Abstract:
First generation (edible) crops used for bioenergy production are generally not economically feasible due to the fact that they rarely offer significantly more energy at the end of the processing chain as compared to the energy required throughout all the production steps. South Africa, being a water scarce country will rely heavily on bioenergy crops with high water use efficiency in conjunction with high overall energy production to effectively produce sustainable and renewable forms of energy, and thus this study also investigated the responses of different bioenergy crops to water regimes. Knowledge is lacking in the areas of water use, water use efficiency, potential energy yield and management of non-edible (second generation) bioenergy crops under South African climatic conditions. The aim of this study was to determine how biomass production and corresponding calorific values are affected by different water regimes and harvesting intervals. A two-factorial split-plot randomised block design field experiment with three water regimes, eight Poaceae species and three replicates (plot size 5.5 m x 6 m) was conducted. Three regimes of increasingly available soil water were applied, namely: dryland (T1), two-weekly (T2) and weekly (T3) irrigation according to soil water content measurements. Harvesting was done monthly for three months in successive sections of each plot and this cycle was repeated three times (summer (C1), autumn (C2) and spring (C3) cycles). The following Poaceae species were analysed in the present trial: Panicum maximum, Pennisetum purpureum, Miscanthus giganteus, Chrysopogon zizanoides, Hyparrhenia tamba, Brachiaria brizantha, Sorghum bicolor (sweet sorghum) and Sorghum bicolor (grain sorghum), with sweet sorghum as the control species. It was noted that more frequent harvest intervals within the same time period compared to a single final harvest generally did not produce greater biomass yields. The average annual water to energy production efficiency (WEPE) values for each species across all water treatments are listed in descending order: H. tamba (S6 � 601 MJ ha-1 mm-1), P. maximum (S1 � 549 MJ ha-1 mm-1), P. purpureum (S2 � 502 MJ ha-1 mm-1), B. brizantha (S7 � 477 MJ ha-1 mm-1), C. zizanoides (S5 � 454 MJ ha-1 mm-1), S. bicolor (S8 � 309 MJ ha-1 mm-1), S. bicolor (S9 � 185 MJ ha-1 mm-1) and M. giganteus (S4 � 113 MJ ha-1 mm-1). The greatest average WEPE across all species was produced at T2 (415 MJ ha-1 mm-1), followed by T1 (398 MJ ha-1 mm-1) and T3 (384 MJ ha-1 mm-1). From preliminary data it can be concluded that H. tamba, P. maximum and P. purpureum are the three most promising Poaceae species for bioenergy production under South African climatic conditions. Also, P. maximum and B. brizantha are likely better alternatives for annual rotation crops as compared to the two sorghum varieties. Lastly, indigenous species convincingly outperformed exotic species.
