Sweet sorghum (Sorghum bicolour L. Moench) has the potential to improve livelihoods of rural communities in southern Africa due to its potential industrial use for bioethanol production. Stem sugars accumulated by the varieties can be extracted and fermented to provide liquid fuels such as bioethanol. However, sweet sorghum’s potential to stimulate rural growth due to its high biomass potential has not received enough attention (Leistritz and Hodur, 2008) especially in tropical Africa. Small-scale and resource-poor farmers in southern Africa reside in marginal and dry tropical lowland environments where sorghum plays an important role due to its drought tolerance compared to other grain cereals (Tsuchihashi and Goto, 2008). Sweet sorghum has a clear comparative advantage over the leading crop, sugarcane (Saccharum officinarum L), in bio-fuel production. It can grow under dryland conditions where sugarcane cannot grow (Tsuchihashi and Goto, 2008). Thus sorghum would require less irrigation water than sugarcane in tropical lowlands and dry mid-altitude environments which accounts for about 16% and 19%, respectively, of the cereal mega- environments in southern Africa (Vivek et al., 2005). These areas are characterised by low, erratic and uni-modal annual rainfall during November to March with a high probability of a mid-season dry spell, mid-season drought or a full-season drought. However, the tropical lowland environments are warm enough to produce sorghum throughout the year provided irrigation is applied during off-season.
Literature on the production of sweet sorghums has been scarcely reported, but all year round production has been demonstrated in Indonesia (Tsuchihashi and Goto, 2008). Woods (2000) demonstrated that it was possible to grow sweet sorghum for both bioethanol and bagasse for electricity generation in the tropical lowlands of Zimbabwe, but only in the summer (in-season), November to April. There is therefore need to generate research information on the potential to produce the crop in the tropical lowland environments, both in- season and off-season, and in the dry mid-altitude environments where sorghum plays an important role. Although only about 5% of the land in Africa is under irrigation, some small- scale farmers in the tropical lowlands in southern Africa have access to irrigation facilities.
This area includes Chokwe, Makhathini, and Muzarabani in Mozambique, South Africa, and Zimbabwe, respectively. Farmers have traditionally grown sweet stem landraces for chewing as snacks, but only on a small scale. This necessitates the inclusion of the tropical lowland off-season during the evaluation of experimental entries. The challenge is to make available
97 appropriate sweet sorghum cultivars for this purpose. This demonstrates the need to develop productive sweet sorghum hybrids for production at a commercial level. With sweet sorghum, industrial use might economically justify the high costs associated with irrigation during the off-season and in-season should the need to supplement arise.
Research has demonstrated stem sugar concentrations of between 14.0 and 18.5°brix in specialized sweet sorghum cultivars (Guiying et al., 2000; Woods, 2000; Tsuchihashi and Goto, 2004), but similar work on dual-purpose cultivars is lacking. Tsuchihashi and Goto (2004) reported stem brix values of about 13 under dryland production in Indonesia. Part the current study reported low stem brix values during the off-season in the tropical lowland environments (Makanda et al., 2009). This suggests that stem sugar concentration can be depressed under both dryland and off-season conditions. However, the varieties used in the former study might not be adaptable to the tropical conditions in southern Africa; especially the off-season production and those in the latter have not been evaluated during in-season.
This necessitates the need to evaluate experimental hybrids across environments to determine performance stability as well as specific adaptations to the different regions. Lin and Binns (1988) devised a measure of performance stability, the cultivar superiority index (Pi), across environments. The Pi is the distance mean square between the response observed in a cultivar in a particular environment and the maximum response observed for the same environments. Therefore, a cultivar with a low Pi is superior in performance across environments and selected over another with a high Pi because it shows consistency in performance across environments (Lin and Binns, 1988). Selection is based on a single value, which simplifies the process. The index does not require check varieties in all the environments unlike the previously proposed indices (Lin and Binns 1985; Lin et al., 1985), which reduces the trial size and cost. The index has been successfully used in screening barley cultivars by Lin and Binns (1988).
Development of a viable breeding programme for sweet sorghum requires a clear breeding strategy. This depends on the understanding of gene action for the traits of interest.
Schlehuber (1945) reported that genes with partial dominance action controlled sucrose content in hybrids. Baocheng et al. (1986) reported that genes with additive and dominance effects influenced stem sugar accumulation. Guiying et al. (2000) reported that recessive genes exhibiting additive effects controlled stem sugar accumulation in sorghum. Following a QTL analysis, Natoli et al. (2002) reported no significant segregation for genes with major
98 effects on stem sugar percentage. However, studies by Ritter et al. (2008) suggested involvement of major genes in addition to genes with minor effects for stem brix. Moderate to high h2 estimates, ranging between 40% and 96% (Baocheng et al., 1986; Guiying et al., 2000), and the predominance of genes with additive effects suggest that brix could be improved through selection.
Knowledge of the combining ability of the parents, especially for hybrid cultivar development is important for the optimization of a breeding strategy. Reports on the combining ability of sorghum lines for stem brix are scarce in the literature. However, there are reports of significant GCA and SCA effects for the associated traits, but their level of importance was dependent on the germplasm that was evaluated. Kenga et al. (2004) reported that SCA effects were predominant over GCA for grain yield and days to anthesis, while Haussmann et al. (1999) reported that GCA effects were more important than SCA effects. Nevertheless, results obtained elsewhere may not necessarily give an indication of the behaviour of the genes in a different environment. Falconer and Mackay (1996) reported that combining ability and heritability information is pertinent to the set of genotypes and the environment where it has been tested.
Sorghum hybrid cultivars have been shown to be more productive than pure-lines (Kenga et al., 2004; Li and Li, 1998). However, the cost of producing hybrids is only justified when their performance surpasses those of their parents and current varieties. A survey of the literature showed extensive reports on heterosis for grain yield but little information is available on stem sugar heterosis in sorghum. Corn (2008) reported better parent heterosis values ranging between -24% and 7% for stem brix, and -27% to 43% for stem biomass. Therefore there is potential to exploit heterosis in new sweet sorghum cultivar development.
Given the foregoing, this study aimed at studying (i) the combining ability effects, (ii) heterosis and (iii) cultivars superiority of experimental entries for stem brix and associated traits across six environments representing the target recommendation domain in southern Africa. The following hypotheses were tested:
i. cultivars that are superior to those on the market can be developed from the current germplasm,
ii. there are high levels of heterosis for stem brix and associated traits that can be exploited in cultivar development from the current germplasm
99 iii. genes with additive effects control stem brix, stem biomass and associated traits in
sorghum, and
iv. genes with non-additive effects control stem brix, stem biomass and associated traits in sorghum.