Performances of two commercial grinders [349 and 463 kW (475 and 630 HP)] were studied to produce coarsely ground particles from round-baled, plastic net-wrapped corn stover and native perennial grasses. The effects of grinder screen size [19.1 mm (0.75 in.) to 127.0 mm (5.0 in.)] and feedstock on the particle size, bulk density, throughput, and specific energy consumption were studied. The bulk density of ground biomass ranged from 69 to 207 kg/m 3 (4 to 13 lb/ft3). The throughput of the grinders ranged from 5 to 45 t/h. The specific energy consumption ranged from 68 to 804 MJ/t. Increasing the grinder screen size decreased the bulk density and specific energy consumption, but increased the particle size and throughput. High moisture bales (25% or higher moisture content) decreased the performance of the grinders. The results from this study would be useful for developing a biomass supply logistics system involving mobile grinding.