The objective of this experiment was to evaluate the effect of the addition of virginiamycin (Vm) in combination with different doses of monensin (Mn) on feedlot performance. Two-hundred 40 crossbred steers (189 ± 3 kg) were used in an 80 d experiment. Steers were blocked by BW and randomly distributed in 24 pens, 9 animals each. Steers were fed a finishing diet (90% concentrate) based on whole-shelled corn and milo silage as roughage source. Three dietary treatments were evaluated: T1, basal diet supplemented with 180 mg Mn/hd/d; T2, basal diet supplemented with 200 mg Vm and 180 mg Mn hd/d; and T3, 200 mg Vm and 120 mg Mn hd/d. Body weight was measured on d 0, 40 and 80. Dry matter intake was measured weekly and G:F was estimated for the entire feeding period. Prior to harvest, on d 80, an ultrasound was performed to measure back fat (BF) and ribeye area (REA). Hot carcass weight was measured at harvest. Data were analyzed using a complete randomized block design with block as random effect and treatment as fixed effect. Fisher's protected test was used for mean separation. Pen was used as the experimental unit. Hot carcass weight and G:F were greater and DMI lower in the T2 group (all P < 0.05, Table 1). Back-fat tended to be greater in T2 (P = 0.09), BW on d 40 and 80, and REA were numerically greater in T2 but not statistically different among treatments. Feedlot performance (ADG, G:F) of T3 was numerically greater than that of T1, but differences were not significant (all P > 0.10). In conclusion the addition of 200 mg Vm to a higher dose of Mn improved feedlot performance (HCW and G:F). Table 1. Feedlot performance and carcass characteristics in cattle supplemented with Vm in diets with two levels of Mn

	T1	T2	T3	P-value
BW d 0, kg	189.4	188.9	188.9	0.80
BW d 40, kg	278.3	282.8	279.6	0.44
BW d 80, kg	302.6	305.8	303.1	0.64
ADG, kg/d	1.42	1.46	1.42	0.73
DMI, kg/d	9.32a	8.59b	9.19a	<0.01
G:F	0.137a	0.150b	0.137a	0.05
Carcass weight, kg	173.5c	178.0d	171.2c	0.05
Ribeye area, cm2	52.1	52.4	52.1	0.95
Back fat, cm	0.49	0.53	0.50	0.09

^abValues with different superscripts are statistically different at P < 0.05. ^cdValues with different superscripts are statistically different at P < 0.10.