Technical Abstract: Corn residue is viewed as an abundant, inexpensive source of biomass that can be removed from fields for ethanol production without deleterious production or environmental effects if proper management is used according to some recent publications. Other publications indicate that corn residue needs to be retained on the land to reduce erosion and maintain or perhaps even improve soil organic matter levels. As researchers attempt to address these questions, one component of corn residue that may be available for immediate use for conversion to ethanol is the cob. Our objective was to determine how much cob biomass or cob biomass (as a percentage of grain biomass) is produced that could potentially be converted to biofuels. Results from two long-term experiments: 1) Rainfed with four cropping systems that included corn with three nitrogen fertilizer rates (20 years), and 2) Irrigated with two cropping systems, with four corn hybrids and five nitrogen fertilizer rates (8 years). Several factors (Cropping System, Hybrids, N fertilizer, and their interactions) significantly affected both cob biomass and cob biomass as a percent of grain biomass but were not of sufficient magnitude to be of practical significance. Most importantly, when N fertilizer was applied at rates sufficient to optimize grain yields in all cropping systems and hybrids, cob biomass as a percent of grain biomass averaged approximately 20%. This consistent relationship allows quick and easy calculation of the cob biomass that could be available for harvest for biofuels if grain yield levels or potentials are known.