The objective was to evaluate effects of virginiamycin (VM) and monensin sodium (MON) in whole corn diet, with or without the use of roughage during adaptation on carcass characteristics of feedlot cattle. The animals were kept in the feedlot in group pens for 106-d with a 21-d adaptation period. One hundred Nellore bulls were used in a randomized complete block design with a 2 × 2 factorial arrangement of 4 treatments, being 2 kinds of additive (VM and MON) and 2 adaptation strategies. Each treatment consisted of 5 pens with 5 bulls per pen. Treatments were defined by VM (25 mg/kg of DM) and MON (30 mg/kg of DM), with or without 10% (DM basis) sugarcane bagasse (SB) during adaptation period as follows: VM 10% SB; VM 0% SB; MON 10% SB and MON 0% SB. After the adaptation period, animals were fed ad libitum once daily with a diet containing 85% whole corn, 15% pelleted feed (soybean, minerals and the additive). Probability of 10% was considered as statistical difference and 15% of probability was discussed as tendency. There were no differences (P > 0.10) between VM and MON treatments for any variables studied (Table 1). However, differences were observed (P < 0.10) for average daily gain (ADG) when the animals were fed with SB in the adaptation period. There was no interaction between the additives and SB for any variables studied; in contrast, animals fed with SB at adaptation period tended (P < 0.15) to have higher carcass gain (CG) and carcass daily gain (CDG) than treatments without SB, independent of the additive used. Therefore, VM and MON had a similar effect on carcass characteristics, but the treatments containing SB showed great carcass gain when the tendency was discussed. The use of roughage source (SB) may increase the performance and carcass gain due to a greater adaptation of the Nellore bulls to whole corn diet. Table 1. Carcass data of bulls feedlot fed with whole corn diet

Variable (kg)	MON			VM		SEM	P-value
10%SB	0% SB	10%SB	0% SB	Additive	Roughage	A × R
Initial carcass weight	197.24	197.37		196.99	196.99	1.375	0.827	0.9631	0.963
Final carcass weight	290.98	297.54		291.48	289.30	5.865	0.521	0.715	0.470
Average daily gain	1.455	1.384		1.577	1.354	0.075	0.551	0.074	0.331
Carcass daily gain	0.895	0.877		0.977	0.842	0.048	0.631	0.138	0.249