Food is the most basic human need. At low levels of income, the utmost concern for the human being is to meet the energy needs to overcome hunger. Cereals (like rice) provide the cheapest source of energy.

Rice is the most staple food crop in world, so it does in Africa where it is the most rapidly growing food source but largely grown on small family farms. It has a long and varied history in Africa. African farmers probably domesticated this grain at the same time as Asian farmers (about 3 000 years ago). African farmers developed the species Oryza glaberrima, while Asian farmers developed Oryza sativa. Oryza sativa was introduced to Africa about 500 years ago, and peasants there have adapted it to their rice production systems, developing many local varieties of the Asian species and turning Africa into an important secondary source of diversity. West Africa was dependant on rice as their primary source of food energy and protein, but the majority of Africa’s rice was imported.

There was the need for the national agricultural research system to develop suitable packages for farmers to enhance the performance of the rice crop to mitigate the threats, for a fresh generation of agricultural technologies; it was a call from Ban KI-moon, the UN secretary, to usher in a second green revolution, one which permits sustainable yield improvements with minimal environmental damage and contributes to sustainable development goals.

Following the Africa rice center (ex. WARDA) was formed with an objective to produce a rice variety which was better suited to harsh conditions in Africa. In 1991 a research team from Africa rice center leaded by Dr. Jones set up a research program based on biotechnological tools. Working in association with partners from across Africa and overseas, the team collected and classified all available rice strains (including a gene bank of 1 500 strains of the native O. glaberrima species, which had been in danger of extinction). They then began the painstaking process of selecting parents for the best combination of characteristics, crossing them to produce offspring and backcrossing the offspring with the O. sativa parent to fix the desired traits; after a series of failures, they turned to “embryo rescue” techniques, in which the cross fertilized embryos, were grown on artificial media. By the mid 1990s they succeeded in producing robustly fertile plants, and so the first New Rice for Africa (NERICA) was born. Field testing of the new rice started in 1994, and with improved techniques many more lines were generated each year. There are now more than 3 000 NERICA lines. NERICA was developed using complex embryo rescue techniques to cross the Asian Oryza sativa rice with the African Oryza glaberrima rice. The first Nerica variety was developed in 1994 by researchers at the Africa Rice center using an Oryza sativa japonica variety (WAB56104) and an African Oryza glaberrima variety (CG14). The researchers have developed several other hybrids, working with Japanese researchers on the Interspecific Hybridization Project (IHP), financed by the Japanese government, the US Rockefeller Foundation and the United Nations Development Program (UNDP). These interspecific hybrids were supposed to combine the best traits of both of the two parents such as the high yield of their Asian parent and the adaptability to local conditions of their African parent.

At first, the NERICA researchers insisted that they did not intend NERICA to replace local diversity. Indeed, the incorporation of new seeds is nothing new for African farmers. New varieties are often mixed with old and become part of the selection process, contributing to the local genetic heritage, and now it is perfectly adapted to the harsh growing environment and low-input conditions of upland rice ecologies in sub-Saharan Africa (SSA), where smallholder farmers lack the means to irrigate and apply chemical fertilizers or pesticides and it responds even better to higher inputs. The NERICA appears to offer a rich source of genetic resistance to drought, weed competition, blast, virus diseases and soil acidity and iron toxicity (Dingkuhn et al., 1998; Diagne, 2006).

Since its creation in the mid-1990s, the New Rice for Africa (NERICA) has carved a special niche for itself among upland rice farmers in sub-Saharan Africa (SSA). Today, it is a symbol of hope for food security in SSA.

1 The new rice for Africa: Origins and nomenclature
1.1 NERICA’s meaning
The term NERICA stands for “NEw RICe for Africa” an extended family of some 3 000 siblings. The agro physiological traits of NERICA can not be generalized because of the large variation existing amongst NERICA varieties. 

NERICA is used to refer to genetic material derived from the successful crossing that combine the best traits of both of the two species of cultivated rice, the African rice (O. glaberrima Steud) and the Asian rice (O. sativa L.), to produce progeny (know as interspecifics) with high yields potential from the Asian parent and the ability from the african parent to thrive in harsh environments. In 2000, the results from this breeding were denominate New Rice for Africa (NERICA).

According to its researchers, NERICA is produced through conventional crossbreeding and is therefore not genetically modified rice. It is a new group of upland rice varieties that perfectly adapt to the rainfed upland ecology in sub-saharian Africa (SSA), where smallholder farmers lack the means to irrigate or apply chemical fertilizers or pesticides. However, NERICA also respond even better than traditional varieties to higher inputs.

1.2 NERICA’s development program
According to the Africa rice center annual reports, the NERICA rice varieties were developed at the Africa Rice Center. In the early 1990s, a team of rice breeders developed stable and fertile progeny from crosses between Asian rice, O. sativa L. and African rice, O. glaberrima Steud (Jones et al., 1997a; 1997b).

The first generation of NERICA varieties 1~11, including the WAB450 progeny, was developed from crosses of the existing released variety CG14 (O. glaberrima Steud.) and WAB56-104 (WAB signifies a variety or line developed at WARDA BOUAKE), which belongs to the subspecies japonica of O. sativa L., an upland improved variety. On the other hand, NERICAs 12 to 18 are progeny of two series of crosses, using the same O. glaberrima CG14 parent but two different O. sativa parents (WAB56-50 and WAB181-18). Morphologically diverse, genetically stable and fully fertile, these interspecific progeny have developed either through the refined method of conventional breeding, or with the use of specifically-developed anther culture and double-haploidization techniques to overcome sterility and to hasten the breeding process. Crosses were made and embryo rescue was used to remove fertilized embryos and grow them in artificial media. Anther culture allowed rapid fixation and helped to retain interspecific lines combining desirable features of the two rice species.

This achievement was indeed a scientific breakthrough. Previous conventional breeding efforts elsewhere in the world to develop interspecific hybrids of rice had failed, yielding only infertile offspring of the two species being used for crossing.

This breakthrough established the Africa rice center’s lead role in interspecific hybridization and anther culture for rice (Jones et al., 1997b). Exploitation of the O. glaberrima gene pool increased the scope for the development of low management input plant types (Jones, 1997a; Dingkuhn et al., 1998). O. glaberrima originates in Africa and is resistant to a number of major African insect pests and diseases such as stem borers and rice blast. O. glaberrima is also very competitive with weeds (Audebert et al., 1998; Johnson et al., 1998), the main constraint to rice production across ecologies in sub-saharian Africa.

1.3 NERICA varieties ratoonability potential
According to the study conducted by Sanni et al. (2007), in the savanna zone in 2006 on the named 18 rainfed upland NERICA indicated a large variation in the ratoon performance among these varieties, with a ratoon yield ranging from 39% (NERICA17) to 13% (NERICA2) of the main crop yield, bringing the total grain yield per plot and per cropping season up to about 6 500 kg/ha. NERICA14, NERICA15, NERICA17, NERICA18 exhibited the highest ratoon yield performance. The mechanism contributing to the ratoonability of these short duration NERICA varieties was not investigated in the reported study. Similar findings on the ratoon yield potential of interspecific progeny, including the NERICA’s was also reported in other zone (kouko et al., 2006).

1.4 High-potential irrigated and rainfed lowlands of NERICA
African lowlands constitute one of the most complex rice ecologies in the world. The rainfed lowlands, where rice is grown in bunded fields that are flooded for at least part of the growing season, are more fertile than the uplands and have the added advantage of opportunities for irrigation. Most of the traditional lowland rice varieties grown in West Africa have a narrow genetic base, which leads to their vulnerability to diseases and pest. Yet lowlands account for about 30% of the area under rice cultivation in west and central Africa.

1.5 Differences in the upland and lowland NERICA varieties development process
The main difference between the development of lowland NERICA and that of the upland interspecifics was in the selection of the Asian rice varieties for the crosses(Figure 1). The Asian O. sativa has two main strains, including the japonica (traditional rainfed or upland rice) and the indica (traditional irrigated or lowland rice). In the creation of upland NERICA varieties, japonica varieties were used in the crosses, while the indica subspecies was used for developing the new lowland rice.

Figure 1 Scheme of the lowland NERICA varieties development

As in the process of upland NERICA development, the initial problem was again hybrid (infertile offspring of the crosses). The sterility blockage was overcome by backcrossing (crossing the hybrid to O. sativa to restore fertility).

The schematic explanation for NERICA development was given by the Africa rice center: “after four backcrosses (O. sativa as recurrent), over 500 highfertile fixed lines (85%~100% fertility) were selected and evaluated in tandem with NARS (shuttle breeding) in observational nurseries under irrigated conditions and rainfed lowland conditions for adaptation, and with Africa rice center plant pathologist for resistance to RYMV (rice yellow mollte virus). These lines where evaluated for phenotypic acceptability, yield potential, RYMV resistance, nitrogen-use efficiency and adaptation of different water regimes.

Through a strong networking approach supporting the shuttle breeding, undertaken with the active involvement of the West and Central Africa Regional Rice Network (ROCARIZ) and the International Network for Genetic Evaluation of Rice (INGER-Africa), the Africa rice center has been able to accelerate the selection process and achieve wide adaptability of the lowland NERICA in West Africa.

The development of interspecific rice varieties for various ecologies is a significant international public good. The Africa Rice Center has generated several hundred NERICA lines, opening new gene pools and increasing the biodiversity of rice available to the world of science. This development of the NERICA varieties is further advancing the farm level agrobiodiversity of rice in the high-impact ecologies.”

By unblocking the treasure store of genes in Africa rice (O. glaberrima), the Africa rice center has presented the global rice research community with an opportunity to exploit the preferred biodiversity.

2 Major rice diseases
2.1 Resistance to diseases and pests
After the NERICA was made, it has been evaluated and characterized for a range of agronomic traits and reaction to key African endemic diseases and pests. The results are like this, first, its varieties generally have shorter growth duration than most traditional rice varieties; second, a number of NERICA varieties process early vigor, which is an important trait for weed competiveness in rice, thus improving the productivity of scarce labor; Moreover, some of them also have tolerance to drought and soil acidity.

In the other hand, the West African common three major diseases of key economic importance and seriously constrain rice production in most rice ecologies, such as the rice yellow mottle virus (RYMV), bacterial leaf blight (BLB) and rice blast: result, only blast is specific to the rainfed upland ecology for which the NERICA varieties were developed. Blast is rice fungal disease caused by Pyricularia grisea (Cke.) Sacc. And is particularly dangerous in upland rice, but also caused serious damage in rain fed lowland and irrigated systems (WARDA, 1999, annual report,  www.warda.org/warda1/main/ Publications/publications.htm). Blast is one of the major constraints to intensification (Figure 2).

Figure 2 Symptoms of some blast diseases on upland rice (www.warda.org)

2.2 Irrigated management of disease
In the low-input farming systems of sub-saharian Africa where resource-limited farmers can hardly ever afford external inputs, the control of the above diseases is mainly through the use of resistant/tolerant varieties in combination with sound management practices, such as good weed control. One of the principal components of an integrated management system for diseases is varietal resistance though this can be unstable in space and in time depending to the structure of the pathogen population.

This constraint should be taken into consideration either when diffusing material to farmers or when breeders are selecting donor lines.

2.3 Varietal resistance and tolerance to blast
According to the Africa rice center annual reports, nine interspecifics, including NERICA9 and NERICA 18, consistently show resistance to blast at various hotspots across four countries in West Africa. The resistance of the interspecific lines is believed to be as stable and durable as that of WAB56-50 and WAB56-104, which are well known for processing horizontal resistance to the blast resistance in West Africa (Talbe 1).

Table 1 Evaluation of resistance to blast in 4 sub-Saharan countries

3 Drought control
To evaluate the drought tolerance on the interspecific, conducted in the dry season, eleven NERICA varieties (N1, N2, N3, N4, N5, N6, N7, N8, N9, N10, and N12) as well as the two parents of NERICA1-7, WAB56-104 and CG14, were screened together with 87 other rice genotypes that included O. sativa spp. indica, O. sativa spp. japonica and interspecifics (O. sativa × O. glaberrima), which were sourced from Africa rice center, CIAT, and IRRI. The screening protocol used involving 21 days drought tolerance at 45 days after sowing (DAS), which coincides with the vegetative/ reproductive phase of crop developpment.

Method: According to the report, the trial was designed as a split-plot protocol with irrigation regime as the main plot factor and genotype as the sub-plot factor. Then two irrigation levels were used: one is full irrigation up to maturity and imposing 21 days, the other is drought stress starts 45 days.

Thinning, fertilizer application, weeding and spraying against pests and diseases were carried out. Soil water status at the trial site was measured in three 20 cm layers of soil from the surface to 60 cm depth.

The soil at the trial site is an Alfisol with a sandy texture (82%~89%) from 0~50 cm depth and hence has low waterholding capacity. Soil moisture content in the top 20 cm towards the end of drought stress was 2.61% in the stressed treatments and 4.5% in the fully irrigated treatment. Thus, the researchers said that, withholding irrigation for 21 days was sufficient to induce severe drought stress in the trial since the effective rooting depth of most rice varieties is the top 20 cm of soil.

Symptoms of drought stress results are showed that Leaf rolling, severity of leaf rolling, leaf drying increased with duration of drought; they noted that drought stress significantly (P<0.05) reduced tiller number, leaf chlorophyll content (SPAD92), numbers and weights of fertile panicles and grain yield but increased leaf temperature and delayed flowering.

The flowering was delayed by 10 days in the stress treatment compared to the non-stressed treatment. Consequently, grain yield per plant was significantly reduced from a mean of 12.42 g in the fully irrigated treatment to 5.063 g in the stress plot.

In conclusion, the experiment shows that the NERICAs exhibited a wide range of responses to drought stress. However, six out of eleven NERICAs screened gave higher than the average yield under drought.

As upland rice, NERICA is not restricted to growing in paddies, thus enabling African farmers to grow rice in places not previously thought possible.

4 Weeds control
Rice is an important staple food in West Africa, largely grown on small family farms usually less than 4 ha in size (Johnson, 1996; Guei and Traore, 2001).

About 57% of the total rice area in West Africa is planted to upland rice in diverse cropping systems (Johnson, 1996). Rice is generally a weak competitor against weeds. Thus, weeds are the major source of yield loss in upland rice and its control is an intensive labour (de Datta and Llagas, 1984; Fischer et al., 2001; Labrada, 2003). Weed infection in rice field is like watering plants with a bucket full of holes. Increase in production is negated when weeds compete with rice for limited light water and nutriment water.

In the SSA, smallholder farmers manage weeds in upland rice using hand hoes and machetes but face high costs as a result of labour shortages. Johnson et al. (2004) noted that weeds can emerge at the same time or before the rice plants, causing serious competition. Consequently, the tasks of planting and weeding often overlap and compete for labour (Rowland and Whiteman, 1993). Many study showed that, herbicides for weed control in upland rice are expensive and often not available to smallholder farmers at the time of need and, when available, farmers lack the requisite knowledge and skill to use herbicides correctly. Although herbicide use alleviates the problem of labour for weeding, incorrect use may bring about other environmental problems (Labrada, 2003).

Therefore, reducing dependence on herbicides may bring down the costs of crop production and retard the development of herbicide resistance in weeds (Lemerle et al., 1996; de Vida et al., 2006).

It was known that the African rice species (O. glaberrima) often has good weed competitiveness and exhibits resilience against some major African biotic and abiotic stresses (Koffi, 1980; Jones et al., 1997a). Thus, the development and integration of more competitive rice cultivars into the upland rice production system may be a viable option for attaining optimum yields in smallholder farms. Although some studies exist of differences in competitiveness, including attempts to relate rice traits to weed competitiveness and yield (Jones et al., 1996; Johnson et al., 1998; Fischer et al., 2001;Gibson et al., 2003; Koarai and Morita, 2003; Zhao et al., 2006), only a limited number of cultivars have been evaluated in the Guinea and Sudan savannas of West Africa. 

The Africa rice center experiments shown that, the interspecific hybrids called New Rice for Africa (NERICAs) have not been evaluated extensively for weed competitiveness in the savannas of West Africa. The potential for the adoption of NERICAs by smallholder farmers may depend in part on their weed competitiveness as well as high yield potential. Adékambi et al. (2006) cited by Rodenburg et al. (2006) reported that cultivation of NERICAs reduced the labour burden on school children because of the shorter growth cycle and high weed competitiveness. Rodenburg et al. (2006) suggested the evaluation of the NERICAs in different ecosystems to confirm possession of weed competitive traits and provide far-  mers with a wider choice of options when cultivating under weedy conditions.

5 NERICAs heading and yield
The concept of the NERICA varieties was to combine the superior traits of O. glaberrima and O. sativa. Thus lines which resemble O. glaberrima during early growth stages and O. sativa during later stages were developed. 

The Africa rice center research’s reports shown that the most popular NERICA lines take only three months to ripen, as opposed to six months for the parent species, thus allowing African farmers to “double crop” it in a single growing season with nutriationally rich vegetables or high-value fiber crops. In addition, NERICAs escape late-season insects and diseases, thus preventing yield losses. NERICAs have raised the yield ceiling of upland rice by 50%. The new rice can produce 6 t/ha in the best condition and up to 2.3 t/ha under drought.

As a further bonus, some of the new lines contain up to 12% protein, compared to about 10 percent in the imported rice sold in the local market.

New NERICA varieties can smother weeds like the African parents, resist drought and pests or can thrive in poor soils. They have strong stem, produce more tillers and bear longer panicles than either parent. The strong stem can support heavy panicles, like its Asian parents NERICA has a high yield. The grain head holds 300 to 400 grains compared to the 75 to 100 grains of traditional varieties grown in the region. Its strong stems and heads prevent shattering, and the taller plants make harvesting easier.

6 Grain quality
Studies on the milled NERICA varieties showed that it has higher protein contents and a better balance of amino-acids compared to both imported varieties and the international Rice standard.

Hence, the high protein content and good balance of essential amino-acids in its varieties can play a significant role in combating malnutrition in many subsaharian countries where rice is the main staple food. One could calculate the Africa-wide benefits of this extra protein from many angles: health, substitution for costlier protein sources, mental development in youth, etc. Number of varieties also shows a high micro nutriment (Iron and Zinc) concentration.

7 Achievements
The Africa rice center reports on the NERICAs project achievements so far, the report is as follow. In Nigeria, the new rice has resulted in over 30% expansion in upland rice cultivation. In Guinea the NERICA area has quickly superseded the modern varieties introduced by the national system.

Since Uganda launched the Upland Rice Project in 2004, in which NERICA is a major component, the Ugandan National Agricultural Research Organization (NARO) reports an almost nine-fold increase in the number of rice farmers from 4 000 to over 35 000 in 2007. At the same time, the country has almost halved its rice imports from 60 000 tones in 2005 to 35 000 in 2007, saving roughly US$30 million in the process.

While farmers cultivate previously released varieties, over 100 examples of new material have been introduced through a PVS (participatory varietal selection) approach during a 4-year period, and many new promising varieties have been identified by farmers: NERICAs 8, 9, 10 and 11 are the most popular; they are extra early and have good grain quality. To facilitate adoption and increase utilization, all named or newly introduced varieties are characterized and results are made available to the public. Seed purity and homogeneity are also addressed regularly in order to ensure the good quality of the seed produced and distributed to end-users. To increase the productivity of the NERICAs, complementary technologies (e.g. fertilizer rate and timing, crop density, weeding regime, sowing depth and harvest timing) are under evaluation: results will be published in appropriate journals.

NERICA was registered as a trademark with the USPTO in 2004, and as the expanding range of NERICA products are adopted by ever more smallholder farmers, CAS-IP notes that it will be increa- singly important to protect the quality associations that have been so carefully established by the Africa rice center, and to ensure that any NERICA seeds acquired by a farmer are the real thing.

8 Conclusion
NERICA is now popular among farmers and can have a strong impact on livelihoods. Detailed characteri- zation of NERICA varieties is therefore required to support farmers’ decision-making. Making seeds available to farmers is the main target for the coming years. While continuing to produce breeder and foundation seed at the Africa rice center and in collaboration with NARS, partners among NGOs, farmers’ organizations and individual seed growers will be identified and encouraged to produce certified seed.

Agronomic and post-harvest technology packages should be developed or released in order to enhance performance and quality. Prerequisites for enabling technologies such as NERICA to raise food security in the region include farmers having improved access to seed and information, as well as favorable policies supporting the development of the agricultural sector. Finally, lowland NERICAs have shown high-yielding ability and efforts are being made to extend them more to farmers.

As the Africa rice center declares with pride on its web pages, the New Rice for Africa, a technology from Africa for Africa, has become a symbol of hope for food security in a region of the world where one-third of the people are undernourished and half the population struggle to survive on US$1 a day or less and the Africa rice center directorgeneral Papa Abdoulaye Seck comments, “NERICA is a powerful weapon on Africa’s fight against hunger and poverty”.

Authors' contributions
HYQ finished paper, TYN modified paper. Both authors had read and consented the final text.

Acknowledgements
This work was supported in partial by a grant of Gates foundation and the National Program of High Technology Development, and a grant from the National Natural Science Foundation of China.

References
Adékambi S.A., Diagn A., and Biaou G., 2006, Impact de ládoption des nouvelles variétés NERICAs sur la scolarisation des enfants au Benin, Mimeo, ADRAO, Cotonou, Benin, pp.28

Audebert A., Dingkuhn M., Jones M.P., and Johnson D.E., 1998, Physiological mechanism for vegetative vigor of interspecific upland rices: Implication for weed competiveness, Japanese Journal of Crop Science, 67(2): 358-359

de Datta S.K., and Llagas M.A., 1984, Weed problems and weed control in upland rice in tropical Asia, In: De Datta S.K. (ed.), an Overview of Upland Rice Research. Proceedings of 1982 Upland Rice Workshop, Bouake, Ivory Coast, IRRI, Los Banos, the Philippines, pp.321-341

de Vida F.B.P., Laca E.A., Mackill D.J., Grisel M., and Fischer A.J., 2006, Relating rice traits to weed competitiveness and yield: A path analysis, Weed Sci., 54(6): 1122-1131 doi:10.1614/WS-06-042R.1

Diagne A., 2006, The diffusion and adoption of NERICA rice varieties in cote dìvoire. The developing Economies, 44(2): 208-231 doi:10.1111/j.1746-1049.2006.00014.x

Dingkuhn M., Jones M.P., Johnson D.E., and Sow A., 1998, Growth and yield potential of Oryza sativa and O. Glaberrima upland rice cultivars and their interspecific progenies, Field Crops Research, 57(1): 57-69 doi:10.1016/S0378-4290(97)00115-9

Fischer A.J., Ramirez H.V., Gibson K.D., and Pinhjeiro S.B., 2001, Competitiveness of semidwarf upland rice cultivars against Palisadegrass (Brachiaria brizantha) and Signalgrass (B. decumbens), Agron. J., 93: 967-973
doi:10.2134/agronj2001.935967x

Gibson K.D., Fischer A.J., Foin T.C., and Hill J.E., 2003, Crop traits related to weed suppression in water-seeded rice (Oryza sativa L.), Weed Sci., 51(1): 87-93 doi:10.1614/0043-1745(2003)051[0087:CTRTWS]2.0.CO;2

Guei R.G., and Traore K., 2001, New approach to germplasm exchange for a sustainable increase of rice biodiversity and production in Africa, Int. Rice Comm. Newsl. FAO pp.49-58

Johnson D.E., 1996, Weed management in smallholder rice production in the tropics, In: Radcliffe E.B., and Hutchison W.D. (eds.), IPM Word Textbook, University of Minnesota, St. Paul, MN Available from: http://ipmworld.umm.edu/chapters/johnson.htm

Johnson D.E., Dingkuhn M., Jones M.P., and Mahamane M.C., 1998, The influence of rice plant type on the effect of weed competition on Oryza sativa and O. glaberrima, Weed Res., 38(3): 207-216 doi:10.1046/j.1365-3180.1998.00092.x

Johnson D.E., Dingkuhn M., Jones M.P., and Mahamane M.C., 1998, The influence of rice plant type on the effect of weed competition on O. sativa and O. glaberrima, Weed Research 38: 207-216 doi:10.1046/j.1365-3180.1998.00092.x

Johnson D.E., Wopereis M.C.S., Mbodj D., Diallo S., Pewers S., and Haefele S.M., 2004, Timing of weed management and yield losses due weeds in irrigated rice in the Sahel, Field Crops Res., 85(1): 31-42 doi:10.1016/S0378-4290(03)00124-2

Jones M.P., Johnson D.E., and Kouper T., 1996, Selection for weed competitiveness in upland rice, Int. Rice Res. Notes, 21(1): 32-33

Jones M.P., Dingkuhn M., Aluko K., and Mande S., 1997a, Interspecific Oryza sativa L.X O. glaberrima Steud. progenies in upland rice improvement, Euphytica, 94(2): 237-246b doi:10.1023/A:1002969932224

Jones, M.P., Mande, S., and Aluko, K., 1997b, Diversity and potential of Oryza glaberrima Steud. in upland rice breeding, Breed. Sci., 47(4): 395-398

Koarai A., and Morita H., 2003, Evaluation of the suppression ability of rice (Oryza sativa) on Monochoria vaginalis by measuring photosynthetic photon flux density below rice canopy, Weed Biol. Manag., 3(3): 172-178 doi:10.1046/j.1445-6664.2003.00104.x

Koffi G., 1980, Collection and conservation of existing rice species and varieties of Africa, Agronomie Tropicale, 34: 228-237

Kouko W.O., Makehara D., Choke J., and Wakhu P., 2006, NERICA research and promotion in Kenya: JICA seminar 6-8 December 2006, Accra, Ghana

Labrada R., 2003, The need for improved weed management in rice, In: Van Tran, Dat, and Duffy, R. (eds.), Sustainable Rice Production for Food Security, Proceedings of the 20th Session of the International Rice Commission Bangkok, Thailand, 23-26 July 2002, FAO, Rome, Italy, pp.1-11

Lemerle D., Verbeck B., Cousens R.D., and Coombes N.E., 1996, The potential for selecting wheat varieties strongly competitive against weeds, Weed Res., 36(6): 505-513 doi:10.1111/j.1365-3180.1996.tb01679.x

Rodenburg J., Diagne A., Oikeh S., Futakuchi K., Kormawa P.M., Sémon I., Akintayo B., Cissé B., Sié M., Narteh L., Nwilene F., Diatta S., Sere Y., Ndjiondjop M.N., and Keya S.O., 2006, Achievements and impact of NERICA on sustainable rice production in sub-Saharan Africa, Int. Rice Comm. Newsl., 55: 45-58

Rowland J., and Whiteman P., 1993, Principles of dry land farming, In: Rowland J. (ed.), Dryland Farming in Africa, Macmillan Press Limited, London, pp.68-92

Sanni L., Alenkhe B., Edosio R., Patino M., And Dixon A., 2007, Tech- nology transfer in developing countries: Capitalizing on equipment de- velopment, Journal of food, Agriculture and environment, 5(2): 88-91

Zhao D.L., Atlin G.N., Bastiaans L., Spiertz J.H.J., 2006, Cultivar weed competitiveness in aerobic rice: heritability, correlated traits, and the potential for indirect selection in weed-free environments, Crop Sci., 46(1): 372-380
doi:10.2135/cropsci2005.0192