Volatile fatty acids (VFA) production was studied using rumen bacteria. Fermentation of pretreated corn stover to VFA. VFA yield increased to 40.8 g/L at 5% total solids (TS) from 22.8 g/L at 2.5% TS. Valeric acid production was seen only at 5% TS but not at 2.5% TS. Composition analysis showed 33% carbohydrates conversion at 2.5% and 50% at 5% TS.