The cost of raw material is important and cannot be overlooked since it governs the total cost which represents more than 60% of total ethanol production cost (Ogbonna et al., 2001). Using cassava (Manihot esculenta) or tapioca starch as substrate in bioethanol production will reduce the production cost since cassava plants are abundant, cheap and can easily be planted. It is a good alternative at low production cost. It is a preferred substrate for bioethanol production especially in situation where water availability is limited. It tolerates drought and yields on relatively low fertility soil where the cultivation of other crops would be uneconomical especially on idle lands. Furthermore, the starch has a lower gelatinization temperature and offers a higher solubility for amylases in comparison to corn starch (Sanchez and Cardona, 2008).

Cassava is one of the richest fermentable substances and most popular choice of substrates for bioethanol production in the Asian region. The fresh roots of cassava contain 30% starch and 5% sugars while the dried roots contain about 80% fermentable substances. Its roots can yield up to an average 30-36 t/ha. Several other varieties of its non edible tubers maybe selected based on the cyanide content which can be categorized as sweet, bitter, non-bitter and very bitter cassava contains 40-130 ppm, 30-180 ppm, 80-412 ppm and 280-490 ppm, respectively (Food Safety Network, 2005). Since fresh cassava tubers cannot be kept long, it needs to be processed immediately or produced ethanol from dried root. Alternatively, its roots can be milled and dried to form pallet or flour. This will prolong its storage time and save storage spaces. Cassava tuber also can be kept in soil after maturating for several months unharvest without deteriorating. Besides the tuber, cassava waste also can be utilized for ethanol production due to its high content of cellulose, hemicelluloses and starch respectively at 24.99%, 6.67% and 30-50% (w/w) (Ferreira-Leitao et al., 2010).

One of the advantages of using starch such as cassava is that most of the plants can be intercropped with other plants such as cover crops (legume plant) or tree crops (such as cocoa plant and palm oil plant) which can simultaneously grow together (Aweto and Obe, 1993; Polthanee et al., 2007). Polthanee et al. (2007) discussed four possible ways of intercropping practice. They are i) mixed inter-cropping, simultaneous growing of two or more crop species; ii) row-intercropping where simultaneous growing of two or more crop species in a well-defined row arrangement in an irregular arrangement; iii) strip inter­cropping, simultaneous growing of two or more crop species in strips wide enough to allow independent cultivation but, at the same time, sufficiently narrow to induce crop interactions and iv) Relay inter-cropping, planting one or more crops within an established crop in a way that the final stage of the first crop coincides with the initial development of the other crops. This will improve the land productivity and better land usage without the need to explore new land which might lead to deforestation. Figures 2 and 3 show the row-

intercropping of immature oil palm plantation intercrop with soyabean and cassava, respectively by Malaysian Palm Oil Berhad (MPOB), Malaysia.