25 with soil fertility also being considered to a limited extent. AquaCrop, however, does not consider other biotic factors such as pests and diseases (Steduto et al., 2009).
The review of modelling has shown that there has been much progress in the development and understanding of crop models. However, a lot of this progress and most of the models currently developed are for the major crops. There have been very limited efforts to develop models for NUS. Perhaps, it is in this regard that AquaCrop leads. Although the model is still in its infancy, it has already been calibrated and validated for some NUS – quinoia (Geerts et al., 2009) and bambara groundnut (Karunaratne et al., 2011). These efforts form stepping stones to modelling of other NUS. A huge gap currently exists in this regard. As such, part of the focus of this study was to also contribute to international efforts on modelling yield response to water availability of NUS through calibrating and validation AquaCrop for local landraces of taro and bambara groundnut.
26 accepted as a form of intercropping. The spatial arrangements of the component crops can be arranged in several ways, which may include, but are not limited to:
i. row intercropping – growing two or more crops at the same time with at least one crop planted in rows. This system is more aligned to conventional agriculture and will serve as the scenario for the case study described in this review,
ii. strip intercropping – growing two or more crops together in strips wide enough to allow for separate, mechanized crop production. This arrangement is often most desirable whereby at least one or both crops are to be machine harvested; however, the component crops should remain in proximity to allow for interaction,
iii. mixed intercropping – growing two or more crops in no distinct row arrangement which is more typical of traditional agro-systems. As mentioned earlier, some text may not necessarily refer to this as a form of intercropping but rather ‘mixed cropping (Ruthernberg, 1971; Andrews and Kassam, 1975; Freyman and Venkateswarlu, 1977;
cited in Willey, 1979) and,
iv. relay intercropping – planting a second crop into a standing crop at a time when the standing crop is at its reproductive stage but before harvesting.
Willey (1979) stated that intercropping had long been recognized as a common agricultural practice in the tropics. This view was also shared by Walker (2009) who stated that, in the tropics, many of the traditional cropping systems comprised more than one crop growing in one field at any given time. However, despite this historical contribution, intercropping has mostly been regarded as a primitive practice. This has meant that there has been limited research on intercropping as opposed to the volumes of research done on sole crops. The status of intercropping as a farming practice is simar to that of NUS. They too form part of historical cropping systems that have been sidelined by research in favour of the major crops – rice, wheat, maize and potato.
27 1.9.1 Resource Utilization
Intercropping is known for its main advantage of allowing plants to efficiently utilise available resources of light, water and nutrients thence increasing productivity (Lithourgidis et al., 2011). Component crops use natural resources differently and therefore make better overall use of them than when grown as sole crops (Willey, 1979). They are able to completely absorb and convert natural resources such as solar radiation, water and nutrients to crop biomass thus improve yield production. This is due to the fact that component crops have different competitive abilities for resources due to variation in crop characteristics such as rates of canopy development, final canopy size, photosynthetic adaptation of irradiance conditions and rooting depth (Midmore, 1993; Marris and Garrity, 1993; Tsubo et al., 2001).
Radiation interception is perhaps the most important factor affecting productivity of intercrops. Since radiation intercepted depends on canopy size and duration (Black and Ong, 2000), this provides a scenario whereby it can be manipulated by varying plant density and spatial arrangements within the intercrop. Thus, selection of crops that differ in competitive ability in space or time is important (Lithourgidis et al., 2011). Previous research has shown that intercropping is more efficient when component crops differ greatly in growth duration (Wien and Smithson, 1981; Smith and Francis, 1986; Fukai and Trenbath, 1993; Keating and Carberry, 1993). Rao and Willey (1980) found that intercropping late maturing pigeon pea with early maturing setaria improved the Land Equivalent Ratio (LER); LER is a measure of the efficiency of intercropping in relation to monocropping.
Intercropping has also been reported to improve water-use efficiency (WUE) (Hook and Gascho, 1988). It was reported to result in increases in WUE (18 - 99%) relative to WUE of sole crops of component crops (Morris and Garrity, 1993). In separate studies, Sani et al.
(2011) observed better water-use in a maize-sorghum intercrop while Oseni (2011) observed it in a sorghum-cowpea intercrop. High leaf area and leaf area index have been identified as some of the factors which contribute to water conservation in intercropping (Ogindo and Walker, 2005). Morris and Garrity (1993) found that intercropping improved water capture by 7% compared with monocropping. Elsewhere, it was found that WUE of a maize-soybean intercrop was higher than that of sole crops (Barhom, 2001). As such, intercropping can be a very useful cropping practice in water scarce areas.
28 Differences in root and canopy architecture of component crops provides a platform for harnessing more solar radiation, improved water- and nutrient- use than root and leaves of a sole crop (Thayamini and Brintha, 2010). Dahmardeh et al. (2009) reported that maize- cowpea intercropping increased soil nitrogen, phosphorus, and potassium content in relation to a maize mono crop. Intercropping between high and low canopy crops can improve light interception and consequently yield (Azam-Ali et al., 1990). In the case of this study, taro and bambara groundnut exhibit different canopy architecture, and size as well as different rooting depths. On one hand, taro is characterised by large leaves compared with the small leaves of bambara groundnut. In addition, taro plants grow taller than bambara groundnut plants. Lastly, taro has shallow roots while bambara groundnut has a deeper rooting system. Hypothetically, intercropping taro and bambara groundnut would create a scenario whereby the root density in the soil is increased. Increased root density implies enhanced soil water capture which would translate to greater biomass production.
1.9.2 Sustainability
Issues of sustainability have taken centre stage in most debates. As such, as we advocate for the promotion of NUS and alternative farming practices, such discussions should also focus on sustainability. Such sustainability should be long-term, enhance the environment as opposed to degradation and still meet the objective of feeding the human population. Here we will define
‘sustainability’ according to Sivakumar et al. (2000) who described sustainability as the balance between utilization to satisfy human needs and maintenance of the environment.
Hansen (1996) also provides some useful definitions of sustainability that are specific to agriculture. Since sustainable agriculture seeks, in most cases, to mimic nature, intercropping offers a rare window to achieve such. Intercropping increases on-farm diversity maximizes on resource use and conversion to biomass as well as improving water- and nutrient-use. Other benefits of intercropping may also include reduced incidence of pests and disease in intercrops as well as reduced requirement for labour for weeding. This amounts to savings (financial and human resource) for resource-constrained farmers. Theoretically, this creates a balance between providing food and fibre for man, enhancing the environment and avoiding
29 environmental degradation. This implies that intercropping is indeed a sustainable farming practice.