Castillo-Lopez, E., Klopfenstein, T. J., Fernando, S. C. and Kononoff, P. J. 2014. Effect of dried distillers′ grains and solubles when replacing corn or soybean meal on rumen microbial growth in vitro as measured using DNA as a microbial marker. Can. J. Anim. Sci. 94: 349-356. The objectives were to evaluate the use of rDNA markers to measure the effects of dried distillers′ grains with solubles (DDGS) and the potential treatment×time interaction on microbial crude protein (MCP) synthesis in vitro and secondly to measure the contribution of yeast based protein originating from DDGS. Treatments were: (1) CONT, control with no DDGS, but with alfalfa hay, corn silage, ground corn (GC) and soybean meal (SBM) included at 25% (DM basis); (2) LOWCORN, 20% DDGS (DM basis) replacing GC; (3) LOWSBM, 20% DDGS (DM basis) replacing SBM; and (4) LOWCORNSBM, 20% DDGS (DM basis) replacing 10% GC and 10% SBM. Treatments (0.5 g) were incubated in 50 mL of inoculum in duplicate. At 0, 4, 16, 32, 48 and 96 h of fermentation total DNA was extracted from each treatment and MCP was measured using rDNA markers. The sum of bacterial crude protein (BCP) and protozoal crude protein (PCP) was considered as MCP. Data were analyzed as a completely randomized design. The treatment×time interaction was tested and the SLICE option was included to evaluate the effect of treatment at each fermentation time point. There was a tendency to a treatment×time interaction (P=0.07) for MCP. Specifically, at 16 h, LOWCORNSBM yielded greater (P<0.05) MCP compared to either CONT or LOWCORN with estimates of 68.5, 33.8 and 23.3±8.9 mg g^-1 DM, for LOWCORNSBM, CONT and LOWCORN, respectively. At 48 h, however, LOWCORN yielded greater MCP (P<0.05) compared with LOWSBM with estimates of 72.2 and 32.5±8.9 mg g^-1 DM, for LOWCORN and LOWSBM, respectively. Yeast crude protein (YCP) was not affected (P=0.21) and averaged 0.04±0.02 mg g^-1 of substrate (DM basis). Overall, rDNA markers were effective for quantifying MCP, but further research on the methodology is needed. With DDGS inclusion, MCP was maintained; however, yeast cells were extensively degraded during fermentation.