Background: Corn stover composition changes considerably throughout the growing season and also  varies  between  the  various  fractions  of  the  plant.  These  differences  can  impact  optimal pretreatment  conditions,  enzymatic  digestibility  and  maximum  achievable  sugar  yields  in  the process of converting lignocellulosics to ethanol. The goal of this project was to determine which combination of corn stover fractions provides the most benefit to the biorefinery in terms of sugaryields and to determine the preferential order in which fractions should be harvested. Ammoniafiber expansion (AFEX) pretreatment, followed by enzymatic hydrolysis, was performed on early and late harvest corn stover fractions (stem, leaf, husk and cob). Sugar yields were used to optimize scenarios  for  the  selective  harvest  of  corn  stover  assuming  70%  or  30%  collection  of  the  total available stover.

Results: The optimal AFEX conditions for all stover fractions, regardless of harvest period, were:1.5 (g NH3 g-1 biomass); 60% moisture content (dry-weight basis; dwb), 90°C and 5 min residence time. Enzymatic hydrolysis was conducted using cellulase, β-glucosidase, and xylanase at 31.3, 41.3,and 3.1 mg g-1 glucan, respectively. The optimal harvest order for selectively harvested corn stover (SHCS)  was  husk  >  leaf  >  stem  >  cob.  This  harvest  scenario,  combined  with  optimal  AFEX pretreatment conditions, gave a theoretical ethanol yield of 2051 L ha-1 and 912 L ha-1 for 70% and 30% corn stover collection, respectively.

Conclusion: Changing  the  proportion  of  stover  fractions  collected  had  a  smaller  impact  on theoretical ethanol yields (29 - 141 L ha-1) compared to the effect of altering pretreatment and enzymatic hydrolysis conditions (150 - 462 L ha-1) or harvesting less stover (852 - 1139 L ha-1).Resources may be more effectively spent on improving sustainable harvesting, thereby increasing potential  ethanol  yields  per  hectare  harvested,  and  optimizing  biomass  processing  rather  than focusing on the selective harvest of specific corn stover fractions.