Corn milling (co)products (n=120) were evaluated for their neutral detergent fiber residue (NDR) and neutral detergent insoluble protein (NDIP) ruminal degradation rates using several in vitro methods. Two (co)products (BPX-DDGS and HP-DDG) were fed to lactating dairy cows (n=44) to evaluate effects on milk production. The Cornell-Penn-Miner Institute (CPM) Dairy model was used to formulate diets and predict milk production. In vitro determined NDR and NDIP rates and were compared to CPM-dairy feed library values, and model predictions were compared with observed milk production. Additionally, BPX-DDGS and HP-DDG were defatted and compared with their intact forms for fermentation characteristics using the in vitro gas production (IVGP) technique. Fermentations were analyzed for rumen bacterial population shifts using the 16S rDNA bacterial tag encoded FLX amplicon pyrosequencing (bTEFAP) technique. Lastly, a novel ruminal in vitro method was described to measure the soluble protein fraction of feeds, with adjustments for microbial contamination. Fermentation rate of the NDR of BPX-DDGS and HP-DDG (0.08 and 0.07 h^-1, respectively) and NDIP degradation rates (0.07 and 0.06 h^-1, respectively) were similar to CPM-dairy feed library NDR and NDIP rates of corn distillers grain (0.07 and 0.05 h^-1, respectively). Model predictions using standard and in vitro determined values did not differ. As BPX-DDGS decreased and HP-DDG increased in the diet, observed milk production tended to decrease linearly (P = 0.08). There was a cubic effect for milk fat percent (P = 0.03) and a cubic trend for milk fat yield (P = 0.09). Milk protein yield also tended to decrease linearly (P = 0.06). CPM-dairy model prediction accuracies were less than 50 percent. Defatting (co)products reduced lag time and fractional rate of fermentation by at least half for BPX-DDG, and had no effect on HP-DDG. Defatting both (co)products increased the fibrolytic (26.8 to 38.7 percent) and proteolytic (26.1 to 37.2 percent) bacterial guild populations and decreased the lactate-utilizing bacterial guild (3.06 to 1.44 percent). The novel ruminal in vitro method determined that the specific activity of ammonia production was not different among (co)products. However, results were within numerical range of previously used methodologies.