To reduce the recalcitrance of lignocellulosic biomass for subsequent biological processing, we pretreated energy crop feedstocks with mild steam treatment (ST; 130 and 150 degrees C for 60 min) and wet disk milling (WDM). We tested two phylogenetically different, but typical energy crop feedstocks: Populus trichocarpa and switchgrass (Panicum virgatum). WDM after ST facilitated the fibrillation of both types of biomass, resulting in an increase of specific surface area, improved enzymatic saccharification yield, and decrease in cellulose crystallinity. After steam treatment at 150 degrees C followed by 17 cycles of WDM, enzymatic hydrolysis resulted in almost complete glucan to glucose conversion in both feedstocks. (C) 2014 Elsevier Ltd. All rights reserved.