Natural variability and antioxidant properties of commercially cultivated switchgrass extractives

•Third growth cycle provided higher total switchgrass extractives and total phenolics.•Early harvest generated biomass with higher total phenolics and increased anti-oxidant activity.•Switchgrass cultivar ‘EG1102’ displayed higher DPPH radical scavenging activity.•Switchgrass IC50 was inversely correlated to free sugars content.•Switchgrass IC50 was positively correlated to flavonols and caffeic acid content.