 210 min. These responses resulted in a quadratic (P < .05) increase in quantity of total and SBM N disappearing in the SI; quantities increased as SBM was heated to 150 min and remained unchanged when SBM was heated for 180 and 210 min. These responses resulted in a quadratic (P < .05) increase in quantity of total and SBM N disappearing in the SI; quantities increased as SBM was heated to 150 min and remained unchanged when SBM was heated for 180 and 210 min. Duodenal flows and SI digestibilities of total and non-bacterial amino acids (AA) and of most individual AA followed patterns similar to those observed for N. Maximum quantities of total and individual AA disappeared from the SI when wethers were fed SBM roasted at 165 degrees C for 150 min. Evaluation of the effects of heat treatment on the nutritive value of a protein source for ruminants should include considerations for both ruminal protein escape and SI availability of escaped protein and for the nature of the AA supply absorbed from the SI.