The purpose of this study was to investigate the effect of feeding different levels of processed soybean meal on the of ruminal microorganisms including three types of cellulitecs: (Fibrobacter succinogenes, Ruminococcus albus and Ruminococcus flavefacience ) and two proteolytics ( Butyrivibrio fibrisolvens , Prevotella ruminicola ) using Real time PCR. The treatments were treatment 1 (starter or concentrate containing 30% common soybean meal), treatment 2 (starter containing 10% processed soybean meal and 20% soybean meal), and treatment (ration containing 15% processed soybean meal and 15% soybean meal) For this purpose, thirty nine Holstein calves were assigned to 3 experimental treatments CB calves per treatment in a completely randomized design. Rumen fluid was sellected at 65 days of age and stored after being combined with RNAlater in a freezer-80?C. The results of this study showed that feeding of treatment 3 (15%) increased the concentration of propionic acid, isovalic, caproic and valeric acid in comparison with the control (0%). feeding 15% FSBM caused an increased in the population of Prevotella.ruminicola bacteria and a decrease in the population of Ruminococcus albus bacteria and Butyrivibrio fibrisolvens compared to the control (0%). feeding processed soybean meal did not affect Fibrobacter succinogenes and Ruminococcus flavefacience . Compared to SBM, M contained a lower amount of large peptides but had higher crude protein and small-sized peptides, protein content was increased from 43.3% for SBM to 48.2% for M (P 0.001). Furthermore, small peptides content was increased (P 0.01), but large peptides content was decreased (P 0.001) in M compared to SBM. The present data exhibited that, there was a greater level of small peptides in M compared to SBM. Acetic acid level and large peptides there was a positive correlation and Ruminococcus albus , Butyrivibrio fibrisolvens . there was a positive correlation between propionic acid and isovaleric acid and small peptides and Prevotella.ruminicola . According to the obtained results, it can be concluded that the consumption of processed soybean can increase the population of proteolytic bacteria by increasing the amount of small peptides, and also increase the concentration of propionic acid. so consuming processed soybean can help to grow rumen in Holstein calves. Keywords : cellulolytic, Proteolytic, Real time PCR, volatile fatty acid, processed soybean
