The generation of knowledge on biodiversity of rumen microorganisms, as well as the behavior of such microbial populations in different feed management situations, allows for increased efficiency of food use, optimizing fermentation and digestion of the diet accordingly. Concomitantly, the detailed knowledge of the ruminal microbiota, as well as the different factors that can directly alter this microbiota, represents a potential increase in the productivity of livestock. The objective with this study is to characterize the changes in the rumen bacterial population, caused by the increase of concentrate in the diet, use of different sources of non-fiber carbohydrates and roughages with different fiber digestibility by determining the relative amount of amylolytic, cellulolytic and lactate fermenting bacteria. Therefore, rumen fluid samples were collected during four research projects conducted at the Beef Cattle Research Laboratory for subsequent relative quantification of microbial populations. Three important rumen microorganisms to degrade cellulose: Fibrobacter succinogenes, Ruminococcus albus and Ruminococcus flavefaciens; two microorganisms important in the degradation of starch: Streptococcus bovis and Ruminobacter amylophilus; and a lactate fermenting microorganism: Megasphaera elsdenii, will be quantified using the technique of Real-time Polymerase Chain Reaction in real-time (RT-PCR) to determine the effect of concentrate level and fiber digestibility on these populations. Results will be expressed and relative populations using total bacteria as control. Data will be analyzed as a replicated Latin square design using the MIXED procedure of SAS, version 9.2. The model includes fixed effects of treatment and their interactions, and the random effects of square, animal and periods. The degrees of freedom and tests will be adjusted using the Kenward-Roger. Significance will be considered at P d 0.05. Data will be analyzed separately for each experiment described. The effects of fiber content and concentrate, which are repeated in three experiments, will be analyzed including the random effect of experiment in the model.
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