N.V. Harris, Fielder Gillespie Limited
Experimental yields indicate that cassava is a potential ethanol source for northern Australia. Climatic and soil constraints have not yet been clearly defined, but it is probable that production areas would be limited to coastal Queensland and its tropical hinterland.
Agricultural systems which involve a high degree of mechanisation and are suited to the economic environment of Australia have been developed. Current problems of pests and diseases are identified.
As a consequence of escalating crude oil prices and forecast shortages, there is increasing interest in the use of agricultural crops as renewable energy sources. Their role has been proposed as ethanol feedstocks to extend and complement petroleum products, particularly within the automotive industry. In tropical regions, similar to northern Australia, one of the crops that is receiving major attention is cassava.
Cassava (Manihot esculenta Crantz / Euphorbiaceae) originated in tropical America and has been used as a food source for over 4000 years. It is found today throughout the tropical world from latitudes 30° North to 30° South, at altitudes up to 2000 metres and is known by other names including mandioca, manioc, tapioca and yuca. The plant is a short-lived perennial shrub varying between one and five metres in height and is grown for its tuberous starchy roots. Cassava has an estab- lished reputation (Jones, 1959; de Vries, Ferweda and Flach, 1967; Rogers and Appan, 1972; Montaldo, 1979) for its efficiency as an energy producer while being able to tolerate poor soil conditions and low rainfall — factors which limit the potential expansion of other crops in many tropical regions.
Annual world production exceeds 100 million tonnes, most of which is grown at a subsistence level for consumption as human food, with surplus being used as an industrial source of carbohydrate. Major producing countries are Brazil, Indonesia, Zaire, Nigeria, India and Thailand, which is a significant exporter providing tuber chips and pellets for the EEC stockfeed market. Brazil, which has an annual produc- tion in excess of 33 million tonnes, used cassava as an ethanol feedstock during the 1930s and recently implemented a national development program to supplement ethanol produced from sugar cane (Hammond, 1977). A commercial cassava ethanol distillery was commissioned in 1977 (Anon., 1978), but has not operated to its nominal capacity of 60 000 litres per day, because of feedstock supply problems.
Cassava was first introduced to Australia during last century and interest in the crop as a potential source of starch and ethanol was reported in 1916 (Anon., 1916).
In 1925 improved cultivars were imported from Indonesia for evaluation as a feed- stock for the power alcohol distillery at Sarina (Anon., 1926) and experimental yields were promising. When the distillery began operations, however, cheap molasses supplies were found to be adequate and interest in large scale cassava production subsequently declined. Cassava has since been grown on a limited scale in coastal Queensland, mainly as a farm pig feed.
In the mid 1970s, high experimental yields, particularly at CIAT, encouraged the re-evaluation of cassava as a potential low cost source of carbohydrate and agro- nomic research was initiated by several organisations, notably the Queensland Department of Primary Industries, Queensland University, CSR Limited and 173
Fielder Gillespie Limited. Current research programs involve cultivar evaluation, growth physiology and climatic responses. Fielder Gillespie Limited have established demonstration plantations at Bundaberg and Maryborough where agricultural systems suited to the economic environment of Australia are being developed.
In this paper, the potential of cassava as an ethanol source for northern Australia is briefly evaluated from recent experimental results and observations.
Climatic Limitations Temperature
Optimum temperatures for growth are between 25°C and 29°C. Temperatures above 35°C adversely affect cassava and yields are depressed below 18°C (Jones, 1959). Leaf area index decreases when minimum temperature is below 13 °C and physiological activity ceases below 10°C. Low temperatures also increase disease incidence, particularly green stem die-back and tuber soft rots. Cassava can tolerate light frosts, but top growth can be damaged although resprouting takes place.
These limitations would suggest that nearly all of coastal Queensland, the Atherton Tablelands and a large part of the Northern Territory have temperatures suitable for cassava production.
Evenson and Keating (1978), however, reported that although the base tempera- ture for cutting germination was 13 °C, rapid field emergence and high percent emergence was not attained until soil temperatures reached 18°C. Continuous plant- ing, therefore, would not be feasible in south coastal Queensland, or on the Atherton Tableland.
Rainfall
After establishment, considered to be the first three months of growth, cassava can withstand prolonged periods of drought. It has been grown in areas of rainfall from 500 mm to 5000 mm but optimum conditions for commercial production are con- sidered to be average annual rainfalls of between 1000 mm and 1500 mm, well dis- tributed over the growing season (Kay, 1973). Areas of more than 1000 mm rainfall are confined to the coastal plain and highlands of eastern Queensland, Cape York Peninsula and in the Northern Territory, areas north of the Daly River. These areas, however, have little meaning unless seasonal distribution and variability are con- sidered.
Within the limits of temperature suitability that have been described, rainfall characteristics as described by Gentilli (1975) are:
• The Monsoonal Wet/Dry North which experiences a short wet season and a long dry season. Cassava growth, without irrigation, will probably be limited, even in areas of high total annual rainfall.
• The Trade-Wind Coast which includes narrow strips of humid climate extends southwards to reach some 60 km north of Townsville. Adjacent is the Atherton Tableland. A second humid coastal strip is found from Proserpine to Koumala.
• Within the Trade-Wind Coast there are frequent invasions of semi-arid climate where severe winter drought and limited summer rainfall occur. These areas, which occur from Townsville to Bowen and south to Koumala, would probably have a limited potential for commercial cassava production without irrigation.
• The subtropical East Coast which is found from Rockhampton south, has a favourable hybrid growing season for cassava production. Supplementary irriga- 174
tion, however, would probably be required as seasonal drought frequently occurs in spring when temperatures are optimal for planting.
Soils
Experimental areas of cassava have been grown successfully on a wide range of soils in northern Australia, with the exception of heavy self mulching soils, saline soils and soils with poor internal drainage. Light textured soils, however, are preferred for ease of harvest. Relatively flat landforms are required, as soil erosion is a major consideration in the climatic areas considered for potential production.
Recent experience indicates that cassava will grow satisfactorily on soils that have generally been considered too poor for extensive cultivation, such as sugar cane production. The crop's potential, therefore, cannot be seen as competitive with established agricultural crops, but as complementary, utilising areas of marginal agricultural potential and grazing lands. The duplex soils which occur over wide areas of coastal Queensland will be of major consideration for cassava production.
Although northern Australia would appear to have large areas of soils suitable for cassava production, detailed land potential and availability surveys have not been completed. It is probable, however, that within eastern Queensland, which is climatically most suited to cassava, no more than 200 000 hectares would be avail- able. This land is concentrated in the Bundaberg-Maryborough district of south coastal Queensland, between Proserpine and Bowen, the south-western Burdekin and the Atherton Tableland.
On marginal soils cassava will need applied fertilisers. Actual requirements, how- ever, cannot be quantified until production areas are defined and nutrient studies are undertaken on specific soil types.
Pests and Diseases
Pests and diseases are major factors limiting cassava overseas (Brekelbaum, Bellotti and Lozano, 1978) but those of major economic importance have not been recorded in Australia. Several pests and diseases have been noted, however, and although little damage occurs under experimental conditions it is possible they could become of economic importance as production areas expand. Pests and diseases currently known are:
• Cutworms {Spodoptera litura, Agrotis spp.) attack emerging shoots, particularly after periods of prolonged drought.
• Green vegetable bugs (Nezara viridula) and mealy bugs (Pseudococcidae) can damage green stem material.
• Leaf eating beetles (Monolepta australis) and spider mites (Tetranychus spp.) can cause damage to cassava leaf and in severe cases defoliation.
• Termites (Coptotermes spp. and Mastotermes darwiniensis) can invade and destroy cassava cuttings and tubers. Mastotermes has been observed causing severe damage in experimental areas in the Northern Territory and will be a major factor limiting production in the Monsoonal Wet/Dry regions of northern Aus- tralia.
Other Pests
• The common wood duck (Chenonetta jubata) can be a major pest during dry winter and spring months if alternative green forage is not available. They graze newly emerged leaf-shoots causing set-backs in crop establishment. When wood ducks are present, they can be a major factor in determining planting times.
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• Rats and other rodents can cause stem and tuber damage, inducing rots. Attack appears to be confined to drought conditions and damage will probably be of similar economic importance to that experienced within the sugar cane industry.
Diseases
• A root rot can cause yield losses on poorly drained soils. Roots decompose com- pletely, exuding a watery liquid typical of Phytophtora spp.
• Stem die-back and lodging has been observed under moist conditions. The causal organism is probably Sclerotinia spp.
• Green stem die-back, associated with a concentric-ring leaf spot similar to Phoma spp., can occur in cool moist conditions.
• Brown leaf spot (Cercospora henningsii) can cause defoliation in warm, moist conditions.
Table 1. Yields of Fresh Tubers, Harvest Index, Tuber Dry Matter and Fermentable Sugars of Cassava at Bundaberg under Drought Conditions
* P < 0.05 ** P < 0.01
1 Harvest Index = Underground Harvested F.W.
Total Plant F.W.
2 AOAC 10th Edn. Method 10.0142 Cultivars and Yield
Within the Australian cassava collection, which includes indigenous entries and material imported from Latin America and South East Asia, are cultivars which have produced satisfactory tuber yields.
The cultivar, M AUS 7, has consistently given high tuber yields since 1974 and has been adopted by the Queensland Department of Primary Industries, Queensland University and commercial interests as a reference by which to compare other culti- vars. The cultivar has performed similarly in experiments throughout northern Aus- tralia and a linear yield response with age is suggested. From 23 experiments con- ducted from 1974 to 1979, with harvest dates ranging from seven months to 24 months, the following correlation can be obtained:
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Fresh root yield (t.ha-1) = 3.42 (age at harvest in months) — 6.42 ( r = 0.876**)
This suggests a yield potential of approximately 35 tonnes per hectare when harvested at twelve months of age and 76 tonnes per hectare when harvested at twenty-four months. Tuber dry matter content, however, varies by up to 25 per cent depending on season, but is maximum in late winter.
Table 1 presents data recently recorded from a cultivar experiment harvested after twelve months by Fielder Gillespie Limited at Bundaberg. The data shows the large yield differences between cultivars and the potential of cassava as a source of ethanol as indicated by the yields of fermentable sugars. The data also demonstrates the degree of tolerance cassava has to drought as the experiment received only 745 mm of rainfall with only 29 mm falling in the last six months of growth.
Agricultural Engineering
As a subsistence crop cassava has been labour intensive with mechanisation almost entirely restricted to land preparation. In economic enviroments similar to Aus- tralia's, however, large scale mechanisation is necessary. Land preparation and crop cultivation systems involve the simple adoption of machinery already being used in other row crops. Much of the machinery used in sugar cane cultivation is suitable.
Recent areas of major development have been in mechanising planting and harvesting which in the past have been carried out almost entirely by hand.
Planting
Modified sugar cane planters are commercially available and have been used to plant more than 400 hectares of cassava. The machines cut whole stems into 20 cm cut- tings which are planted horizontally. The cuttings are sprayed with fungicide and fertiliser is applied in bands beside the cuttings.
Machines are available as single row models with a minimum row spacing of 1.4 metres and multirow planters with minimum row spacings of 0.9 metres.
Automatic planters, similar to those being adopted within the Queensland sugar cane industry are under development in north Queensland.
Harvesting
Mechanical harvesters have been developed that are capable of lifting cassava tubers from a depth of 35 cm. These machines remove trash and dirt on a series of elevators and deliver roots to an independent trailer moving alongside. Two commercial models are currently available.
Weed Control
Weed control has been found to be a major problem with mechanical cultivation.
They not only reduce cassava yields, but also cause serious difficulties when harvest- ing, presenting a product with unacceptable levels of trash and dirt for processing.
There are obvious limits to the success of mechanical cultivation and chemical weed control measures are necessary.
Alachlor, a pre-emergence herbicide, provides reasonable weed control for up to sixty days, but still allows weed growth to commence before cassava has reached a stage where it can effectively compete. Post-emergence knockdown chemicals can be used, but are expensive. Herbicides that provide long term crop weed control — more than ninety days — have not yet been identified and indicate a major shortfall in research.
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Conclusions
The experimental yields so far obtained with cassava in northern Australia indicate that it could be a potential source of ethanol, particularly on marginal agricultural soils and land areas presently being utilised for grazing purposes. Climatic require- ments, however, may restrict potential production, without considering major irrigation inputs, to areas of east coastal Queensland and parts of the Atherton Tableland.
Before large scale production of cassava can be considered, several areas of research and development would need to be clarified. These are:
• More closely defined climatic limitations, particularly the effects of rainfall and probably more importantly the role of soil-moisture relationship of cassava.
• The demand for fertiliser of cassava on defined soil types.
• Effective economic means of controlling weed growth.
• Detailed assessment of costs of production of units of various sizes. Present esti- mates are forecast with a degree of licence to be between $10 and $15 per tonne fresh weight tubers, indicating gross margins similar to those expected from crops such as peanuts, soya beans and navy beans.
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