194 Substituting DDGS with soybean meal in feedlot diets: Impacts on small intestinal amino acid flow and digestibility

Objectives of this study were to evaluate the effects of substituting DDGS with soybean meal (SBM) on nutrient and amino acid (AA) intake, flow, and digestibility. Red Angus steers (n = 4; BW = 360 ± 9 kg) with ruminal, duodenal, and ileal cannulas were used in a 4 × 4 Latin square. Diets consisted of dry-rolled corn (70%), hay (10%), liquid supplement (5%), and test ingredients (15%). Treatments were (DM basis): 1) 15.0% DDGS (CON); 2) SBM in replacement of 50% of DDGS (SBM50); 3) SBM in replacement of 75% of DDGS (SBM75), and 4) SBM in replacement of 100% of DDGS (SBM100). Each period lasted 18-d, consisting of a 10-d diet adaptation followed by an 8-d collection period. Steers were dosed intraruminally with chromic oxide (16 g/d) as an indigestible marker from d 8 to d 15. Total fecal collections were conducted from d 11 to d 15 of each period. Duodenal and ileal collections were taken from d 13 to d 15, totaling 8 samples per site in 9-h intervals, representing 24-h sampling. Data were analyzed using the MIXED procedure of SAS, with treatment included as a fixed effect and period and animal as random effects. Linear, quadratic, and cubic contrasts were applied to assess the response patterns to varying levels of DDGS and SBM. The apparent digestibility of total AA (individual, essential EAA [EAA], and nonessential AA [NEAA]) and microbial AA (individual, EAA, and NEAA) in small intestine was calculated by difference between AA flow in duodenum and ileum, divided by duodenal flow. True intestinal digestibility of total AA (individual, EAA, and NEAA) and microbial AA (individual, EAA, and NEAA) was estimated by linear regression model fitted between the AA absorbed in small intestine (Y; g/d) and their respective duodenal flow (X; g/d). The intercept of the equation represented the endogenous losses, and the slope represented the true digestibility of the total AA or microbial AA. The amount of essential AA (EAA) and nonessential AA (NEAA) reaching the small and large intestines was not affected by treatment (P ≥ 0.37). Total tract digestibility of EAA and NEAA was not affected by treatment except for lysine (P = 0.03, linear effect), tryptophan (P = 0.03, linear effect), and arginine (P = 0.05, linear effect), which increased linearly with the inclusion of soybean meal. True intestinal digestibility of individual EAA varied from 79.46% for histidine to 89.11% for lysine, while for NEAA it varied from 70.43% for cysteine to 89.65% for tyrosine. In conclusion, the flow of lysine to the small intestine (feed or microbial origin) did not vary between the protein sources evaluated in this study; however, increased inclusions of SBM increased lysine digestibility.