LAUSR

The objective was to determine the interaction of replacing forage with coproducts and the number of step-up diets when adapting feedlot cattle to a finishing diet on the ruminal microbiome. Simmental × Angus and Angus steers (n = 24; 234 ± 38 kg) were blocked by initial body weight (BW) and allotted to 8 pens as a subset of a larger group. Pens were randomly assigned to 1 of 2 dietary treatments: 1) decreasing coproduct (CO; soybean hulls and modified wet distillers grains) inclusion while increasing dry-rolled corn over 36 d or 2) decreasing forage (FO; alfalfa and grass hay) inclusion while increasing dry-rolled corn over 36 d. Pens were fed either 5 (5S) or 2 (2S) step-up diets for each dietary treatment during the 36 d adaptation period in a 2 × 2 factorial design. Steers were fed a common finishing diet for the remainder of the trial (34 d). The ruminal microbiome was evaluated on d 36 and 70. Alpha diversity was greatest (P < 0.01) for FO/2S steers on d 36. A diet × day effect (P = 0.02) was observed for beta diversity as the microbial community was more similar (P < 0.01) between d 36 and 70 for steers fed CO compared with FO. Relative abundance of Bacteroidetes was greater (P = 0.03) for 2S steers compared with 5S steers. Prevotellaceae tended to be greater (P = 0.07) in 2S steers compared with 5S steers. Lactobacillaceae was greater (P < 0.01) on d 36 than d 70. Limited observed day effects for individual taxa suggest the microbiome was well adapted by treatments for the finishing diet on d 36. Greater coproduct inclusion in adaptation diets may alter the ruminal bacterial community to be more similar to the community while feeding high levels of dry-rolled corn.