Ruminal Degradability Kinetics of Old World Bluestem “WW-B Dahl” Substrate on Beef Steers Offered Grower or Finisher Diets with Saccharomyces Cerevisiae

Abstract The effects of live yeast inclusion and beef cattle diet type on ruminal degradation kinetics of “WW-B Dahl” Old World bluestem (Bothriochloa bladhii Retz T. Blake) substrate were evaluated. Ruminal cannulated Angus crossbred steers (n = 8, BW = 550 ± 20 kg) were used in a duplicated 4 × 4 Latin square design 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 placed in gel capsules (0.25g, as-is) twice daily via the ruminal cannula. Pre-dehydrated (55C for 72 h) WW-B Dahl was ground (2mm) and placed into 10 × 20 cm (28μm) nylon nags (5g, as-is). The substrate bags were placed within a nylon mesh into the rumen and reversely removed at 0, 2, 4, 8, 12, 20, 32, 48, 64, 72, and 96 h after incubation. Bags were rinsed, dehydrated for 72 h at 55C and composited by steer, period, and incubation time, and used to fit a first-order kinetics model using the NLIN procedure of SAS (animal as the experimental unit). No interaction (P ≥ 0.17) diet × yeast was observed for ruminal degradability variables. Regardless of dietary type, live yeast increased (P = 0.02) the rate of degradation (kd, %/h) of WW-B Dahl substrate NDF and tended (P = 0.12) to increase for hemicellulose; decreased (P = 0.05) the lag time for dry matter (DM) and tended to decrease for NDF (P = 0.06) and ADF (P = 0.08); and tended (P ≤ 0.14) to increase the effective degradability (ED) of NDF (at a kp rate of passage of 5 and 6%/h) and hemicellulose (P ≤ 0.12; kp of either 4, 5, or 6 %/h). Regardless of live yeast inclusion, steers offered the grower diet had greater (P 0.01) WW-B Dahl substrate potentially degradable (B) fraction, ED (kp at 4, 5, or 6%/h), and lesser undegradable (C) fraction for DM, organic matter (OM), NDF, ADF, and hemicellulose; greater (P ≤ 0.05) lag time for DM, OM, and hemicellulose, while lesser (P 0.01) lag time for ADF, compared to steers offered the finisher diet. Treatments did not affect (P ≥ 0.23) WW-B Dahl substrate rate of degradation of DM, OM, and ADF; while only subtle differences were observed for the soluble fraction (A). The model replicated expected results between grower and finisher diets when using a low-quality forage substrate. Live yeast improved the ruminal degradation kinetics of WW-B Dahl, especially those related to fiber fractions, as well as tending to trigger overall dry matter, NDF, and ADF degradation sooner than diets without live yeast.