Combined cultivar and harvest time to enhance biomass and methane yield in sorghum under warm dry conditions in Pakistan

Abstract Methane from agricultural biomass contributes to achieving self-reliance and sustainability in alternative energy sources. In this framework, a two-year experiment was set up in Punjab, Pakistan, combining four cultivars (CV) of biomass sorghum with five harvest times (HT) (from 60 to 120 days after sowing) (DAS). HT and CV, both alone and together strongly influenced morphological, compositional and yield traits. Dual purpose (grain and biomass) CV (Jawar-2011 and JS-263) attained higher dry biomass yield (DBY) than fibre CV (Hagari and JS-2002) (+22%), possibly due to a stronger sink for assimilates at late growth stages. Delayed harvest (120 vs. 60 DAS) determined the largest differences in all traits: increase in DBY (+48% DBY), hemi-cellulose (+80%), cellulose (+85%) and lignin (+41%), and concurrent decrease in protein (−22%) and ash (−28%). These compositional changes indicate a strengthening in plant structure with advancing maturity that negatively reflected on specific methane yield (SMY) (−17%). However, average SMY obtained with sorghum in the warm dry conditions of Pakistan (290 l N  kg −1 VS) is consistent with data observed at higher latitudes under temperate climate. The significant HT × CV interactions for DBY, SMY and their product, methane yield per hectare, were fitted by means of functions describing CV trends in time. Based on this, suitable combinations of HT and CV can be envisaged to balance methane yield and bio-degradability over a reasonable harvest window. On concluding, data obtained in this experiment disclose favourable prospects for sorghum as bio-methane feedstock in warm dry areas of the world.