A first‐generation energy sorghum hybrid with enhanced photoperiod sensitivity and long growth duration accumulated more than twice as much biomass as grain sorghum. The energy sorghum produced more leaves (~45 vs 17–20), longer stems (~4 vs 1.5 meters) and had a higher stem‐to‐leaf biomass ratio than grain sorghum. At the end of the season, energy sorghum stems represented 83% of the plant's shoot biomass. The greater biomass accumulation was due to longer growth duration, a higher leaf area index, greater radiation interception, and higher radiation use efficiency. When grown under dryland or limited irrigation conditions, the rate of biomass accumulation by the energy sorghum hybrid was reduced in mid‐season. This decrease was most apparent in August due to summer water deficit; however plants recovered when rain occurred in September. Crop growth modeling and biomass accumulation rates measured under optimal field conditions show that energy sorghum, like other C4 energy grasses, has excellent biomass yield potential. The greenhouse gas offset values of energy sorghum hybrids, grown in large and small field plots, and under fully irrigated and dryland conditions, ranged from 63–78% for cellulosic ethanol production and 88–95% for power generation. This study shows that drought‐tolerant, annual energy sorghum hybrids have the genetic yield potential to contribute significantly to bioenergy production. Sorghum is a genetically tractable, diverse species with a good genomics platform, making energy sorghum a promising genetic model for the design of C4 grass energy crops. © 2012 Society of Chemical Industry and John Wiley & Sons, Ltd

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