PS13-9. Ruminal Degradability Kinetics of a Wheat Silage Substrate in Beef Cattle Grower and Finisher Diets with or Without Live Yeast.

Abstract The ruminal degradability kinetics of wheat silage (in situ substrate) placed into the rumen of beef steers consuming grower and finisher diets with or without live yeast were evaluated. Beef steers fitted with ruminal cannulas (n = 8) were used in a duplicated 4 × 4 Latin square design (four 38-day periods) following a 2 × 2 factorial arrangement of treatments: A) presence of live yeast (Saccharomyces cerevisiae [CNCM I-1077], at 1 × 1010 CFU/animal-daily); and B) diet type (steam-flaked corn-based grower or finisher diets [ad libitum intake]). Live yeast was delivered via gel capsules (0.25 g, as-is) twice daily through the ruminal cannula. Pre-dehydrated (55ºC for 72 h) wheat silage substrate was ground (2 mm) and placed into 10 × 20 cm (28 µm) nylon bags (5g, as-is). Bags were placed in the ruminal ventral sac and reversely removed at 0, 2, 4, 8, 12, 20, 32, 48, 64, 72, and 96 h after incubation. Residues were rinsed and dehydrated, while duplicates were composited, corrected for moisture, and used to fit a first-order kinetics model using the NLIN procedure of SAS. The GLIMMIX procedure of SAS was used with animals as the experimental unit, the fixed effects (diet, period, interaction), and the random effects of animal ID within square, square, and period. The substrate acid detergent fiber (ADF) ruminal degradation lag time increased (Diet × Yeast interaction, P = 0.02) when steers were consuming a finisher diet with live yeast, compared to those consuming the control finisher diet, while such an effect was not observed for grower diets. Regardless of diet type, live yeast increased (P = 0.02) the substrate ADF rate of degradation, and tended (P ≤ 0.09) to increase the effective degradability of dry matter (DM), organic matter (OM), and ADF at 4, 5, and 6%/h calculated rate of passage. Regardless of live yeast inclusion, steers offered the grower diet had greater (P < 0.01) wheat silage substrate potentially degradable fraction, lesser undegradable fraction (P < 0.01), and greater (P ≤ 0.04) effective degradability calculated at 4, 5, and 6 %/h rate of passage for DM, OM, neutral detergent fiber (NDF), ADF, and hemicellulose, while also tending (P = 0.07) to decrease the substrate degradation lag time for hemicellulose compared to finisher diets. The current ruminal in situ model replicated expected results for beef cattle grower and finisher diets when using a forage substrate. Live yeast seemed to improve ruminal effective degradability of a high-quality substrate (wheat silage) DM, OM, and ADF fractions in a beef cattle diet (regardless of a finisher or grower). While live yeast delayed the degradation of the smallest and most difficult fraction to be degraded (ADF) of wheat silage on a finisher diet only.