LAUSR

Switchgrass is a perennial C 4 grass that is undergoing development as a dedicated biomass feedstock for conversion to bioenergy. Increasing biomass yield is the principal goal of most breeding programs. Delayed flowering time is one of the principal approaches toward achieving increased biomass yield. The objective of this study was to conduct and evaluate up to three cycles of phenotypic selection for altered flowering time in two populations of upland switchgrass, one tetraploid, and one octoploid. Selection was conducted in southern Wisconsin and evaluations were conducted at four locations ranging from 42 to 46° N latitude (USDA hardiness zones 4a to 5b). Realized heritability for anthesis date was high, approximately 0.7 in both populations, with selection responses averaging 5 days cycle −1 . Biomass yield was strongly influenced by delayed flowering, averaging + 0.03 to 0.13 Mg ha −1 for each day delay in anthesis date. However, the biomass yield responses to delayed flowering were strongly associated with latitude of the evaluation location, decreasing by 75% as latitude increased from 42 to 46° N. There is likely a threshold latitude, above which the growing season is too short for later flowering to have a significant impact on increasing biomass yield of upland populations. Future increases in biomass yield for USDA hardiness zone 3 may be extremely difficult or impossible, or at least require different strategies.