Size reduction, or comminution, is a fundamental processing step for lignocellulosic biomass feedstocks used as a raw material for conversion to biofuels. Grinding or milling of herbaceous materials such as corn stover, switchgrass, miscanthus and other biofuel feedstocks consumes a lot of energy, produces particle shapes that are difficult to handle, requires dry raw material, and often results in excessive dust. New comminution methods are needed that produce more uniform small particles from herbaceous biomass across the moisture content range of full field moisture to dry, and consume minimal energy. The proposed rotary shearing technology has the potential to reduce the comminution energy to near the theoretical minimum and also enable production of flowable particles of uniform size, shape, and surface area.