Substitution of dried distillers' grains with canola meal in cattle finishing diets affects amino acid intake but does not affect post-ruminal amino acid flow and digestibility.

The objectives of this study were to evaluate the effects of substituting dried distillers grains with solubles (DDGS) with canola meal (CM) on nutrient and amino acid (AA) intake, flow, and digestibility. Jersey steers (n = 6; body weight [BW] = 257 ± 16.5 kg) with ruminal and duodenal cannulas were used in a duplicated 3 × 3 Latin square design. Diets consisted of (DM basis) dry-rolled corn (65%), hay (10%), liquid supplement (5%), and test ingredients (20%). Treatments were: 1) 20% DDGS inclusion (CON), 2) CM replacing 50% of DDGS (CM50), and 3) CM replacing 100% of DDGS (CM100). Each period lasted 17-d, consisting of a 10-d diet adaptation followed by a 7-d collection period. Data were analyzed using the MIXED procedure of SAS, with fixed effects of diet and random effects of period, square, and animal nested within square. Linear and quadratic contrasts were used to assess responses to varying levels of DDGS and CM. Intakes of dry matter (DM), organic matter (OM), and non-fiber carbohydrates (NFC) were greater (P ≤ 0.03, quadratic) for CON and CM100 compared to CM50. Despite the quadratic effect on dry matter intake (DMI), no differences were observed for nitrogen (N) intake (P = 0.17 linear and P = 0.15 quadratic), ruminal digestibility, duodenal flow from either feed or microbial origin, post-ruminal digestibility, and N balance (P ≥ 0.12). Intakes of arginine, lysine, tryptophan, and glycine (P ≤ 0.05) linearly increased with CM inclusion. Tyrosine intake was greater for CON and CM100 compared to CM50 (P ≤ 0.054, quadratic). Despite that, the amount of essential AA and non-essential AA reaching the small intestine, as well as the post-ruminal digestibility of AA, were unaffected by treatment (P ≥ 0.20). This response can be explained by AA utilization in the rumen, which was similar between treatments (P ≥ 0.13). Most AA exhibited AA utilization indexes greater than 1.0, indicating net ruminal gain. Propionate responded quadratically to dietary inclusion of CM (P < 0.01) with greater concentrations for CM100 followed by CON and CM50. The opposite pattern was observed for acetate (P < 0.01, quadratic). Consequently, the acetate to propionate ratio also responded quadratically (P < 0.01; 1.79, 2.46, and 1.49 for CON, CM50, and CM100, respectively). These results demonstrate that CM can fully replace DDGS in finishing cattle diets without detrimental effects on DMI, N utilization, or duodenal AA supply. Despite inherent differences in protein source and AA composition, ruminal microbial metabolism effectively buffered dietary variation, resulting in comparable MP supply and N retention across treatments.