In Exp. 1, Brachiaria ruziziensis (11.1 \% CP) was inoculated or not with two sources of monensin, resulting in three treatments: 1) no monensin inoculation (CONT), 2) 20 mg of monensin sodium-A/kg of DM (Elanco Animal Health; MON-A), and 3) 20 mg of monensin sodium-B/kg of DM (Shandong Qilu King-Phar Pharmaceutical Co. Ltd.; MON-B). Three rumen-fistulated Jersey steers were offered a cool-season forage-based diet and were used as the rumen inoculum donors. Volatile fatty acids concentrations were evaluated at 0, 6, 12, 24, 30, and 48 h after treatment inoculation. Overall, acetate and butyrate concentrations were reduced in MON-A vs. CONT (P <= 0.02), whereas both monensin products reduced Ac:Pr ratio vs. CONT (P <= 0.01); however, MON-A also (P = 0.05) reduced the Ac:Pr ratio vs. MON-B. A treatment x hour interaction was detected for rumen propionate concentration (P = 0.01), primarily because MON-A resulted in greater propionate than CONT and MON-B at 24 and 48 h (P <= 0.03), but no differences were observed between CONT vs. MON-B (P >= 0.27). In Exp. 2, 240 Nellore bulls (initial BW = 363.2 +/- 40.9 kg) were ranked and blocked according to initial BW, and within blocks animals were allotted into pens (n = 10 pens/treatment). Pens were randomly assigned into one of three treatments: 1) corn-based diet with no monensin (CONT), 2) CONT plus 28 mg of MON-A/kg of DM, and 3) CONT plus 28 mg of MON-B/kg of DM. The CONT diet was composed of sugarcane bagasse, ground corn, DDGS, urea, and a mineral-vitamin mix. The experimental period lasted 106 d and was divided into a 21-d adaptation period and an 85-d finishing phase. During the adaptation phase, both monensin sources increased (P <= 0.01) BW change, ADG, and F:G, as well as reduced DMI variation (P = 0.02). When the entire experimental period was evaluated, no treatment effects were detected for final BW, DMI, and ADG (P 0.26). Nonetheless, DMI variation was reduced as monensin was included (P = 0.01) and only MON-A improved the efficiency by reducing F:G vs. CONT (P = 0.05) and biological efficiency vs. MON-B (P = 0.05). Additionally, carcass ADG tended (P = 0.10) to be greater for MON-A vs. MON-B, whereas no other differences in the carcass characteristics were observed (P >= 0.53). In summary, the source of monensin inoculated in vitro and offered to Nellore bulls during the feedlot phase significantly affected the energetic efficiency and the performance of the animals.