A South African perspective investigatingfive nitrogen application levels for optimumsweet sorghum juice yields needed for theproduction of bio-ethanol

Abstract

The rasionale behind this study was merely to determine whether the tested sweet sorghum genotypes can be utilised as a renewable bio-ethanol resourse and whether different nitrogen (N) application levels have an effect on production (biomass yield, Brix% and juice yield). It was not to quantify and qualify sweet sorghum production and not to quantify and qualify the effect of different N application levels on the production of sweet sorghum. However, the results obtained during the study did indicate a performance profile of the genotypes that was discussed in Chapter 4. A shortage of scientific information exists in South Africa regarding the propagation of the best sweet sorghum genotypes and the application of optimum levels of nitrogen (N) fertilisers in the cultivation of the feedstock to produce bio-ethanol (EtOH) for blending with fossil fuels. Data presented here will address this gap and I trust it will add scientific knowledge that could aid all present and future stakeholders involved in the biofuel genre. Due to the involvement of the Agricultural Research Council: Grain Crops Institute (ARC: GCI) in the Sweetfuel Programme, sweet sorghum genotype evaluation trails were planted in South Africa since 2010. Dryland agricultural practises were applied at various locations and the genotypes were selected at random as to include as many genotypes as possible. An average of 20 genotypes were planted at the various locations across a number of years to determine the best lines for biomass yield, juice yield and Brix% values to be introduced into the sweet sorghum based EtOH production environment. Nitrogen trials were also conducted under dryland conditions and in a glasshouse. The same genotypes were planted and their reaction to the different N levels were recorded to determine whether N has an effect on biomass yield, juice yield and the Brix%. Rondomised block designs with three replications were used in the genotype trial layouts and two replications were applied in the N application trials. The amounts of fermentable and non-fermentable sugars produced by the sweet sorghum were determined by high-pressure liquid chromatography by the North West University (Potchefstroom, South Africa) and these values were used to calculate the potential EtOH that can be produced from sweet sorghum and be blended into the existing fossil fuels. During 2010 / 2011, one trial was planted at the ARC: GCI at Potchefstroom (North West Province) and one at Taung (Northern Cape Province). Thereafter, the genotype trails were extended and trials were planted at the Agricultural Research Institute (ARC: SGI) at Bethlehem (Freestate Province), the Agricultural Research Institute (ARC: IIC) at Rustenburg (North West Provinve), Vaalharts (Northern Cape Province), the ARC: GCI and Wilgeboom (10 kilometres outside Potchefstroom, North West Province), to cover different climatic and soil conditions. The best performing genotypes (between 18 to 20) were planted consecutively over three years, stretching across 2011/12 to 2013/14. This trial-based data was collected and analysed. In an attempt to allow comparisons regarding the data amongst the genotypes and the countries involved in the Sweetfuel project, the layouts of the trials were determined by the Sweetfuel Consortium in attempted to standardise the agronomical specifications across the six countries who were involved in the Sweetfuel project (www.sweetfuel-project.eu). Fertilisers applied for the genotype trials applied was merely to standardise the soil nutrient content and to supply the necessary additional nutrients that were required for proper plant growth. The applications also took the clay content of the different soils into consideration. Planting started as soon as 50 mm of rainfall measured, usually from mid October to mid December. Different randomisation of the genotypes was applied at each location. The genotypes were planted in four rows of 5 m each. The inter-row spacing was 0.6 m and the intra-row spacing was 8 cm. A plant population of 207 500 plants per hectare was achieved. Chemical and mechanical weed control were executed and insecticides used to control stalkborer and aphids were applied when necessary. Harvesting was done when the seed reached the physiological maturity stage, which usually was from day 90 to day 120, depending on the genotype. Representative samples (54 stalks) from each genotype were processed and the data was recorded and anaysed. A three-roller hydraulic press was used to extract the juice from the stalks. During the genotype evaluation trials, the biomass yield (mass), the juice yield (mass) and Brix% were determined, and the potential EtOH production was calculated from the synthesised sugars. The best biomassa yield produced by ss 003, ss 007, ss 017, ss 120, Hunnigreen (HG) and Supa. The highest calculated total EtOH potential produced from the bagasse was 71.1 kL ha-1 and obtained from HG during the 2014 season in Potchefstroom, as well as the highest calculated amount of EtOH (83.09 kL ha-1) from bagasse, juice and residual sugars. Supa produced the best juice yield (57.38 t ha-1) with a Brix% value of 20.84% at Rustenburg in 2014. To study the effect of different N fertiliser application levels on the genotypes, overall eight N fertiliser application rates were applied with five levels per locality. Although ss 007 produced best at 200 kg ha-1, it was clear from the recorded data that except for a few outliers, the effect of N fertiliser applications did not produce economical viable higher EtOH yields at very high N levels. However, when looking at the conclusions drawn from this dissertation, sweet sorghum proved to be most viable on the subject of the production of EtOH in South Africa, when compared to other crops such as sugarcane and sugar beet compared to sweet sorghum (Table 18). When the decision by the stakeholders is in favour of the industry, it will be worthwhile to cultivate sweet sorghum.