Further characterization rumen digestion could aid in development of a more efficient diet. The objective was to determine the early effects (0-6h) of concentrate type on rumen pH and forage digestion. Two forages, orchard grass and corn (Zea mays L.) stover, were combined in a 50:50 ratio with 5 concentrate treatments (no concentrate, corn meal, corn gluten meal, barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.)). Samples were incubated in vitro in a 1:4 buffer to rumen fluid mixture for 420 min. The pH was measured at intervals throughout the trial and samples were collected for volatile fatty acid (VFA) analysis. Forage samples alone had significantly lower pH (P<0.05) than those samples containing concentrate. For corn stover samples, this correlates with a high mean lactic acid concentration (535 ppm). The final study measured change in pH and the difference in neutral detergent fiber (NDF) digestibility of four concentrate treatments (no concentrate, corn meal, corn gluten meal, and barley). Two sample distribution methods were tested; forage and concentrate were mixed together in the same filter bag or separated into individual bags. Barley had the largest inhibitory effect on fiber digestion and caused the largest decline in pH. Treatments with feeds in the same bag showed larger differences in pH and digestibility. Results suggest a diet containing a mixture of forages or, a mixture of corn and forage minimizes declines in rumen pH. They also suggest the study of associative effects is impeded by separation of feeds through filter bags.