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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 5, Num. 2, 1997, pp. 209-217
African Crop Science Journal
Vol.5. No.2, pp.209-217, 1997

- FORUM -

Strategies and approaches to mangrove swamp rice varietal improvement in west Africa

R.G. Guei, C.A. Dixon and M.A. Sampong

WARDA 01 B.P. 2551 Bouake 01, Cote d'Ivoire

(Received 26 February, 1997; accepted 17 June, 1997)


Code Number: CS97027
Sizes of Files:
    Text: 40K
    Graphics: Line drawings (gif) - 16K  

ABSTRACT

West African Mangrove swamps are located on the flood plains of rivers and creeks and along the Atlantic coast. The environment is highly variable in terms of rainfall, soil and environmental stresses. In 1976, the West Africa Rice Development Association (WARDA) initiated a mangrove swamp rice varietal improvement programme. Although some progress has been achieved, it is necessary to put in place strategies and approaches to consolidate these results. Any varietal improvement programme for mangrove swamps in West Africa should distinguish two classes of swamps: high rainfall belt or wet mangrove swamps and low rainfall belt or dry mangrove swamps. In high rainfall zones (Guinea, Sierra Leone, Nigeria) selection of short, medium, and long duration varieties of good grain quality should be emphasised. In dry zones (Senegal, Gambia, northern Guinea-Bissau), however, selecting short duration varieties, with multiple stress tolerance ( salinity, sulphate acidity, drought and blast) will be essential. Environmental as well as socio-economic characterisation is important and should be undertaken on country basis. A lot of information could be obtained from a systematic collection of all local and improved varieties grown by farmers in the region through national research programmes. Characterising these varieties will help define farmers' preferences in terms of plant type, growth duration, environmental stress tolerance, as well as grain and cooking quality.

Key Words: Acidity, blast, drought, mangrove, salinity, swamp

RESUME

Les marais de mangrove en Afrique de l'Ouest sont situes dans des plaines inondees de fleuves et de rivieres tout au long de la c“te atlantique. Ce milieu est tres variable en terme de pluviometrie, sol et condition du milieu. En 1976, l'Association pour le Developpement de la Riziculture en Afrique de l'Ouest (A.D.R.A.O.) avait initie un programme d'amelioration du riz de mangrove. Bien qu'un progres certain aie ete realise, il convient de mettre en place une approche strategique de l'amelioration varietale afin de consolider ces acquis. Pour cela, tout programme d'amelioration varietale doit distinguer deux classes de marais de mangrove: marais de zones seches et marais de zones humides. Ainsi les objectifs de selection doivent differer selon les zones. Dans les zones humides (Guinee, Sierra Leone, Nigeria), les varietes de cycle court, moyen ou long de bonne qualite seront mieux adaptees. Les cycles courts doivent avoir une certaine resistance a la salinite tandis que dans les zones seches (Senegal, Gambie), toutes les varietes doivent etre de cycle court, resistantes a la salinite, aux conditions sulfate-acides, a la pyriculariose, a la secheresse et etre de bonne qualite. La caracterisation du milieu et des conditions socio-economiques au niveau de chaque pays est importante. Beaucoup d'informations peuvent etre obtenues en faisant une collection systematique de toutes les varietes locales et ameliorees cultivees par les paysans dans la sous-region et par le biais des programmes nationaux de recherche. La caracterisation de ces varietes aidera a definir la preference des paysans en matiere de type de plante, cycle, tolerance aux contraintes du milieu et de la qualite de graine.

Mots Cles: Acidite, pyriculariose, mangrove, marais, salinite, secheresse

INTRODUCTION

West African mangrove swamps are located on the flood plains of rivers and creeks and along the atlantic coast. They cover the dry tropical and humid tropical zones of the region between latitude 5 degrees N and 20 degrees N. The environment is highly variable in terms of quantity and distribution of rainfall and temperature which has resulted in a wide variability in flooding, salinity, acid sulphate conditions and soil development. The individual effect of these factors or their interaction greatly affect rice production in the region, necessitating use of adaptable rice cultivars and cultural practices for sustainable productivity.

The West Africa Rice Development Association (WARDA)'s regional mangrove swamp rice varietal improvement programme started in 1976. More than 6000 improved varieties have been introduced principally from Asia. These varieties have been screened for adaptation to the West African environmental conditions, and the best have been used either directly as varieties or as breeding lines. From 1980 to 1987, more than 300 crosses were made. The most popular WARDA varieties in the region are WAR 77-3-2-2 derived from a cross between an IRRI variety IR4595-4-5 and a local variety tolerant to salinity, PaMerr 108A; Kuatik Kundur a variety introduced from Sri Lanka; Rohyb 6-WAR6-2-b-2 a variety selected by WARDA from crosses made by the Sierra Leone National Research Station in Rokupr; WAR1, derived from a cross between CP4 an Asian variety and IR 442 an IRRI variety.

Although many varieties have been introduced or developed by WARDA, only a few have reached farmers in the region. Several reasons could be given among which are: (1) The non adaptation of most of introduced materials to the West African mangrove ecology; (2) Over emphasis on breeding for yield potential and stability while little consideration is given to other varietal characteristics of interest to farmers such as grain quality and cooking quality; (3) Limited knowledge of the mangrove environments as it can vary from one country to another, and from site to site along the same river; (4) Shortage of well trained rice scientists in national research institutions to effectively carry out work generated at WARDA; and (5) Poor or nonexisting extension service which is often linked with poor agricultural policy of the local government. Lately, considerable effort has been made by the WARDA Mangrove Swamp Network Project through varietal distribution to National Agricultural Systems (NARS) and farmers (Table 1). To sustain this, we need a better knowledge and understanding of the mangrove environments and a strategic approach to mangrove swamp rice varietal improvement. This is to provide farmers with a range of varieties capable of significantly out-yielding local varieties in stress environments (salinity, acid sulphate conditions), and of appropriate growth duration, panicle size, height, resistance to lodging, with good grain quality (long and slender or short and round grain, tasty and with good cooking quality).

TABLE 1. Mean grain yields (t ha^-1) and percent gain of WARDA improved varieties over the local checks in on-farm trials conducted in the region

---------------------------------------------------------------------------
Variety    Guinea       Nigeria    Sierra  Leone    Senegal      Gambia 
       ------------  ------------  ------------  ------------  ------------
       Yield  %gain  Yield  %gain  Yield  %gain  Yield  %gain  Yield  %gain
---------------------------------------------------------------------------
WAR 77   2.5   37     2.3    44     3.3    15     2.0    18     3.9    58
WAR 1     -     -     2.3    44     2.6    22     2.0    16     2.5    35
ROK5     2.7   27     1.3    15     4.2    30     1.7    10     3.4    52
ROHYB 6  3.0   32     1.3    15     3.0    17      -      -      -      -
KUATIK
KUNDUR   3.4   41     2.0    33     3.4    26      -      -      -      -
---------------------------------------------------------------------------

Description of the mangrove environments.

Mangrove swamps are strongly affected by tidal movement of sea water. The prevalent vegetation consists principally of Rhizophora racemosa, Rhizophora mangle, Rhizophora barisoni and Avicennia nitida. Avicennia sp. are prevalent in highly saline conditions along the coast while Rhizophora sp. are predominant further inland along rivers and creeks. Upon empoldering and drying, Rhizophora-dominated fields tend to become too acid for rice. The soils from these fields contain twice as much phosphorus, calcium to nitrogen (C/N) ratio and higher water-holding capacity than Avicennia-dominated soils. According to Hesse and Jeffrey (1963), Jordan (1964), and Dent (1986), the mangrove swamp soils are rich in pyrites and therefore potentially acid sulphate soils or cat-clays with weakly differentiated horizons. Once cleared of the original mangrove vegetation, the salt tolerant grass Paspalum virginatum become the predominant vegetation.

Classes of mangrove swamps.

Mangrove swamps have been classified according to their direct exposure or not to the tidal movement of sea water. Whether swamps are exposed to the tidal movement of sea water or not influences soil characteristics, the severity of soil stresses (salinity, acidity or iron toxicity), soil fertility, and type of weeds. Different technology packages including varieties, soil and water management have been developed to fit the different classes of mangrove swamps. Although no extensive work has been reported on the classification of the mangrove, basically two classes of mangrove swamps are recognised depending on the "salt free" period and flooding conditions: the Tidal mangrove swamps and the Associated mangrove swamps.

Tidal mangrove swamps.

In this class, three categories of swamps are recognised according to the variation in salinity and flood levels.

Category 1: Mangrove areas which experience tidal movement of sea water twice daily, and, according to the amount of fresh water available in the season, are flooded with low salt content water for less than four months in a year. The length of this "salt-free" period is determined by the interplay of the volume of fresh water available (rainfall) and salt water intrusion from the sea. These swamps are located nearer to the sea coast. Traditionally short duration (less than 4 months) varieties are grown in these areas to escape salinity stress.

Category 2: Mangrove areas under tidal flow but the "salt-free" period can last longer, i.e., from four to six months. In these areas, medium duration (4 to 6 months) varieties are usually grown.

Category 3: Areas which are subjected to tidal flow for most of the year, but with a "salt-free" period of more than 6 months. The areas are located further away from the coast, therefore, receive larger quantity of fresh water both as direct rainfall and as river floods. Long duration varieties (more than 6 months) are grown.

Associated mangrove swamps.

These swamps are areas adjoining tidally affected swamps. They are located between the tidal swamps and the upland (Agyen-Sampong et al., 1986). They are not subject to tidal floods but get submerged with up to 20cm depth of water received as direct rainfall, runoff and seepage from adjoining uplands as well as through the upwelling of the ground water table. They are characterised by excessive grass and sedge weeds with fewer broad leaved ones (Agyen-Sampong et al., 1986). They are often taken for inland valley swamps.

This classification is basically adopted for varietal improvement purpose in high rainfall zone mangrove swamps where empoldering is not used as a cultural practice to avoid sea water intrusion in the field. In some countries where bands can be constructed by farmers around rice fields (e.g. Guinea), three classes of mangrove swamps are often used by local scientists:

Class 1: Open Mangrove Swamps. They are the same as Tidal Mangrove Swamps defined above.

Class 2: Semi-open Mangrove Swamps. They are the same type of swamps as above but they are not directly exposed to the tidal movement of sea water. Farmers construct polders between the rice field and the saline water source (sea or river) with timely opening of the bonds to control salt intrusion and acidification. This cultural practice is also well known to farmers in Guinea-Bissau where the technology originated from some one hundred years ago.

Class 3: Closed Mangrove Swamps. They are the same as Associated Mangrove Swamps. In the Sahel regions, however, because of the limitation in rainfall which increases soil stresses, all rice fields are empoldered.

Thus, the classification of mangrove swamps based on direct exposure to the tidal movement of sea water becomes not applicable to all mangrove ecologies. In addition, short duration varieties are basically grown in the sahel because only a "salt-free" period of less than 4 months is available. Furthermore, the Associated mangrove swamps, not saline in high rainfall belt, has developed hypersalinity (Tannes) under drought conditions, making it extremely difficult to grow rice.

Clearly, what brings the difference between the southern mangrove swamps ( Guinea, Sierra Leone up to Nigeria) and the sahelian mangrove swamps (The Gambia, Senegal and part of Guinea-Bissau) is the amount of rainfall received annually which influences the intensity of production constraints such as soil stresses, drought, diseases as well as cultural practices. Thus, for strategic breeding purposes and the development of more appropriate technology packages, the vast mangrove swamps of West Africa should be classified based on the amount of rainfall. Two classes of mangrove swamps should be considered: The High Rainfall Belt or Wet Mangrove Swamps, and the Low Rainfall Belt or Dry Mangrove Swamps. Breeding objectives and cultural practices may vary from one category to another.

High Rainfall Belt Mangrove Swamps.

TABLE 2. Rainfall data in mangrove swamps of West Africa

---------------------------------------
Country         Area      Rainfall (mm)
---------------------------------------
Nigeria         Calaba      3010

Sierra Leone    Freetown    3842
                Rokupr      3011
                Balancera   2166

Guinea          Conakry     4292

Senegal         Ziguinchor  1546
                Saloum       807

Gambia          Jenoi        743
---------------------------------------

These are southern mangrove swamps encountered in Guinea, Sierra Leone up to Nigeria. They are characterised by high rainfall (2000 to 4000mm) (Table 2). They include the Tidal mangrove swamps (open and semi-open mangrove swamps) with the three "salt-free" zones, and the Associated Mangrove swamps or Closed mangrove Swamps as described above. The following conditions are recognized within a catenary sequence extending from the river to the older terraces adjoining the uplands which give rise to different cultural practices (Fig.1):

    - Areas adjacent to rivers and creeks in tidally affected swamps that have developed levees owing to the continual deposition of mud. They are relatively elevated but fertile. Flood levels may not exceed 40-50 cm during the cropping season. The soils are partially ripe with pH of 5.0 - 6.0.

    - Depressed areas beyond the levees which experience deeper and more prolonged submergence. Flood levels range between 60 - 70 cm. The soils are unripe with pH of 5.5 - 6.2.

    - Areas on the limit of tidal intrusion in the dry season that are dependent on rainfall and runoff from adjoining uplands. These areas may be submerged up to 20 cm in the cropping season. The soils are ripe, strongly mottled with pH of 4.5 - 5.5.

    - Areas adjoining uplands and strongly affected by seepage and materials eroded from uplands. The soils are ripe with a distinctive Ap horizon; and strongly mottled at depths of 50 - 60 cm.

    Figure 1 - Schematic presentation of flooding conditions and soil groups along a Mangrove Swamp Topsequence-high raainfall zone.

Low Rainfall Belt Mangrove Swamps.

These swamps are encountered in low rainfall zones (sahel zones). With some variations in the severity of drought, they cover the northern part of the West African Atlantic coast from Guinea-Bissau to the Cassamance region in Senegal. Rainfall per annum is between 600mm and 1600mm (Table 2). Basically, short duration (120 days or under) or short to medium duration varieties (120 to 135 days) are grown. Owing to declining rainfall, there is a build up of soil stresses making it practically impossible to grow rice. Thus, rice production is heavily influenced by environmental stresses such as salinity, acidity, mineral toxicity (Aluminium), diseases, high temperature and drought. Farmers in these regions grow rice on ridges to leach salt and other toxic elements downwards, creating better environmental conditions at the rooting zone for plant growth. However, unless heavy investments are used in soil and water management to claim salinised lands, most of the farmers in the region are gradually abandoning rice production. Therefore, specific and well thought breeding programme for the region should focus on developing rice varieties of very short duration with multiple stress tolerance.

APPROACHES TO MANGROVE SWAMP RICE VARIETAL IMPROVEMENT IN WEST AFRICA

Growth duration.

Success of rice production in these swamps depends on the development and utilisation of varieties that fit into the different "salt-free" periods. Breeding for appropriate growth duration is of great importance, but, because of farmers' increasing request for shorter duration varieties, more attention should be given to developing varieties of 120 days or less (Table 3). In addition, short, medium and long duration zones should be defined in all mangrove swamps for better utilisation of salinity escape mechanism. For the low rainfall belt mangrove swamps, there is practically no room for the utilisation of long duration varieties (160-200 days). The amount of rainfall per annum is so small that a "salt-free" period of that length is impossible to reach. Rather, short duration varieties (120 days or less) of tall stature are needed. Short to medium duration varieties (120-140 days), however, can still be used in some areas in The Gambia, and in southern Guinea-Bissau. The erratic nature of rainfall in the region is more crucial in the Cassamance, Senegal and in northern Guinea-Bissau where farmers, National Research Programmes and development agencies are continuously requesting varieties of shorter duration (less than 100 days). It is clear that greater effort should be put in developing varieties of appropriate maturity date. Breeding for short duration varieties should be the number one priority of any mangrove swamp rice development programme, to properly target "salt free" zones for salinity escape especially in dry mangrove swamp areas.

TABLE 3. Prioritisation of varietal characteristics as selection criteria according to the class of mangrove swamps

--------------------------------------------------------------------------
Varietal           High rainfall    Priority^a    Low rain      Priority
characteristics      Mangroove                    Mangroove 
--------------------------------------------------------------------------- 
Growth duration        short            **         Short            **
                                     Medium         *                      
                                     Long           *
Plant hieght           tall              *         tall              *
Culm                   stiff            **         Stiff             * 
Tillering ability      high             **         high             **
Panicle size           Big              **         Big              **
Salinity tolerance     short duration   **         all varieties   ***
Acidity/iron toxicity  
   tolerance           all varieties     *         all varieties    -
Drought tolerance        -               -         to be considered  *
                                                   in some areas
Resistance to key 
diseases               all varieties     *         all varieties    **
Resistance to crab     all varieties    **         all varieties    **
Insect resistance      all varieties     -         all varieties     -
Grain quality:
 -size/shape           all varieties     *         all varieties     *
 -good taste           all varieties     *         all varieties     *
 -High amylose content all varieties    **         all varieties    **
 -Treshability         all varieties    **         all varieties    **
Yield                  all varieties   ***         all varieties   ***

^a Not critical; * priority; ** high priority; *** highest priority
---------------------------------------------------------------------------

Salinity stress tolerance.

In high rainfall belt mangrove regions, breeding for salinity stress tolerance is important but not a number one priority as in the sahel regions. Salinity escape as a resistance mechanism should still be utilised especially for varieties developed for medium and long "salt-free" period areas which are located further away from the sea. Short "salt-free" period zones are located closer to the sea water, and there is a risk of salination in case of delay in rain or in sowing date. Therefore, salt tolerant varieties of short duration should be used in these areas as well as in the low rainfall belt mangrove swamps. This approach could minimise the cost of technology generation by combining breeding objectives in both the low and high rainfall mangrove swamps concerning the development of short duration varieties (Table 3). It is important to recognise that a decision to invest in a breeding programme for salinity resistance should be based on the nature of the target environment in which the resistant genotype is expected to provide income to the farmer.

Drought/high temperature stresses.

Usually, people talk about drought in the mangrove swamp in terms of lack of sufficient rainfall to leach out salt from the field. It is well kown that the lack of adequate rainfall will amplify salinity and acidity stresses. But, the combined effect of the stress complex:salinity-acidity-drought-high temperature, on plants' physiological functions is very difficult to dissociate. It will be useful that in mangrove swamps, especially in the low rainfall belt mangrove, soil stresses are not looked at separately, since they interact with one another. It should be noted that, when plants grow in saline conditions, they are subjected to three types of stress: (1) water stress caused by the osmoticum, (2) mineral toxicity stress caused by the salt, and (3) disturbances in the mineral nutrition of the plant (Blum, 1988). According to Blum (1988), the soil solution of a wet saline soil may be at a water potential of -24 bar (which is the value for sea water) and up. Plants challenged by this potential develop a large soil-to-leaf gradient of potentials, are unable to meet transpirational demand, and ultimately, may wilt and dessicate. Symptoms of water deficit can be observed in salinised plants, especially under conditions of high evaporative demand. Water stress is therefore considered a component of salt injury (Blum, 1988). This shows that, plants can suffer from a salinity induced physiological water stress, even though they are standing in water. Drought and high temperature stresses therefore, should be looked at as a component of the global environmental stresses in dry mangrove swamp rice production. In addition, drought stress due to the lack of water in the rooting zone, may be encountered on the upper fringe of the valley in the hydromorphic zone, and rice varieties grown in this area should possess drought tolerance genes. Although water and salt stresses are interrelated, there is still no evidence that salt tolerant rice varieties will show drought tolerance and vice vrsa. This is yet to be explored by rice breeders and physiologists.

Iron toxicity and acidity stress tolerance.

Apart from its interaction with other metals in causing toxicity problems, direct iron toxicity is associated, in rice, with leaf bronzing which often occurs under permanent flooding. Drainage could correct the disorder. The soil environment which promotes Al and Mn toxicities is generally conductive to conditions of Fe toxicity (Blum, 1988). For this reason, the direct treatment of Fe toxicity by plant breeding is difficult. Acidity stress is determined by acid sulphate condition. Patches of iron and acidity stresses can be found in the high rainfall mangrove swamps, generally in the associated mangrove swamps. Although the level of the stress might not be as high in wet mangrove swamps as in the sahel regions, breeding for acidity and iron toxicity is justified for both ecologies.

Diseases.

A survey of wet mangrove swamps diseases situation was conducted by Fomba in Sierra Leone, Guinea and Nigeria in 1984 (Fomba, 1984). Generally, it was observed that disease severity was less pronounced in mangrove swamps than in other rice growing ecologies e.g. upland. Rice seedling blast (Pyricularia oryzae) was reported to be severe in dry upland beds. Even so, the economic importance of the disease at the moment lies in the fact that badly blasted seedlings are weakened and become susceptible to crabs after transplanting (WARDA, 1976, 1980). Among several diseases encountered under field conditions, four key diseases were reported which can cause crop losses ranging from 10 to more than 50%. These are Brown spot (Helmin-thosporium oryzae), Leaf scald (Rhynchosporium oryzae), Narrow brown leaf spot (Cercospora oryzae), and Sheath rot (Acrocylindrium oryzae). These diseases were reported to be more severe in associated mangrove swamps due to early drying up of these areas at the onset of the dry season (Fomba, 1984). Soil stresses, soil nutrient imbalance and drought stress influence the severity of diseases (Bidaux, 1977). In that regard, diseases, especially blast incidence in the dry mangrove swamps could be more severe than in the wet mangrove swamps because of its association with drought stress.

The incidence of diseases in rice fields in the mangrove is highly unpredictable as the environment itself is variable. Soils stresses, nutrient inbalance, relative humidity and drought stress are not homogeneously distributed throughout the region. Rice varieties grown in the region could be exposed to greater disease pressure in one area in the region than in other areas. Also, because of the rapid changes taking place in the mangrove swamp environments, diseases are also becoming more and more of a problem which should be seriously taken into consideration in a breeding programme. Varieties developed for this ecology should have stable resistance to some key diseases. Rice blast damages in wet mangrove swamps are heavier at the seedling stage while for dry mangrove swamps, leaf and panicle blast should seriously be taken into consideration in varietal improvement programmes (Mbodj, 1989).

Pests.

A wide range of rice pests has been recorded in mangrove swamp of West Africa. The most important and widely spread pests are Stemborers especially the white African borer Maliarpha separatella, found in Nigeria, Sierra Leone, Guinea-Bissau and The Gambia (WARDA, 1976); and Crabs (Sesarma huzardi) (WARDA, 1977). The extent of crab damage has been estimated at 19.0 to 34.0 kg ha^-1 for every 1.0 percent crab damage (WARDA, 1978). Crabs are common problems to the whole West African mangrove ecology. They are found across and along rivers and creeks up to the tidal limit (WARDA, 1977). They can be controlled through cultural practices such as good land preparation, increased plant density, use of older and vigorous seedlings, and through the use of resistant varieties. Crab damage is so severe that in recent years farmers have been using unidentified lethal chemicals in their fields. Despite the importance of crab damage in the mangrove no work has been undertaken in the past to develop resistant varieties. Past screening work undertaken by Agyen-Sampong (personal communication) showed that genetic variability exists in mangrove rice varieties for crab resistance. Based on field observations, it is now known that crabs prefer young or weak seedlings. But as seedlings become older, they probably become less succulent and more lignified for crabs to cut them. Varietal characteristics such as fast growth at seedling stage, the rate of lignification of xylem tissue, and silicon content should be investigated as selection criteria for crab resistance. Breeding for crab resistance should be among the top priorities in breeding programmes designed to improve farmers productivity in both the low and high rainfall belt mangrove swamp regions.

Grain quality.

Next to yield, grain quality is the most important factor to be considered by plant breeders. If consumers do not accept the taste, texture, aroma, or appearance of a newly developed variety, its usefulness is greatly impaired. Quality in rice may be categorised into four broad areas: (1) milling quality; (2) cooking, eating and processing quality; (3) nutritive quality; and (4) specific standards for cleanliness, soundness and purity (Webb, 1979). In the mangrove areas of West Africa, consumers, especially farmers' requirement for quality characteristics is highly variable, which has a great influence on varietal adoption. Grain quality requirement varies from Forecariah region in Guinea (short and round grain) to The Gambia (long and slender grain). Farmers' preference is linked with their rice growing, processing, cooking and eating habits. For instance, one of the improved mangrove swamp rice variety Rok 5 widely adopted throughout West Africa for its high yielding ability under soil stress conditions, was rejected by Guinea-Bissau farmers because they think that, compared to their local varieties, it does not stay longer in the stomach; pointing out the fundamental problem of criteria for varietal adoption by farmers in Africa. It has been argued for a long time that, in developing countries, food availability was more important than quality. This may not be true today. African urban areas are increasingly eating high quality imported rice. It is not uncommon to find farmers accepting improved rice varieties only as a cash crop, while they still hold on to their old landraces with very good taste and cooking quality. The example of ROK 5 among others is an indication that any breeding programme which capitalises only on yield and yield stability without including grain quality characteristics as selection criteria is bound to fail.

The development of mangrove swamp rice varieties should include the testing for grain quality at early stages of the selection process. Selection criteria should include at least the ease of threshing, size (long slender, but also some varieties with medium and short grain), cooking and eating qualities (largely determined by the properties of the starch). Several tests are conducted in standard laboratories to determine the cooking characteristics such as gelatinisation, temperature, amylose content, gel consistency, grain elongation, and aroma (Webb, 1976). Several varieties of each grain type (long, medium and short ) should be developed. It is essential for new varieties of each grain type to have the same or improved milling, cooking and processing qualities as the varieties they replace. However, regardless of whether a new variety is developed for greater yielding ability, greater nitrogen responsiveness, improved plant type, diseases and soil stress resistance, the necessary grain quality characteristics acceptable to farmers should be included.

CONCLUSION

The mangrove environments in West Africa are highly variable and to facilitate effective targeting of new rice technologies to appropriate recommendation domains, a complete inventory of the environmental conditions should be made. Within each agroclimatic zone (wet and dry mangrove swamps), major agroecological and sub-agroecological zones should be delineated to reflect the regional variation in climatic parameters, principally rainfall and relative humidity, as well as salinity conditions, depth and duration of flooding, depth and annual fluctuation of the ground water table, and period of availability of fresh water for rice production. A regional map of varietal characteristics could be made which could guide in breeding for more targeted areas. The mangrove environment is so variable that we should abandon our quest for varieties that are widely adaptable and begin to select varieties that produce well in particular environments and are acceptable to local consumers.

REFERENCES

Agyen-Sampong, M., Prakeh-Asante, K. and Fomba, S. N. 1988. Rice improvement in mangrove swamps of West Africa. In: Selected Papers of the Dakar Symposium on Acid Sulfate Soils, Dakar, Senegal, 6-11 January, 1986, 163pp. International Institute of Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.

Bidaux, J.M. 1977. Annual Report on Rice Pathology for 1976. IRAT, Bouake, Cote d'Ivoire.

Blum, A. 1988. Plant Breeding for Stress Environment. CRC Press, Inc. Boca Raton, Florida.

Dent, D. 1986. Acid sulphate soils: a baseline for research and development. Publication 39, ILRI, Wageningen, The Netherlands.

Fomba, S.N. 1984. Rice disease situation in mangrove and associated swamps in Sierra Leone. Tropical Pest Management 30:73-81.

Hesse, P.R. and Jeffrey, J.W.O. 1963. Some properties of Sierra Leone mangrove soils. L'Agronomie Tropicale 8:803-805.

Jordan, H.D. 1964. The relation of vegetation and soil to development of mangrove swamps for rice growing in Sierra Leone. Journal of Applied Ecology 1:209-212.

Mbodj, Y. 1989. La lutte integree contre les maladies du riz en Casamance, au Senegal. These de Doctorat en sciences Agrono-miques. Universite Catholique de Louvain.

WARDA, 1976. Special project. Annual Research Report. West Africa Rice Development Association, Freetown, Sierra Leone.

WARDA, 1977. Annual Report. Regional Mangrove Swamp Rice Research Station, Rokupr, Sierra Leone.

WARDA, 1978. Annual Report. Regional Mangrove Swamp Rice Research Station, Rokupr, Sierra Leone.

WARDA, 1980. Special project. Annual Research Report. West Africa Rice Development Association, Freetown, Sierra Leone.

Webb, B.D. 1979. Rice quality and grades. In: Rice Production and Utilization. AVI Publishing Company, Inc. Wesport, Connecticut. 15pp.

Copyright 1997 The African Crop Science Society


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