This experiment was conducted to evaluate the effects of adding salinomycin (SL), virginiamycin (VM), or their combination to finishing diets of Nellore steers on the ruminal population of Streptococcus bovis, Megasphaera elsdenii, Selenomonas ruminantium, and methanogenic archaea. Eight ruminally cannulated Nellore steers (322 ± 26 kg initial BW) were allotted to a 4 × 4 replicated Latin square design with four 16-d periods. Experimental diets had 80% concentrate (DM basis), and treatments were arranged in a 2 × 2 factorial, with 2 SL levels (0 and 13 ppm) and 2 VM levels (0 and 15 ppm) in the diet DM. Animals were housed in individual pens and fed once daily at 0800 h. Ruminal fluid samples were collected at 0 and 4 h post-feeding on d 13 of each period, and DNA was extracted from samples using the Qiagen DNA stool mini kit. Ruminal bacteria and methanogenic archaea populations were assessed by real-time PCR. Statistical analyses were performed using the MIXED procedure of SAS. No interactions between SL or VM levels, nor between time of collection and SL or VM levels were observed. The relative population of S. bovis tended to decrease (P = 0.09) for animals fed diets containing 15 ppm VM in comparison with those not receiving the additive, but no effects of dietary treatment were observed for any other microbial species (Table 1). There was an effect of time of collection in the relative populations of M. elsdenii, S. ruminantium, and methanogenic archaea, which were greater (P ≤ 0.01) in samples collected at 0 h post-feeding. In conclusion, adding SL, VM, or both to the diets of Nellore steers had little effect on the populations of ruminal bacteria and methanogenic archaea. Table 1. Relative populations of ruminal bacteria and methanogenic archaea in Nellore steers fed diets with 2 levels of salinomycin (SL) or virginiamycin (VM)

Item	0 ppm SL			13 ppm SL		SEM	P-value
0 ppm VZM	15 ppmVM	0 ppmVM	15 ppmVM	SL	VM	SL × VM
S. bovis	1.00	0.58		0.86	0.49	0.03	0.61	0.09	0.97
M. elsdenii	1.00	1.09		1.09	0.69	0.02	0.43	0.44	0.26
S. ruminantium	1.00	0.79		1.22	0.97	0.04	0.33	0.26	0.99
Methanogenic	1.00	0.64		0.71	1.24	0.08	0.61	0.84	0.11