Conservation and Use Strategies

Vegetable crop diversification and the place of traditional species in the tropics
Conservation through increased use: Complementary approaches to conserving Africa's traditional vegetables
Documenting Indigenous Knowledge - a challenge for all

Vegetable crop diversification and the place of traditional species in the tropics¹

¹ Dedication. This paper is dedicated to researchers, producers and consumers of African Traditional Vegetables. I thank Dr Franklin W. Martin, formerly of Mayagues Institute of Tropical Agriculture, Puerto Rico, for inspiring me to write it.

Abstract

The nutritional dilemma caused by decreasing food choice and the inadequate prioritization of balanced nutrition is a justification for diversified vegetable crop production. The merits and problems associated with diversification in the tropics are discussed. The importance of traditional vegetable species as a category of secondary food crops is illustrated from the agronomic, economic, nutritional, ecological and technical sides. Reasons for their neglect are discussed, and research needs and approaches are suggested while recognizing the regional specificity of the species and the intrinsic problems associated with the promotion of good nutrition as an incentive in itself.

Crop agriculture and the nutritional paradox

The agricultural revolution that occurred probably 20 000 years ago radically altered human economic systems and nutrition patterns. Ultimately, it permitted the development of urban societies. While plant (and animal) domestication allowed social and technological developments, it also initiated a basic human nutritional paradox (Grivetti et al. 1987). As reliance upon domesticated food increased, dietary diversity diminished. It follows that, as food choice has diminished, the probability that all essential nutrients can be obtained from locally available foods also has diminished. Food trade is supposed to offset this effect. However, trade considerations may dominate food/nutrition issues to the detriment of some, particularly the poor.

The principal effort in agricultural development heretofore has been improvement of productivity. Other rural development aims are directed toward improving the economic livelihood of individual families and regions. Yet economic improvement may not lead to nutritional or health improvement. Increased access to cash, for example, does not necessarily result in better food selection: a positive correlation between income and nutrition may not reflect the qualitative imbalances in the diets of the affluent.

As Martin (1977) has observed, while agriculture was invented as a response to a human need - the need to eat, as a means to an end, where eating well means eating a well-balanced diet - the needs of states have led agriculture to be viewed differently - as a way of earning money or reducing foreign imports. Thus agriculture becomes a means to other ends, and good nutrition is forgotten. Nutritious plant materials are usually rejected unless they appeal to the consumer, whether or not they have nutritional value. No farmer will grow what cannot be sold. Even when examining the cultivation of a new crop such sequences of operations from production, delivery and sale are usually scrutinized in the reverse order - examining first whether there is a market (Wills and Seberry 1990). This is what is termed 'consumer sovereignty'.

Thus the orientation toward the economic aspects in place of the nutritional permits some irrational activities to occur. For example, research resources are always scarce, but they are frequently just as available for vegetables of low as well as of high nutritional value. In fact, nutritional values are often not even considered when resources are allocated.

Further, the highly nutritious vegetables that cannot be sold are likely to be deprecated. For example, leafy vegetables are regarded as animal fodder by many people in certain regions. The orientation toward producing for sale leads to disdain for she who produces to eat, for the latter does not make a positive impact on the Gross National Product (GNP).

Keys to good nutrition can be found among the leafy vegetables. These species are particularly suited to small-scale production, and can easily be made available year round. Thus, with limited resources a vegetable farmer with some knowledge can feed herself well - and that is what agriculture is all about. Unfortunately, however, some of the poorest vegetables nutritionally are also the most popular. This trend should change with better education that could reverse the choice of vegetables in favour of traditional species.

The principle of diversification

Broadly, crop diversification means the cultivation and marketing of new crops as well as 'standard' crops out of the normal season. It is essentially a change from a one-sided production schedule to a broadened diversity in the production system (Woll 1987) aimed at increasing consumption rate by incorporating new products into production and marketing systems (Fritz 1989). Mixed cropping is crop diversification on a single piece of land. Diversification in vegetables can be achieved through:

· Breeding novel varieties from existing ones. Since vegetable crops are variable, specific varieties with desired traits can be developed, e.g. carrots with high carotene for drying.

· Changing the time of supply. Vegetable forcing or hydroponic production may introduce a new commodity otherwise not grown before. For example, cool-season vegetable production in the tropical highlands brings about increased variation in the vegetables available to consumers in the lowlands.

· Utilization of species which were utilized in the past but are now neglected.

· Bringing into cultivation new species which occur in the wild and/or were not consumed before, e.g. maize cobs as 'baby corn', chayote leaves.

Traditional vegetables: an overview

According to FAO (1988), traditional vegetables are all categories of plants whose leaves, fruits or roots are acceptable and used as vegetables by rural and urban communities through custom, habit and tradition. Before the introduction of exotic species, they were widely consumed, particularly during famines or natural disasters. While most of them are gathered when in season or are grown in home gardens as intercrops with staples, they may find their way to urban markets. Accounts of those utilized in Africa have been given by, for example, Okigbo (1977, 1990), FAO (1988) and van Sloten (1984), and a partial list is shown in Table 1.

At first, it may appear a simple matter to distinguish in such lists the wild from the domesticated species, cultivated from uncultivated, dietary from non-dietary. On close examination, however, boundaries are blurred. Numerous wild plants are carefully tended, domesticated species dot long-abandoned human settlements and many medicinal plants are ingested and provide important nutrients in human diets.

Table 1. A partial list of some traditional vegetables in Africa.

Vegetable type	Botanical family
Leafy vegetables
Corchorus spp.	Amaranthaceae
Gynandropsis gynandra	Capparidaceae
Celosia argentea	Amaranthaceae
Hibiscus sabdariffa	Malvaceae
Telfairia occidentalis	Cucurbitaceae
Vernonia anygdalina	Asteraceae
Solanum nigrum	Solanaceae
Cucurbita pepo	Cucurbitaceae
Brassica carinata	Brassicaceae
Basella alba	Basellaceae
Talinum triangulare	Portulacaceae
Moringa oleifera	Moringaceae
Fruit vegetables
Solanum aethiopicum	Solanaceae
Sesbania aegyptiaca	Leguminosae
Cucumis metuliferus	Cucurbitaceae
Sechium edule	Cucurbitaceae
Telfairia pedata (seeds)	Cucurbitaceae
Root and tuber vegetables
Plectranthus esculentus	Lamiaceae
Sphenostylis stenocarpa	Leguminosae
Disa satyria	Orchidaceae
Legume vegetables
Macrotyloma geocarpum	Fabaceae
Mucuna urens	Fabaceae
Sphenostylis stenocarpa	Fabaceae
Vigna subterranea	Fabaceae

Traditional vegetables have important merits, which include the following considerations (FAO 1985; Mnzava 1989, 1993; Braun 1991; Sattaur 1991): nutritional value, ecological value, agronomic value, food security, cultural value and employment opportunities.

Nutritional value

Traditional vegetables are a valuable source of nutrition in rural areas where exotic species are not available, and contribute substantially to protein, mineral and vitamin intake. They are compatible in use with starchy staples and represent a cheap but quality nutrition to the poor sector of the population in both urban and rural areas where malnutrition is widespread. Some of the nutrient values of species commonly utilized in tropical Africa and Asia are shown in Table 2. They provide good nutrition at low cost, in contrast to exotic species. The nutritional importance of traditional vegetables is discussed further below.

Table 2. Nutritional value (per 100-g edible portion) of 24 tropical leafy vegetables. Vegetables are ranked by dry matter content (after Grubben 1977).

Vegetable	DM (%)	Protein (g)	Fibre (g)	Ca (mg)	Fe (mg)	Carboh. (mg)	Vit. C (mg)	ANV† DM
Chinese cabbage	5.8	1.7	0.7	102	2.6	2.3	53	120.5
Lettuce	6.4	1.4	0.6	56	2.1	2.0	17	83.6
Celon spinach	6.4	1.6	0.6	106	1.6	3.5	86	123.4*
Spinach	7.0	2.4	0.7	62	3.9	3.6	56	120.6
Spinach beet	7.4	1.8	0.9	70	2.9	1.7	22	71.8
Waterleaf	7.6	1.9	0.6	90	4.8	3.0	58	108.9*
Mustard	8.2	2.4	1.0	160	2.7	1.8	73	98.3
New Zealand spinach	8.5	2.8	0.8	178	3.8	3.5	27	98.0
Chayote	8.8	4.0	0.8	62	1.4	1.5	24	57.0
Kangkong	10.0	2.7	1.1	60	2.5	2.9	47	75.7
Amaranth	10.7	3.6	1.3	154	2.9	6.5	23	105.3*
Pumpkin	10.8	4.0	2.4	477	0.8	3.6	80	129.4*
Yardlong bean	11.6	4.2	1.7	108	4.7	2.4	35	79.7
Sweet potato	13.3	3.2	1.6	86	4.5	2.7	21	64.5
Roselle	13.6	1.9	1.3	116	1.5	7.6	34	81.2
Cowpea	15.0	4.7	2.0	256	5.7	8.0	56	118.3*
Jute	15.9	5.6	1.7	266	7.7	7.9	53	116.7*
Amaranth	16.0	4.6	1.8	410	8.9	5.7	64	103.3*
African eggplant	17.9	4.8	2.4	523	6.0	6.4	67	109.9*
Taro	18.6	4.1	1.2	162	1.0	5.5	63	60.3
Cassava	19.0	6.9	2.1	144	2.8	8.3	82	87.7
Drumstick	22.4	7.4	1.2	297	3.6	8.9	167	91.7
Sesbania	24.0	8.7	2.2	404	5.0	6.2	58	58.9
Gnetum	24.6	3.8	2.6	128	2.7	3.6	113	50.5
Mean	12.9	3.8	1.3	187	3.7	4.5	57	92.3

^† ANV = Average Nutritive Value/100-g edible portion
= (g protein/5) + g fibre + (mg Ca/100) + (mg Fe/2) + mg carotene + (mg vit C/40)

i.e. if raw food is consumed, vitamin C is calculated as mg Vit C/20
if much oxalic acid is present, Ca is expressed as (mg Ca/200)/200.

* Note the particularly high ANV for some of the African traditional species.

Since exotic temperate-zone vegetables thrive best during the cool season and are predominantly grown in the cooler highland regions in the tropics, they become scarce and therefore expensive during the off-season, i.e. during the hot/wet months. Traditional species, on the other hand, thrive during the hot/wet season, thereby filling the scarcity gap. The occurrence of traditional species in two ecologically contrasted regions in Zambia (Tables 3a, 3b) illustrates the ecological diversity that can exist. Also, many of these species are environment-friendly in that they do not require high inputs of pesticides or fertilizers in their production.

Table 3a. Seasonal availability^† of vegetables in the dry part of Southern Zambia (Kayuni area, Gwembe valley) (after Ogle et al. 1990).

Vegetable

J

F

M

A

M

J

J

A

S

O

N

D

Abelmoschus esculentus

P

A

A

P

Adansonia digitata (lvs)‡

A

A

Alternanthera nodiflova

A

A

A

A

A

A

A

A

A

A

A

Amaranthus spp.

A

A

A

A

Ampelocissus obtusata

A

A

A

Bidens spp.

A

A

A

Brassica carinata

p

P

A

A

A

Brassica napus

P

P

A

A

A

Cassia obtusa

A

A

Ceratotheca sesamoides

A

A

A

A

**

**

**

**

**

**

A

A

Cleome gynandra

A

A

A

P

P

Corchorus olitorius

A

A

A

A

A

A

Cucumeropsis edulis

A

A

A

A

Cucumis spp.

P

A

A

**

**

**

**

**

**

P

Cucurbita pepo (lvs)

P

P

A

A

P

P

Erythrococca menyharthii

A

A

A

Ipomoea batatas (lvs)

P

A

A

A

A

A

A

P

Manihot esculenta (lvs)

A

A

A

A

A

A

A

A

A

A

A

A

Moringa oleifera

A

A

A

A

A

A

A

A

A

A

A

A

Mukia maderastatona

A

A

A

**

**

**

**

**

A

Triptochiton zambesiacus

A

A

A

A

A

A

A

Vigna unguiculata (lvs)

P

P

A

A

A

**

**

P

^† P = planting; A = available fresh; ** = preserved.
^‡ lvs = leaves.

Traditional vegetables are often grown as intercrops with staples in backyard gardens and are less demanding of management, including inputs (Tables 4a, 4b). For example, it is easy to get large quantities of seeds, in contrast to exotic species whose seed, owing to problems in local production, has to be imported, at prices which can be prohibitive for low-income farmers. A comparison of the crop management needs of vegetable amaranth and the head cabbage, for example, both of which are popular in tropical Africa and Asia (Table 5), indicates clearly the advantages of the former over the latter.

Food security

During periods of relish shortage, especially in the dry season (the relish-gap period), traditional vegetables previously preserved by drying become very important in household food security (see Tables 3a, 3b). Their ability to grow quickly and become harvestable within a short period makes them useful in sustaining nutrition-intervention programmes. They offer variety and can contribute to broadening the food base (Okigbo 1977). Being accessible to the low-income communities in rural and urban areas, they offer an opportunity of providing affordable nutrition to avert malnutrition. Their role in food security and alleviation of malnutrition is illustrated in Figure 1.

Table 3b. Seasonal availability^† of vegetables in high-rainfall northern Zambia (Milambo, Luapula Province) (after Ogle et al. 1990).

Vegetable

J

F

M

A

M

J

J

A

S

O

N

D

Abelmoschus esculentus

A

A

A

P

P

Amaranthus spp.

A

A

**

**

**

A

A

Brassica carinata

A

A

A

A

A

P

P

Brassica chinensis‡

P

P

A

A

A

A

Brassica napus‡

A

A

P

P

A

A

A

A

P

Brassica oleracea var. capitata‡

P

P

A

A

A

A

Bidens pilosa

A

A

A

A

Cleome gynandra

A

A

A

A

Corchorus olitorius

A

A

A

A

A

Cucumis anguria

A

A

**

**

**

**

P

P

Cucurbita pepo (lvs)§

A

A

A

A

A

A

A

P

P

Disa spp.

A

A

A

A

A

A

A

Fagara chalydea

A

A

A

A

A

A

A

A

A

Ipomoea batatas (lvs)

A

A

A

A

A

A

A

A

A

P

P

Ipomoea spp.

A

A

A

A

A

A

P

P

A

A

Manihot esculenta (lvs)

A

A

A

A

A

A

A

A

A

A

A

A

Phaseolus vulgaris (lvs)

P

A

A

**

**

**

**

**

P

A

Sesamum calycinum

A

A

A

A

Solanum aethiopicum

A

A

A

P

P

Vigna unguiculata (lvs)

A

A

A

P

P

^† P = planting; A = available fresh; ** = preserved.
^‡ Exotic species.
^§ lvs = leaves.

Vegetable	Seed needed (kg/ha)	Density (plants/ha x 1000)	Duration (days)	Yield (t/ha)
				Low	High	Mean
Amaranthus cruentus	1†	250	20-80	10	40	20
Amaranthus tricolor	2‡	500	39-90	10	40	25
Basella alba	10‡	50	60-180	10	80	20
Beta vulgaris	20‡	1000	40-100	10	40	20
Celosia argentea	1†	250	40-90	10	30	20
Colocasia esculenta	-§	30	60-270	3	9	6
Corchorus olitorius	5†	250	45-80	3	10	8
Hibiscus sabdariffa	6‡	20	120-180	10	25	16
Ipomoea aquatica	5‡	120	60-360	8	120	80
Ipomoea batatas	-§	120	90-180	5	20	15
Lactuca sativa	5	200	30-60	5	30	15
Manihot esculenta	-§	100	60-270	30	90	50
Spinacia oleracea	150‡	1500	45-60	8	25	12
Solanum macrocarpon	1†	90	60-120	15	35	25
Talinum triangulare	5†	250	60-180	10	60	45
Vigna unguiculata	200‡	500	30-60	5	15	10
Xanthosoma sagittifolium	-§	20	60-270	10	30	20

^† transplanted; ^‡ direct sown; ^§ vegetatively propagated.

Vegetable	Mean DM yield (kg/ha)	1000-seed weight (g)	Seed yield (g/plant)	Yield (kg/ha)			Multiplication factor (kg/kg)
				Low	High	Mean
Amaranthus cruentus	3200	0.3	40	800	3000	1500	1500
Amaranthus tricolor	3000	0.7	15	300	800	500	250
Basella alba	4000	40	25	1000	2000	1200	120
Beta vulgaris	1600	16	10	400	800	600	30
Celosia argentea	2400	1	12	200	700	450	450
Colocasia esculenta	1000	-					10
Corchorus olitorius	1600	2	2	200	400	300	60
Hibiscus sabdariffa	2600	25	20	200	600	400	70
Ipomoea aquatica	7000	40	15	100	500	300	60
Ipomoea batatas	2000	0					20
Lactuca sativa	1100	1.1	2	100	400	200	400
Manihot esculenta	8000	-					10
Spinacia oleracea	1000	10	6	300	1200	900	30
Solanum macrocarpon	400	3.3	30	120	300	200	350
Talinum triangulare	3600	0.3	4	100	300	200	400
Vigna unguiculata	1500	250	20	600	3000	1200	6
Xanthosoma sagittifolium	4000	-					10

Table 5. Comparisons between vegetable amaranth and cabbage production under tropical environments.

Aspect	Amaranth	Cabbage
Seed availability	abundant	scarce
Seed requirement	small	higher
Number of crops/year	multiple	few
Crop gestation period	4 weeks	16 weeks
No. of harvests/crop	multiple	single
Nursery practices	none	needed
Soil fertility requirement	medium	high
Pest/disease control requirement	low	high
Soil moisture requirement for optimum yield	medium	high
Temperature requirement for optimum yield	high	low
Average nutritive value	very high	low
Compatibility with existing systems	good	poor

Cultural value

The use of these species is part of the cultural heritage, playing an important role in customs and traditions, and in maintaining equity within the family structure since their appearance on the family table depends largely on the activities of women. If demand for these foods, as well as their status and cash value, can be increased by improved cultural practices and better processing of the products, then the cash income and status of the food producers themselves would also improve accordingly.

Employment opportunities

While multiple land use based on the cultivation of traditional crops can bring about material increase in productivity, such intensive farming systems also have ecological advantages and provide employment. However, producers may not readily increase labour inputs even if labour is available unless they can foresee commensurate returns in the form of an attractive cash income. Traditional vegetables do have considerable potential as cash income earners provided that this potential is recognized and supported by appropriate policies and inputs. Regular cash income generation can, for example, contribute significantly to food security strategies at the household level and enable women to attain a degree of financial independence within the limitation of the family budget.

Fig. 1. Factors related to food security and nutrition and the place of traditional vegetables.

Despite these important merits, traditional vegetable research has been neglected. A number of reasons have been put forward for this (Mnzava 1989, 1993).

· Lack of demand. Changed food habits in favour of introduced 'improved' vegetables have lowered the demand for local species, which fetch low prices in local markets. Whether they are nutritious and may have other virtues, the pecuniary incentive is one of the overriding factors in motivating farmers to produce. While there is increasing demand for 'modern' style foods, traditional species are considered 'out of fashion'. They thus enjoy less social prestige, being associated with the low-income group. As the poor seek to imitate the eating habits of the affluent and are exposed to the more fashionable exotic species, the traditional species become neglected.

· Strongly localized importance. Their use is often community-specific and their occurrence restricted to a particular region and season.

· Urbanization. Dietary changes are more rapid with increased urbanization and enable the diffusion of varied 'ethnic' food habits, but there is abandonment of traditional ways of life in urban centres, including traditional foods.

· Large number of species.

· Their status as weeds or wild species denotes their perpetual abundance with an inexhaustible seed pool. The tendency has therefore not been to conserve them but to eradicate them by weeding and ad lib foraging without deliberate sowing.

· Lack of proper knowledge, especially of their nutritive value, methods of production, preservation and utilization, is an important deterrent to their wider utilization. Information about traditional vegetables is no longer systematically transferred from one generation to the next so that the knowledge gap between the older generation in the rural areas and urban youth in particular is widening. As a consequence of 'modernization', rural life styles are also gradually changing. Further, as agricultural curricula in school and colleges have not addressed local species to any appreciable extent, and the local food service systems do not include local recipes in their menu, general awareness of traditional species is not being emphasized.

Seed supply and production systems

Most traditional vegetable species which occur in the wild, on fallow land and in other crops as weeds perpetuate themselves untended, and their utilization by foraging is an age-old practice. They are harvested wherever they occur, in particular where manure or household refuse has been left in piles near homes, and whenever they are available, which is mainly during the wet season. This system of procurement heavily depends on the soilborne seed pool and the ability of these species to re-seed themselves. Various seed dormancy mechanisms further ensure regeneration when conditions are optimal. Thus during weeding these 'weeds' are harvested and some are selectively protected to provide seed for subsequent volunteers.

A system of autoregeneration with protection has thus emerged in many areas of Africa. It is not uncommon to see a few plants of amaranths or Cleome amidst staple crops intentionally protected to yield seed for their self re-establishment. A conscious effort to protect the species is made while the major crop is given priority and attention.

Certain species are used more frequently and may have greater market demand. Farmers regularly cultivate such species in backyard or market gardens. To obtain a good crop, manuring and watering are done and sowing schedules are timed to provide sufficient quantities for household use as well as for sale. This system of production of species such as vegetable amaranths, Corchorus or African eggplant is in some ways indistinguishable from the cultivation of exotic species, with the exception of pesticide use.

Seed production has virtually remained in the hands of the farmers, although seed sale in markets is common. Recently, some production of traditional vegetable seed has been attempted by seed companies in some countries following a general observation that seed availability was a problem for vegetable farmers. But unless the seed represents an improved selection of some desired species, farmers will continue to prefer using their own seed, which is available free. As would be expected, the simultaneous promotion of consumption with production was necessary to get people to purchase seed. The sale of seed alone may boost production to some extent, whose further increase will be limited by the level of consumption (or purchasing power).

Traditional vegetables in the diet

Although our knowledge of human biology and biochemistry may tell us much of what we need to know about nutritional requirements, it tells us little about why we eat what we eat, and nothing about how to change food habits (Mintz 1989). Anthropologists have remarked that foods must be 'good to think' before they can be 'good to eat', in which case a relationship between bodily needs and their satisfaction with socially learned behaviours (culture) exists.

It has been suggested that three key factors (which may change through time) determine the type of foods eaten: what is available, what can be afforded and what one is used to. A strong emphasis is placed on the last, signifying the importance of culture (Pirie 1981). Associating traditional vegetables with poverty and famine may deter their greater use but since their utilization in rural area begins from childhood, they are already part of the culture in regions where they are consumed and therefore their wider utilization may be determined by other factors.

Dietary habits can be summarized as comprising a carbohydrate complex complemented with a 'flavouring principle' of legume, vegetable or other relish. The 'flavouring principle' forms a kind of set of aggregate tastes and together with the carbohydrate staple, constitutes the meal. The staple is, according to Mintz (1989) the 'core', while the relish is the 'fringe'. The existence of alternative components in the 'fringe' diminishes the significance of the usually cheap vegetable component relative to animal products. This is clearly illustrated in a case study in Zambia (Ogle et al. 1990) in which seasonal variation of composition of the relish reflects substitutions depending on availability (Fig. 2).

Fig. 2. Composition of relish during wet, cool/dry and hot/dry seasons in two areas in rural Zambia. Note the seasonal substitutions (LV = local vegetables). (After Ogle et al. 1990).

It is generally assumed that traditional vegetables are of a 'supplementary' or 'emergency' nature in the diet, implying that they are not eaten regularly or that they are somehow peripheral to the diet as a component of the 'fringe'. However, they determine the intake of the 'core' while they contribute essential nutrients. It may be true that their use as food is occasional, seasonal or ceremonial, but as shown in Tables 3a and 3b and in Figure 1, and as argued by Longhurst and Lipton (1989), these species are not peripheral to the diet, and are important seasonally (and some can be made available year round). They complement energy and nutrients obtained from other domesticated crops and play a role in maintaining nutritional quality of diet during droughts.

Strategies for the promotion of traditional vegetables

A strategy for the promotion of traditional vegetables must include appropriate measures to increase both production and consumption. This is because it would be futile to try to promote consumption without ensuring adequate supply. In the same vein, sufficient availability without market demand will create gluts and thus act as a disincentive to further production. Success of promotion programmes, therefore, will depend on how these factors can be synchronized (FAO 1985).

To achieve this objective, a promotional strategy must include the following:

· Adequate baseline data to quantify the contribution of traditional vegetables in ensuring national food security, to help identify the constraints to their further development, and to forecast future demands and potentials. It should also help to determine the type of policies, programmes and support that would be necessary for promotional activities.

· A strong advocacy to obtain political commitment and government support to long-term programmes of promotion.

· A well-planned research and extension programme and development of appropriate technologies for increasing production, marketing and preservation. High priority should be given to the development of new recipes that will increase market value and competitiveness against exotic species. Recipe-driven increases in demand are exemplified by industrial processing of many vegetable products.

· Measures to increase public awareness on nutritive value, economic benefits and social prestige in the national diet.

Justification for their research: some critiques

On the basis of nutritional function, traditional vegetables are classified as secondary food crops (SFCs) (Tudge 1988). This is also recognized by economists such as Longhurst and Lipton (1989) who stated that:

· They play an important role as supplementary crops resulting in high cross-elasticities of supply with major crops. Their contribution in energy intake from the 'core' is greater in nutritional terms.

· Their income elasticities of demand are less than unity, a characteristic of inferior goods.

· They have a high marginal utility for producers and consumers because they provide income or a source of food at critical periods of seasonal scarcity, primarily for poor households.

· They provide high quantities of micronutrients that are otherwise deficient in the diet. It has been argued whether research resources should be devoted to SFCs for reasons of malnutrition. This depends on the product of three expected values or probabilities: that of success, that of adoption area given success, and that of value-added per unit of adoption area (Longhurst and Lipton 1989). However, success in research may be at high fixed research costs to acquire small amounts of knowledge. As risk-reducing plants, this characteristic may prevent rapid adoption of innovations. They are adapted to heterogeneous environments, and thus chances of making research breakthroughs are small, i.e. if only small areas of any one of numerous vegetables are grown, any advance through research is likely to bring small benefits.

· Their congruence ratio is low, i.e. the ratio of the commodity's proportion of research expenditure to its share of economic value in the crop mix. Congruence ratios of various crops in Africa are as follows: sweet potato 0.19, cassava 0.09, rice 1.05, wheat 1.30 and soybean 23.6 (Longhurst and Lipton 1989). Given the importance of SFCs like sweet potato and cassava, for example, they are receiving less research funding than they deserve, wheat is over-researched and soybean is also receiving more support than it warrants. Because the congruence ratio is based on value of the product in money terms and does not include poverty or seasonal considerations, it may not be an appropriate indicator for the overall decision-making on whether research funding is deserved.

· Their past neglect in research presents greater scope for quick advances. As an example, a cultivar evaluation leading to the identification of a better-tasting and yielding vegetable amaranth or African eggplant may make a noticeable difference in production, as observed in Tanzania and Zambia.

Selection criteria and future strategy: a synthesis

Vegetables fulfil multiple roles in the diet, not just as unique carriers of certain nutrients, but also adding flavour, colour and texture that relieve the monotony of an otherwise bland starchy diet. As a group they provide improved nutrition for people of all economic levels and are important sources of income for small farmers. Vegetable production and marketing, particularly of the traditional species, frequently offer income-generating opportunities for women. Some species of traditional vegetables are fully domesticated, others partially domesticated and others almost untouched. Yet among the partially domesticated and the untouched, there may be some with great potential either with or without further improvement. For those species that cannot play major roles, there may be special niches where they can be useful.

The discovery of appropriate roles for little-known plant species will depend on careful examination of promising types in many localities. For this purpose, comparative evaluation criteria suggested by Arnold et al. (1985) could be applied. These are:

· Nutritional composition: chemical constituents with value >20% of daily requirement for that constituent.

· Domestication potential: taking into account such characteristics as time to first harvest, ease of harvesting, distribution (habitat), ease of propagation, handling and storage qualities.

· Relative yield: size of edible part, number of edible parts per plant.

· Desirability: potential acceptability of plant part as food; palatability or nature of edible part.

Researchers may be ready to redress the biases of national systems against the remote, the minor, the 'inferior' and the seasonal traditional species as the need to provide a variety of nutrient-rich food to an increasing population becomes stronger, particularly in developing countries. Looking for suitable crop species developed elsewhere for introduction to tropical environments is one of the ways to sustain a diversification programme (Villareal and Opena 1976), but the development of traditional species should not be ignored.

References

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Grivetti, L.E., C.J. Frentzel, K.E. Ginesberg, K.L. Howell and B.M. Ogle. 1987. Bush foods and edible weeds of agriculture: Perspectives on dietary use of wild plants in Africa, their role in maintaining human nutritional status and implications for agricultural development. Pp. 51-81 in Health and Disease in Tropical Africa (R. Akhtar, ed.). Harwood Academy Publ., Switzerland.

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Jonson, U. 1985. Causes of hunger. (Cited in Ogle, B. 1988. Nutrition in the Forests. Tree and People Project in Zambia Report. Swedish University of Agricultural Sciences, Uppsala.)

Longhurst, R. and M. Lipton. 1989. The role of agricultural research and secondary food crops in reducing seasonal food variation. Pp. 285-297 in Seasonal Variability in Third World Agriculture: The Consequences for Food Security (D.E. Sahn, ed.). IFPRI, The John Hopkins University Press, Baltimore & London.

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Ogle, B.A., L. Milambo, D.S. Mingochi, A. Nkomesha and I. Malasha. 1990. Traditional Vegetables in Zambia: A Study of Procurement, Marketing and Consumption of Traditional Vegetables in Selected Urban and Rural Areas in Zambia. Rural and Development Studies No. 28. Swedish Univ. of Agric. Sciences, Uppsala.

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Okigbo, B.N. 1990. Vegetables in Tropical Africa. Pp. 29-52 in Vegetable Research and Development in SADCC Countries (R.T. Opena and M.L. Kyomo, eds.). Proceedings of a workshop held at Arusha, Tanzania, 9-13 July 1990. AVRDC Publication No. 90-328. Asian Vegetable Research and Development Centre, Taiwan.

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Conservation through increased use: Complementary approaches to conserving Africa's traditional vegetables

Pablo Eyzaguirre
IPGRI, Rome, Italy

Introduction

Working with countries to develop strategies for the conservation of agricultural biodiversity and plant genetic resources is at the centre of IPGRI's mission. In developing conservation strategies, IPGRI has largely concentrated on ex situ approaches, by helping to establish national and regional genebanks and supporting their activities. More recently, a complementary, in situ approach has been increasingly considered that aims to conserve agricultural biodiversity and crop genetic resources in their habitats - that is, the agro-ecosystems managed by farmers and agrarian communities.

Many of Africa's traditional vegetables, particularly the leafy green vegetables, are weedy, semi-cultivated species, or crops requiring very little in the way of management and inputs. Kitchen and home gardens, fallows, watercourses, field margins and disturbed areas along the rows and watercourses are typical of the kinds of sites where these species are found. They thus occupy isolated, hidden, fragmented, ephemeral micro-environments. They are often managed, used and sold by women and in some cases by children. For these and other reasons they have been neglected by the formal scientific and development system, though they are important in local production and consumption systems.

IPGRI's interest in neglected crops in general and Africa's traditional vegetables in particular is leading to the crystallization of a third complementary element of an overall conservation strategy, namely conservation through promotion and increased use. This approach focuses on the production and consumption systems for traditional crops. It looks at the agronomic, economic and cultural factors that constrain the expansion and competitiveness of these crops, and tries to identify ways of overcoming these barriers to ensure that useful genetic diversity is not lost forever.

Traditional vegetables in Africa have suffered from neglect by formal-sector agricultural and conservation institutions. The social and environmental conditions where they are grown have led to their relative neglect by development agencies as well. Although the consumption and use of traditional vegetables is still firmly rooted in the practices and knowledge systems of Africa's rural peoples, the rapid pace of social and cultural change suggests that the current neglect may soon translate into disuse and the eventual genetic erosion and loss of these vital nutritional and economic resources. The effect on women and the poor in both rural and urban areas is likely to be great. Strategies are needed for the conservation of these genetic resources, strategies that are unlikely to be effective if based on a single approach. This paper will explore some of the different, complementary options that are open for the conservation of the biodiversity of this important group of plants.

Ex situ conservation

A major institutional line of defense against the genetic erosion of Africa's most important genetic resources has been the system of national genebanks for ex situ conservation of crop genetic resources. In Kenya, for example, the National Gene Bank has been a model for the maintenance and management of collections of the major crops grown in the country. It also has a mandate for the conservation of useful wild species that are indigenous to Kenya. One of the problems with relying entirely on an ex situ genebank for the conservation of a group of species such as Africa's traditional vegetables is the sheer number of taxa involved. The Indigenous Food Plants Database of the National Museums of Kenya lists 69 species used as leafy vegetables in perhaps 25 families, ranging from annual herbs to trees. Many of these species are poorly known taxonomically, let alone in terms of their genetic diversity and reproductive biology. It is unreasonable to expect a national genebank to adequately collect, characterize, evaluate, store and multiply sufficient material of all these species to ensure their ex situ conservation and subsequent use. Different, complementary approaches are needed.

In situ conservation

Recent initiatives at the global, national and local levels have led to an increasing focus on in situ conservation, which is concerned with maintaining populations in the habitats in which they occur. The United Nations Convention on Biological Diversity (CBD) describes in situ conservation as: "the conservation of ecosystems and natural habitats and the maintenance and recovery of viable populations of species in their natural surroundings and, in the case of domesticated or cultivated species, in the surroundings where they have developed their distinctive properties." Conservation of habitats and ecosystems in nature reserves, national parks and the like is an important means for conserving wild plant species. In situ conservation of cultivated species is primarily concerned with the maintenance of traditional crop varieties (or 'landraces') in the fields and home gardens of local subsistence farmers - so-called 'on-farm conservation'. Active participation by farmers and other users of genetic resources is essential for in situ conservation of cultivated species.

Farmer participation in maintaining weedy species, including many of the traditional vegetables, is going to be different from what would be needed for in situ conservation of crop landraces. Since these vegetables often require very little in the way of farmer management and inputs, and because they occupy patchy and often ephemeral niches - such as the boundaries between farm and bush, or between field and fallow, or between crop rotations - it is difficult to demarcate specific areas and declare them to be in situ conservation sites.

In situ conservation of patches, boundaries and even home gardens poses an interesting problem for the existing set of plant genetic resources conservation approaches. Conservation of traditional vegetables needs to focus not only on the habitats, but, more importantly, on the entire production and consumption system of these useful species. It is in production, marketing and consumption that the constraints to their continuing or expanded use are most apparent. This new conservation strategy is fundamentally concerned with identifying and overcoming these constraints to extend the use of traditional vegetables as a means of conserving them. We can call it conservation of plant genetic resources through promotion and improvement for greater use.

Conservation through promotion

Conservation of traditional vegetables in Africa through promotion and improvement entails identifying the constraints or factors that are responsible for their decline, limited use or confinement to restricted locales. These factors can be biological, cultural, or pertain to the production/use system. The mix and relative importance of each factor will of course depend upon the species and its uses.

Biological constraints

It is imperative to improve our knowledge of these species; they are numerous and often poorly known scientifically. Among the inputs that are needed are floristic studies to improve our basic knowledge of diversity at the species level and documentation of genetic variation as a basis for selection and improvement. However, it is important to recognize that despite scientific neglect, much information is often available on these species, in the form of the knowledge of the local people in the communities where they grow and are consumed. An effective conservation strategy will build on this indigenous knowledge.

Cultural constraints

Understanding gender and human cultural diversity is essential for understanding the uses of plant diversity at the species and genetic level. Participatory research methods are the most appropriate for this type of research. Cultural changes associated with development may have the unfortunate consequence of associating introduced foods with higher status and 'modernity' at the expense of the better-adapted, less costly and in many cases more nutritious local vegetables. Cultural programmes that increase people's awareness of the benefits and value of traditional foods are also components of conservation.

Production/use constraints

Several aspects of the production and use system of traditional vegetables need to be researched and documented to overcome constraints to the increased use of these traditional vegetables and to assess their conservation status. Where the species concerned are wild and semi-wild, an important possibility is to consider whether domestication could be a viable option. Knowledge of the genetic diversity of particular species gained through formal research and ethnobotany could be useful in improving the selection and breeding strategies of local producers. Post-harvest techniques (including storage and processing) need to be conserved, improved and diffused to aid in the consumption of these species. A combination of research and development interventions to support marketing systems could make species of traditional vegetables more appropriate to the eating habits of urban populations and could create and satisfy a broader consumer demand.

Conservation approaches and techniques

The need for practical participatory and development-oriented research on Africa's traditional vegetables can forestall the loss of biodiversity which results when the use of these species declines because of their decreasing competitiveness, cultural changes and other factors. The research entailed would be aimed at identifying the components that can be combined into a complementary conservation strategy. The following elements could all be part of such a conservation strategy:

· Community genebanks: in areas and communities where a given species or group of species is particularly important.

· National genebanks: to characterize and evaluate germplasm, including validation of IK (Indigenous Knowledge).

· Community organizations, NGOs, genebanks: exchange of species and varieties among communities and countries to increase the genetic diversity available for local selection and adaptation.

· Local producer and marketing organizations, extension and development projects: improved production, processing and marketing systems, to enable traditional vegetables to compete in urban areas.

Documenting Indige