Appropriate harvesting stage of sweet-sterm sorghum (Sorghum bicolor (L) Moench) cultivar for optimum bio-ethanol production in Namibia
There has been growing global concern over dwindling fossil fuel supply. This development has led to increased interest in exploration of alternative energy sources, especially bio-fuel. Sweet sorghum (Sorghum bicolor), sugar cane (Saccharum officinarum), corn (Zea mays), wheat (Triticum aestivum), rape seed (Brassica napus), barley (Hordeum vulgare), sugar beet (Beta vulgaris), cassava (Manihot esculenta), oil palm (Elaies guineensis), sweet potato (Ipomoea batatas) and many more have been recognized as feedstock crops for bio-fuel production. Like most bio-fuel crops, sweet stem sorghum has the potential to reduce carbon emissions. In addition, this crop shows stronger tolerance than other crops under hot and dry climatic conditions. Its bagasse can be exploited as a by-product including burning, material for electricity generation, paper or fibre board manufacturing, silage for animal feed or juice for ethanol production. While there is abundant availability of sweet stem sorghum in Namibia, its potential as feedstock crop for bio-fuel has not been fully explored. One of the major problems associated with sorghum as a feedstock crop for bio-fuel is the issue of knowing the right harvesting stage for optimum brix sugar yield. In Namibia, great potentials exist for the development of the energy sector through adoption of bio-fuel production. Therefore, this study aimed at ascertaining appropriate harvesting stage of sweet-stem sorghum (Sorghum bicolor (L) Moench) cultivar for optimum brix sugar yield for bio-ethanol production in Namibia. Two trials (February to June 2014) and (October 2014 to February 2015) were conducted in the demonstration plot of the University of Namibia (UNAM), Hifikepunye Pohamba Campus. A total of six harvesting stages (booting stage, and one after every week for five weeks) were used to determine the following response variables: plant height, stem diameter, stem biomass, percentage juice extract, and percentage brix sugar. Data obtained were analyzed using SPSS statistical software and inferences were made at p = 0.05. Mean plant height at 50 % booting was 74.9 cm in trial I and 73.7 cm in trial II with respective coefficients of variation of 20.3 and 21.0. The t-test analysis for plant height was not significant (f = 22.5, p = 0.05). Mean stem weight was 71g in trial 1 (dry season) and 65g in trial 2 (raining season). Mean brix value of the extracted juice was 9.8 % in trial I and 11.8 % in trial II, with coefficients of variation of 5.0 % and 14.5 % respectively. The brix value increases significantly until it peaked at 5 weeks after booting. The study confirms that sweet stem sorghum IS 2331 cultivar could be grown in both dry and wet seasons in the study area. They could be harvested for optimum brix sugar yield for bio-ethanol production at five weeks after booting. Therefore, the study recommends that the Namibian government should raise awareness among crop farmers of the potential of growing sweet stem sorghum in both dry and wet seasons. Future studies should determine the brix sugar yield beyond 5 weeks after booting until possible grain harvest stage.
Harvesting stage, Sweet-sterm sorgham, Rthanol production
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