173. Using the Ruminal Degradability Kinetics of a Corn Silage Substrate to Apprise the Effects of Saccharomyces Cerevisiae in Beef Cattle Grower and Finisher Diets

Abstract The ruminal degradability kinetics of a corn silage substrate incubated into the rumen of beef steers consuming beef cattle grower and finisher diets with or without Saccharomyces cerevisiae were evaluated. A total of eight surgically prepared beef steers (ruminal fistulas) 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 delivered via gel capsules (0.25 g, as-is) twice daily through the ruminal cannula. Pre-dehydrated (55ºC for 72 h) corn silage substrate was ground (2 mm) and placed into 10 × 20 cm (28 µm) nylon bags (5g, as-is). Substrate bags were incubated within the rumen by placing it at the ruminal ventral sac within a nylon mesh (with weights) and removed at 0, 2, 4, 8, 12, 20, 32, 48, 64, 72, and 96 h. Samples were removed, rinsed, and dehydrated for 72 h at 55ºC. Residues were composited by period, steer, and incubation time, corrected for residual moisture, and were used to fit a first-order kinetics model using the NLIN procedure of SAS (with animal as the experimental unit). No interaction (P ≥ 0.26) diet × yeast was observed for degradability variables, except for a tendency (P = 0.06) of yeast to increase the corn silage substrate NDF rate of degradation (kd, %/h) for steers consuming finisher diets. Regardless of dietary type, live-yeast increased (P ≤ 0.04) the corn silage substrate effective degradability (ED) of organic matter (OM) at either calculated 4, 5, and 6 %/h rate of passage (kp); while tended (P = 0.08) to increase ED of dry matter (DM) at a calculated kp of 4 %/h; tended (P = 0.09) to increase the rate of degradation of ADF; while did not (P ≥ 0.12) affect lag time (h). Regardless of live yeast inclusion, steers offered the grower diet had greater (P = 0.01) corn silage substrate potentially degradable (fraction B) for DM, OM, NDF, ADF, and hemicellulose; greater (P ≤ 0.03) ED (at either kp of 4, 5, or 6 %/h) for OM and fiber fractions, while increasing DM at a kp 4 %/h only; reduced (P < 0.01) undegradable fraction (fraction C) for all variables; and reduced (P = 0.02) the lag time (h) for ADF, compared to those offered the finisher diet. The model replicated expected results between grower and finisher diets when using a corn silage substrate. Live yeast improved the ruminal degradation kinetics of corn silage OM fraction regardless of diet type, while finisher diets may receive extra benefit on NDF rate of degradation.