Effect of starch-based supplementation level combined with oil on ruminal microbiota of finishing Nellore steers grazing tropical pasture during dry season

The objective of this study was evaluate the effect of oil supplementation combined with high- or low-starch on rumen microbial profile of young Nellore steers grazing Brachiaria brizantha cv. Xara?s during the finishing phase. The experiment was conducted in the Department of Animal Science, State University of Sao Paulo - UNESP from May to July 2013, in the dry season. Eight ruminal cannulated Nellore steers (514.5 kg ? 30.1) at 24 mo of age were used in a replicate 4 x 4 Latin square with a 2 x 2 factorial arrangement of treatments (high or low starch, with or without a source of oil) and an experimental period of 21 days. The supplements were corn combined with soybean grains (SG; 12.4% of EE and 5.08 Mcal/kg of DM); corn without SG (3.62% of EE and 4.62 Mcal/kg of DM); soybean hulls (SH) combined with SG (11.8% of EE and 4.98 Mcal/kg of DM); and SH without SG (2.58% of EE and 4.45 Mcal/kg of DM). All supplements were isoproteic, approximately 23% CP. Animals were supplemented at the rate of 1.0% body weight. For the quantification and identification of rumen bacteria, fifty grams of ruminal contents were collected on the day 18, after 3 h of supplementation. For quantification of total bacteria and relative quantification of cellulolytic bacteria, the technique used was qPCR. Statistical analyses were performed using the MIXED procedures (SAS Inst. Inc., Cary, NC, USA). Supplementing the animals with low-starch without oil increased the numbers of Ruminococcus albus (P = 0.0120). Low-starch supplement, with or without oil, decreased the number of Selenomonas ruminantium in Nellore (P = 0.0003). Moreover, oil supplementation decreased the number of Fibrobacter succinogenes, Ruminococcus flavefasciens, and Archeas, but increased of Anaerovibrio lipolytica (P < 0.0001). On the other hand, animals supplemented with high-starch showed lower population of Archeas than those supplemented with low-starch (P < 0.0001). The use of SH without oil supplementation may be effective to increase R. albus population in Nellore steers. Oil supplementation decrease the number of F. succinogenes, R. flavefasciens, and Archeas, but increase of A. lipolytica. High-starch supplementation decrease population of Archeas in finishing Nellore steers grazing tropical pasture.