Additional N application and ecotype affect yield and quality of ratoon harvested sorghum x sudangrass hybrid for temperate regions

Sorghum x sudangrass hybrid (Sorghum bicolor (L.) Moench) has excellent biomass yield and requires low maintenance. It can be harvested multiple times during the growing season. It has great potential to be used for cellulosic biofuel production in the temperate regions, including the Korean peninsula. However, N requirements during the regrowth have not been evaluated yet. Three years of the study with two ecotypes (early-flowering vs. late-flowering) were compared. After summer harvest, additional N (0, 50, 100, 150 kg N ha⁻¹) were applied. Early-middle-late ratoon harvest treatments were executed to evaluate the ideal period for biomass yield and Theoretical Ethanol Yield (TEY). The late-flowering ecotype had greater biomass yield, TEY, and N-Agronomic Efficiency (NAE) due to longer vegetative growth. Maximum biomass yield was 7.44 Mg D.W. ha⁻¹ (equivalent to TEY of 2657 L ha⁻¹) at the additional 100 N kg ha⁻¹ of middle ratoon harvest in 2018 when weather condition was unfavorable. However, with the favorable weather condition in 2019, additional 150 N kg ha⁻¹ of late ratoon harvest outyielded (10.66 Mg ha⁻¹; TEY of 4081 L ha⁻¹). NAE was highest at 50 N kg ha⁻¹ and then decreased with higher N rates. It indicates that additional N application is essential to achieve biomass yield and TEY under multiple harvest conditions. The appropriate harvest window for the ratooning crop was up to 14 weeks. Thus, additional N application and decision-making for harvest window are crucial factors affecting biomass yield and quality for biofuel with a choice of late-flowering ecotype.