AbstractThis experiment compared ruminal, physiological, and productive responses of feedlot cattle receivingYucca schidigeraextract to replace or fed in conjunction with monensin + tylosin. Angus-influenced steers (n= 120) were ranked by body weight (BW; 315 ± 3 kg) and allocated to 4 groups of 30 steers each. Groups were housed in 1 of 4 drylot pens (30 × 12 m) equipped with GrowSafe feeding systems (4 bunks/pen) during the experiment (day −14 to slaughter). On day 0, groups were randomly assigned to receive a diet containing (2 × 2 factorial): 1) no inclusion or inclusion of monensin + tylosin (360 mg and 90 mg/steer daily, respectively) and 2) no inclusion or inclusion ofY. schidigeraextract (4 g/steer daily). Steers were slaughtered in 3 groups balanced by treatment combination (36 steers on day 114, 36 steers on day 142, and 48 steers on day 169). Blood was sampled on days 0, 28, 56, and 84, and the day before shipping to slaughter. On day 41, eight rumen-cannulated heifers (BW = 590 ± 15 kg) were housed with steers (1 pair/pen). Pairs rotated among groups every 21 d, resulting in a replicated 4 × 4 Latin square (n= 8/treatment combination) with 14-d washout intervals. Heifers were sampled for blood and rumen fluid at the beginning and end of each 21-d period. Monensin + tylosin inclusion decreased (P< 0.01) feed intake and improved (P= 0.02) feed efficiency of steers, but did not alter (P≥ 0.17) steer BW gain or carcass merit traits. Inclusion ofY. schidigeraextract did not impact (P≥ 0.30) steer performance and carcass characteristics. Plasma glucose, insulin, insulin-like growth factor-I, and urea-N concentrations were not affected (P≥ 0.16) by monensin + tylosin, nor byY. schidigeraextract inclusion in steers and heifers. Ruminal pH in heifers was increased (P= 0.04) by monensin + tylosin, and also by (P= 0.03)Y. schidigeraextract inclusion. Rumen fluid viscosity was reduced (P= 0.04) byY. schidigeraextract, and rumen protozoa count was increased (P< 0.01) by monensin + tylosin inclusion. The proportion of propionate in the ruminal fluid was increased (P= 0.04) by monensin + tylosin, and tended (P= 0.07) to be increased byY. schidigeraextract inclusion. Hence,Y. schidigeraextract yielded similar improvements in rumen fermentation compared with monensin + tylosin, but without increasing performance and carcass quality of finishing cattle. No complimentary effects were observed when combining all these additives into the finishing diet.