Abstract
This study evaluated the effects of monensin source on performance and carcass characteristics of Nellore bulls offered a high-concentrate diet during the finishing phase. On d 0 of the study, 240 Nellore bulls (initial BW = 363.2 ± 40.9 kg) were ranked and blocked according to initial BW, whereas within blocks animals were allotted into pens and randomly assigned into 1 of 3 treatments: 1) no ionophore feeding (CONT), 2) CONT diet plus 28 ppm (mg/kg DM) of monensin sodium-A (Rumensin ™ -200; Elanco Animal Health), and 3) CONT diet plus 28 ppm monensin sodium-B (generic). The experimental period lasted 104 d and was divided into a 21-d adaptation period and an 83-d finishing phase. During the adaptation phase, both monensin sources increased (P ≤ 0.01) BW change, ADG, and F:G ratio, reducing the DMI variation (P = 0.02). When the entire experimental period was evaluated, no treatment effects were detected (P ≥ 0.26) on final BW, BW change, total DMI, DMI as % BW, and ADG. Nonetheless, DMI variation was reduced as monensin was included in the diet (P = 0.01) and, as expected, only MON-A improved the efficiency of the animals by reducing the F:G ratio (P = 0.05) compared with CONT and by reducing (P = 0.05) the biological efficiency when compared with the MON-B group. Additionally, carcass ADG tended (P = 0.10) to be greater for MON-A compared with MON-B, no other differences in the carcass characteristics were observed (P ≥ 0.53). In summary, monensin feeding reduced DMI variation in the feedlot, but the source of monensin is important for the feedlot scenario, given that only MON-A significantly benefited the performance during the finishing phase of Nellore bulls offered a high-concentrate diet.
Keywords: food efficiency, ionophore, Nelore