The Sahelian and Sudano-Sahelian zones of Africa have suffered extensive land degradation due to over-exploitation of native vegetation and recurrent drought. Many of the peoples living in these areas are chronically undernourished and exposed to famine. Certain Australian tropical dry-zone acacias have a considerable, essentially untapped, potential for production of human food and fuelwood in these zones. These species, including Acacia colei, A. elachantha (ms) and A. tumida were successfully introduced into West Africa in the early 1970's, but it is only in recent years that their potential development as new food sources in Africa has received attention and become the focus of necessary research and development work. This report provides updated information concerning the taxonomy and performance of priority species and outlines some of the various studies in progress, including human nutrition trials, genetic and silvicultural research. Recommendations are made on future research priorities and the need to make information and seed of these Acacia species more widely available for evaluation in dry tropical Africa.
Despite considerable progress in increasing world food production, more than 800 million people in developing countries remain undernourished (FAO 1996). The problem is most acute in sub-Saharan Africa where in 17 countries 40-60% of the population do not have adequate food and where the number of undernourished is projected to increase by about 100 million to over 300 million by the year 2010 (FAO 1995). A multipronged approach including raising food production, monitoring population growth and a more equitable distribution of food is needed if widespread food shortage and famine is to be avoided. An essential component for ensuring national food security in sub-Saharan African countries is to increase local food production capacity, but this will need to be achieved in an environmentally sustainable way. Under-utilised and new food plants, especially drought-tolerant trees and other woody perennials, have the potential to make an important direct contribution to food production and security. Trees and forests also contribute indirectly to food security through providing a wide range of environmental benefits, such as soil protection and amelioration (e.g.N-fixing species), and through positive socio-economic linkages, such as providing sources of income and employment (FAO 1989). A recent report commissioned by FAO has highlighted the great importance of Acacia species for a whole range of purposes, including human food and fodder for livestock, for peoples living in dry Africa (Wickens et al. 1995). The present report focuses on the potential of certain Australian acacias to contribute to food production and security in sub-Saharan Africa.
Australian dry-zone acacias - introduction into West Africa and development as food sources
In the early 1970's a large number of Australian Acacia species were introduced into dry tropical parts of West Africa, and extensively evaluated in species, provenance and other trials (Cossalter 1987). A number of species, including Acacia colei Maslin & Thomson (under the name A. holosericea), A. elachantha (manuscript name) (under the name A. cowleana Tate), A. bivenosa DC, A. coriacea DC and A. tumida F. Muell. Ex Benth., proved to be well adapted to Sahelian conditions and suitable for providing fuelwood, soil protection and low windbreak function. The seeds of these and some 50 other Australian Acacia species have been used as seasonal foods by Australian Aboriginal people (Latz 1995). During 1989 a team of East African and Australian scientists visited West Africa for the purpose of exchanging information and ideas on the potential and research needs for Australian dry-zone acacias in dry, tropical Africa (Thomson 1989). It was evident during this visit that certain species, especially A. colei, A. elachantha (ms) and A. tumida, produced heavy seed crops commencing from a young age (<18 months) and had potential to make a contribution to the diets of people living in the Sahelian and Sudano-Sahelian zones.
Since 1990 the Maradi Integrated Rural Development Project (MIDP; based in Maradi, southern Niger) led by the non-governmental organization SIM International, has explored the potential use of Australian acacias as a new food in the region. During April-May 1990, seed of A. colei was collected from local, Government-established windbreak plantings in order to test this species' potential and acceptance as food. Various traditional dishes were made using the acacia seeds, both on their own and in blends with other grains and pulses with highly satisfactory results (T. Rinaudo, observations.). No phase of seed preparation (harvesting, threshing, cleaning, grinding or cooking) required new technology or special skills, and those who tasted the foods found them agreeable. Encouraged by these reports, the Australian Tree Seed Centre (ATSC, CSIRO Forestry & Forest Products) organised a workshop in Glen Helen, Australia to examine the idea of further developing the food value of the seed of Australia's dry zone acacias (House and Harwood 1992). This workshop generated considerable interest in the subject and served as a catalyst for a number of important research efforts and ongoing projects including:
1. Genetic Investigations - Development of DNA probes and studies of population genetics (in progress - P. Butcher and G. Moran, CSIRO Forestry & Forest Products).
2. Taxonomic studies - Collaborative studies between the Western Australian Herbarium and ATSC (Maslin and Thomson 1992; Maslin and McDonald 1996; McDonald and Maslin 1996, in press and McDonald and Maslin 1996, in prep.).
3. Field trials in Niger - Trials to assess seed yields in Niger - collaborative studies between MIDP and ATSC (Rinaudo, Burt and Harwood 1995; Rinaudo 1996).
4. Replicated field trials - Trials of provenances and progenies of a number of dry zone Acacia species have been established in Western Australia and the Northern Territory of Australia, by ATSC in collaboration with the Western Australian Department of Conservation and Land Management, the Australian Nature Conservation Agency and local Aboriginal communities to compare growth performance, seed production and taxonomic features of a wide range of genetic material.
5. Detailed analyses of the chemical constituents of Australian acacia seeds, especially examining possible toxic and anti-nutritional components (Harwood, 1994). No toxic or anti-nutritional components have been found which might impair their use as human foods. Acacia colei seeds contain about 23% crude protein, 57% carbohydrate and 8% fat (Harwood 1994, Adewusi et al. 1997, submitted). Some of the nitrogen is non-protein, and over half of the carbohydrate is in the form of dietary fibre. The amino acid profile indicates that tryptophan, methionine and cysteine are the limiting amino acids. Clearly, acacia seed flour is not a complete food, and should supplement rather than replace existing diets.
6. Animal feeding trials. In a series of trials commencing in 1993, carried out by Adewusi and co-workers at Obafemi Awolowo University, Nigeria, laboratory rats have been fed diets incorporating up to 40% acacia seed flour. Safety trials, trials to test complementarity of acacia flour with millet and sorghum feeds, and reproductive performance trials have now been completed and results will shortly be published (Adewusi and others, in prep.). In summary, animals have fared well on diets incorporating up to 25% acacia flour, while in long-term trials incorporating 40% acacia flour as the only protein source, amino acid imbalance causes health problems in the animals (Adewusi and others, in prep.)
7. Human dietary trials in Niger. A dietary trial has been conducted by Adewusi and co-workers, with support and assistance from CSIRO and MIDP, and ethical clearance from the Niger Government. Volunteers consumed diets incorporating 0%, 15% and 25% Acacia colei seed flour for a three-week intensively monitored trial period. Detailed analysis of blood and urine samples has not been completed, but no adverse health effects were evident and the acacia-incorporated foods were fully acceptable to the volunteers (Adewusi and others, in prep.)
The interest in the human food potential of groups of allied taxa in the Acacia Section Juliflorae from the Australian dry-zone, has provided a stimulus to re-appraise their taxonomy and more fully assess their genetic resources:
The major Australian Acacia species planted in West Africa, viz. A. holosericea sens. lat., has been shown to comprise at least three separate taxa. These taxa differ in many ways including ploidy level, isozyme patterns, morphologically, as well as in their ecological preferences and field performance (Maslin and Thomson 1992) and have been recognised as separate species, viz A. neurocarpa A. Cunn ex. Hook. (diploid), A. holosericea A. Cunn. Ex Don (tetraploid) and A. colei Maslin & Thomson (hexaploid) The most successful and by far the most widely planted of these three species in West Africa is A. colei. At the time of its description, A. colei was believed to have only curved pods, but more recent field research has identified a less common form, found in the southern Kimberley and Pilbara regions in Western Australia, which has tightly coiled pods similar to the pods of A. holosericea or A. neurocarpa. This taxon is to be formally described as a variety of A. colei (A. colei var. ileocarpa manuscript name, McDonald and Maslin, in prep.). Both varieties are well-adapted to Sahelian conditions in West Africa.
Of the Australian Acacia species being tested around Maradi, Niger A. colei has proven to be the most suitable for human food production. A. colei is relatively easy to establish, is very fast growing and has greater longevity than other species in the trials. It survives and produces useful seed crops in adverse conditions. Yields of up to 6 kg of seed/plant (average yield: 1.8 kg per seeding plant) from 19 and 32 month old trees (wide spacing and rainfall of 400 mm) indicate that planting of A. colei for human food production is a potentially worthwhile activity. This is especially so in light of the highly variable harvests of the staple food crop, millet. At lower rainfall (296 mm), 20 month old plants yielded up to 3.5 kg of seed (with an average of 0.8 kg per seeding plant; 6m spacing within rows, 25m between rows).
Around Maradi farmers have shown a clear preference for the coiled-pod A. colei var. ileocarpa (ms). This form has good harvesting characteristics: all pods mature within a short period of time, and pods have a low level of shattering, the seeds being held in the pods after ripening. The compact clusters of pods are easy to pick without spillage, and although separating the seeds from the pods requires pounding in mortar and pestle, only a light pounding is necessary.
There is considerable morphological variation within A. colei var. ileocarpa (ms) (McDonald and Maslin, in prep.) but little is known of variation between provenances in field performance. However typical A. colei var. colei, has considerable provenance variation in field performance and apparent provenance by environment interaction (Souvannavong and de Framond 1992). These findings indicate the importance of making available an array of adapted genetic material, especially coiled-pod types but also better-performing provenances of the curved-pod type, such as Hooker Creek (Northern Territory) and Carranya (Western Australia), to those interested in testing or cultivating the species for human food.
The following are the current recommendations being made to farmers around Maradi. For best seed yields plant coiled-pod A. colei at a wide spacing (10m x 10m). In areas with less than 300 mm of rainfall, planting should be restricted to sites receiving supplementary water (run-on sites and drainage lines). In order to reduce wind throw and other damage from wind it is recommended that older plants (> 33 months) be pruned at breast height after seed harvest, and also when loss of vigour or die-back occurs. Cultivate around the base of trees regularly and, apply fertiliser, if available, in the rainy season.
Acacia elachantha (ms)
McDonald and Maslin (1996, in press) have revised the taxonomy of Acacia cowleana and A. oligophleba Pedley. The species which has been widely planted under the name of A. cowleana in Africa and elsewhere, has been named as A. elachantha (ms). A study of 15 populations of A. elachantha (ms) (under the name A. cowleana) revealed almost no isozyme differentiation either within or between populations (Moran et al. 1992). However, variation in morphological traits and field performance may be considerable, including pronounced provenance by environment interaction (Thomson 1992). The more robust, related species which had been generally planted in trials under the name of A. oligophleba, is referable to A. cowleana.
Around Maradi, Niger A. elachantha (ms) has shown promise with seed yields on good sites in good rainfall years being equal to those of A. colei. However, compared with A. colei, establishment and survival rates for A. elachantha (ms) are lower and seed yields reduced more by lower rainfall and on poorer sites. A. elachantha (ms) displays poorer harvesting characteristics: different pods on a single tree ripen over a long period of time, requiring frequent harvesting of small amounts of seed, and seeds are easily lost from the pods both on the tree and when being picked. This contrasts with seed collections from natural populations in Australia in which plants usually produce prolific seed crops which mature synchronously. Also the seeds dangle from, but adhere to, the pods via their oily aril and very few seeds are lost during harvesting.
Acacia thomsonii Maslin & McDonald
Acacia thomsonii is a recently described species with a discontinuous range within the tropical dry zone of Australia, extending from northeast Western Australia, through Northern Territory to northwest Queensland (Maslin and McDonald 1996). It grows on dissected plateaux, rocky low hills, on diffuse drainage lines (often actively scoured) on low, rocky hills and on stony or sandy plains. The soils are almost invariably skeletal and acidic (pH 5.0-6.0). In the past A. thomsonii was confused with A. elachantha (ms) but is now considered most closely allied to A. colei. The species is generally quite morphologically invariant over its wide geographic range (Maslin and McDonald 1996). A. thomsonii (referred to as A. sp. aff. cowleana) has been noted as having unique attributes for human food production due its high fecundity and ecological preference for harsh, stony sites (Thomson 1992). However, A. thomsonii has not performed as well as A. colei on most sites near Maradi, Niger and has similar poor harvesting characteristics to A. elachantha (ms).
Acacia tumida complex
Acacia tumida has high potential for human food production in dry, tropical Africa (Thomson 1992), although the thick seed coat and associated high levels of dietary fibre (over 50%) dilute the nutrient levels of the whole seed (Harwood 1994). The species includes very large provenance variation for economically important attributes, and as currently circumscribed would appear to include at least two different taxa. There is an urgent need for genetic studies and taxonomic revision of the species complex: this should include both field work and molecular studies (isozyme or DNA analysis). A. tumida has not performed well in trials at Maradi, Niger. Although survival and growth of some provenances has been good and heavy flowering has occurred, seed set has been poor. In Senegal, survival was enhanced in successive generations following initial introduction of the species, perhaps indicating rapid development of a land race of A. tumida better adapted to Sahelian conditions.
1. Exploration, taxonomy and genetic variation
There is a need for genetic studies and taxonomic revision of A. tumida, studies the relationship of the curved and coiled-pod varieties of A. colei; and the relationships of morphological variants in A. elachantha (ms). These studies should include field work at time of flowering and fruiting, molecular studies (isozyme or DNA analysis) and assessment of ploidy level.
2. Evaluation of seed production in different environments/seasons and under different forms of management
There is a need to more accurately quantify seed yields of promising Acacia species/ provenances in different environments selected to cover the major soil and rainfall regimes present in the Sahelian and Sudano-Sahelian zones. This should especially include seed production levels for plants established at wide spacings, on non-arable or marginal crop lands (especially sandy and rocky sites), around villages and in years of below-average rainfall. The development of suitable agroforestry layouts, e.g. an alley cropping design suitable for the semi-arid tropics, for incorporating acacias into millet culture, is another area in need of investigation.
In particular a more detailed assessment of the silviculture and management of the coiled-pod types of A. colei for human food production is needed. Research is also needed on the effect of rhizobia and fertilizers on growth and seed production, and the effect of different pruning regimes on longevity, biomass and seed production.
3. Development of seedling propagation techniques and direct seeding.
A constraint to planting trees in West Africa is the cost of seedling production: this will be especially the case for relatively short-lived poorly-coppicing species which may require replacement every 8-10 years. More research is needed to develop low-cost, appropriate technologies for seedling establishment, including village nurseries (using locally available materials) and to evaluate potential for transplanting bare-root seedlings. Direct seeding has been successful in trials using in A. colei in Senegal and in northern Nigeria, but has been to date unsuccessful near Maradi, Niger. Factors which have contributed to the failure of direct seeding around Maradi include lack of rain following sowing; grasshopper attack; burial by wind-blown sand and competition from weeds.
More research is needed to define the conditions under which direct seeding is likely to be successful, and how it should be done.
4. Weediness potential
The Australian acacia species display little ability to regenerate naturally in the West African Sahelian zone (mean annual rainfall 300-600 mm) but in higher-rainfall tropical environments such as Sabah, Malaysia and coastal Tamil Nadu, India, A. holosericea and A. colei bear heavy seed crops, reproduce freely from shed seed and have the potential to establish and become weeds on open land that is not regularly cultivated. Care should be taken to evaluate weediness potential whenever these species are introduced to a new environment (Hughes and Styles 1987).
5. Breeding systems and reproductive biology
Isolated individuals of A. colei can produce very heavy seed crops, but further research is needed to determine the breeding system of this and other related species. It is possible that apomixis in involved (see Moran et al. 1992) and this would have major implications for selection and breeding of A. colei. Factors contributing to the low seed set of A. tumida in Niger need to be investigated.
The acacias with human food potential can be seen as a back-up, a potentially significant component to regional food security in the Sahel, rather than as a new "miracle" crop. Acacias have limitations, and like all crops require care for optimum yields. Proper espacement and weeding/cultivation are critical. Accordingly there is a need to make both information and seed of these promising new food sources for sub-Saharan Africa more widely available. Greater involvement of development assistance agencies, NGOs and others concerned with issues of sustainable development and food security should be sought. It is suggested that FAO (technical assistance), ATSC (technical assistance, research and seed), CILSS and MIDP/SIM International (extension and development) can provide important foci for such action which could include:
1. Production of a manual describing the cultivation and use for food of Australian dry-zone acacias (initially in English, French, Hausa and Swahili).
2. Development of a kit, including the manual plus semi-bulk quantities of seed of the most promising species for particular agro-ecological zones, for distribution to interested projects and groups in Africa.
3. Arranging visits of interested parties to MIDP to observe first-hand the utilisation of Australian acacias for human food in the Sahel.
This work has been built on the foundation of traditional knowledge developed by Australian Aboriginal people, who have freely shared this knowledge with ethnobotanists and other scientific researchers. Funding support for much of the work described above has been provided by the Australian Centre for International Agricultural Research and the Australian Agency for International Development. FAO has supported species and provenance collections in Australia. SIM International has also provided substantial support. The Australian Nature Conservation Agency, the Western Australian Department of Conservation and Land Management and several local Aboriginal communities have supported field trials in Australia.
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