Substituting dried distillers' grains with solubles with soybean meal in finishing diets: effects on small intestinal amino acid flow, digestibility, and nitrogen balance.

The objectives of the study were to evaluate the effects of substituting dried distillers' grains with solubles (DDGS) with soybean meal (SBM) on nutrient and amino acid (AA) intake, flow, and digestibility. Red Angus steers (n = 4; body weight = 360 ± 9 kg) with ruminal, duodenal, and ileal cannulas were used in a 4 × 4 Latin square. Diets consisted of (DM basis) dry-rolled corn (70%), grass hay (10%), liquid supplement (5%), and test ingredients (15%). Treatments were (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. Data were analyzed using the MIXED procedure of SAS, with treatment included as a fixed effect and period and steer as random effects. Linear, quadratic, and cubic contrasts were applied to assess the response patterns to varying levels of DDGS and SBM. Nitrogen intake (P = 0.038, quadratic) decreased from CON to the intermediate SBM inclusion levels (i.e., SBM50 and SBM75), and then increased to SBM100. The duodenal flow of nonfiber carbohydrates linearly decreased (P = 0.050) as SBM inclusion increased. The total tract digestibility of neutral detergent fiber exhibited a cubic pattern (P = 0.033), with the greatest value observed for CON, the lowest value for SBM50, and intermediate values for SBM75 and SBM100. The amount of essential (EAA) and non-essential (NEAA) AA reaching the small and large intestines were not affected by treatment (P ≥ 0.12). When DDGS was exclusively fed, most AA exhibited AA utilization indexes greater than 1.0, indicating a net ruminal gain. Conversely, AA utilization indexes were near 1.0 when SBM was the sole protein source, suggesting minimal net ruminal change for most AAs. Total tract digestibility of lysine tended to increase linearly (P = 0.058) with the inclusion of SBM, while tryptophan (P = 0.063) and arginine (P = 0.088) displayed a quadratic tendency, increasing from CON to SBM50 and SBM75 and then decreasing to SBM100. True intestinal digestibility of individual EAA varied from 79.5% for histidine to 89.1% for lysine, while for NEAA it varied from 70.4% for cysteine to 89.7% for tyrosine. This study demonstrated that the extent of absorption in the small intestine varied among AA, with lysine and arginine exhibiting greater small intestinal availability in finishing cattle. Ultimately, DDGS and SBM deliver similar metabolizable amino acids in beef cattle finishing diets. This allows producers to prioritize cost-effectiveness and regional availability when selecting a protein source.