Macleaya cordata, is a plant of the Papaveraceae family that contains quaternary benzophenathridine and protopine alkaloids, showing systemic anti-inflammatory effects. Despite Macleaya cordata plant extract being used in Europe as feed additive for livestock (Sangrovit RS; Phytobiotics, Eltville, Germany), its effects on feedlot cattle performance are not well documented. Hence, 60 bullocks (368 ± SD 44.7 kg) were used to determine the effect levels of M. cordata plant extract Sangrovit-RS (S-RS) supplementation level on feedlot performance and carcass traits of finishing bullocks. Animals were crowded by breed in groups of 5 placed in 12 dirt-floor pen and randomly assigned to receive treatments as follows: 1) A 90% concentrate finishing diet (Control); 2) Control plus daily supplementation of 3 g of Sangrovit-RS (SRS)/ bullock (SRS3); and 3) Control plus daily supplementation of 8 g of SRS/bullock (SRS8). Results were analyzed as a completely randomized design. Pen was the experimental unit. Body weight at d −21 and hot carcass weight were used as the associated co-variables, for performance and carcass characteristics, respectively. The influence of SRS level was analyzed using unequal spaced polynomials. Feed efficiency (Gain/feed ratio) responded quadratically to inclusion level of SRS (P = 0.04), with means of 0.17, 0.19 and 0.15 kg/kg for Control, SRS3, and SR8, respectively; bullocks in SRS3 shown the best response, with 12% improved feed efficiency compared with Control (0.19 vs. 0.17 kg gain/kg DM). Dry matter intake tended (P = 0.08) to respond quadratically to SRS supplementation level, with the lowest value for SRS3. Hot carcass dressing was augmented linearly (P = 0.03) by SRS level. KPH-fat tended to decreases linearly as SRS intake was increased (P = 0.11). Results of this experiment suggest that feeding Sangrovit-RS in doses close to 3 g/day improves feed efficiency of finishing bullocks.