The effect of roasting temperature applied to whole soybeans on site of digestion by steers. I. Organic matter, energy, fiber, and fatty acid digestion

A 4 X 5 Youden square design was used to determine the effects of form of oil and roasting temperature of soybeans on site and extent of OM, fiber, energy, lipid, and fatty acid digestion and the degree of ruminal biohydrogenation of C18 unsaturated fatty acids. Four steers (373 kg +/- 30.7) with ruminal, duodenal, and ileal cannulas were fed each of five diets during five periods. Diets contained (percentage of DM) corn silage (50%), alfalfa hay (24%), and a corn-urea concentrate (6%). Cornstarch grits (4%) and whole full-fat soybeans (16% of diet DM), raw or roasted to an exit temperature of 141 degrees, 149 degrees, or 157 degrees C in a commercial roaster, constituted the remainder of the RAW, 141, 149, and 157 treatments. Soybean oil equivalent to the quantity in the whole soybeans (3.4% of diet DM) and corn starch grits (16.6% of diet DM) constituted the basal treatment. Diets provided 6.0% ether extract (EE) and 4.6 Mcal of GE/kg of DM. Flows of nutrients were determined by reference to Cr2O3 as an external marker. Ruminal pH, molar proportions of acetate and butyrate, and total ruminal VFA (mM) were not affected by treatment. Ruminal digestibilities of OM, GE, EE, NDF, and ADF were not different among treatments (average 49.5, 41.7, -3.6, 47.4, and 50.1%, respectively). Small intestinal disappearances of OM, GE, and EE were not different among treatments (average 18.4, 24.8, and 61.6% of intake, respectively). Overall, C18:0, C18:1, C18:2, and C18:3 acids averaged 4.4, 25.3, 60.6, and 8.5% of total C18 fatty acids in the diet, respectively; digesta at the duodenum contained 79.0, 16.5, 4.4, and .2%, respectively. The unsaturated C18 fatty acids of soybean oil from whole soybeans, regardless of roasting temperature, were hydrogenated to a similar degree as free soybean oil in the basal diet (average 78.0%). Small intestinal digestibility of total fatty acids was greater (P = .04) for roasted than for raw soybeans (70.9 vs 60.4%). Total tract digestibilities of total C18 fatty acids and total fatty acids were greater (P = .01) for roasted soybeans than for raw soybeans (88.0 vs 82.4 and 86.3 vs 81.0%, respectively). Total tract apparent digestibility of energy was lower (P = .07) for the basal diet than for soybean-containing diets (69.2 vs 70.0%, respectively). Diets containing whole soybeans, regardless of roasting temperature, or free SB oil had similar effects on ruminal characteristics and nutrient digestion. Roasting had no effect on site of lipid or energy digestion in steers but increased fatty acid digestibility. Further, roasting did not spare the unsaturated fatty acids in soybeans from hydrogenation in the rumen.