Rice (Oryza sativa
L.) is the most important staple food crop in the world. About 3.5 billion people depend on rice globally, since this staple provides in excess of 20% of their daily calorie intake [1
]. In Africa alone, where currently rice consumption is the most rapidly growing food source, about 30 million t more rice will be required by 2035, thus representing an increase of 130% in rice consumption from 2010. Nigeria would require almost one third of this additional rice [2
]. More than 90% of West African rice farmers are smallholders (mostly women) who cultivate less than 1 ha and whose crop yields depend on rainfall. Crop production from these small plots is often insufficient to provide a reasonable household income for maintaining a minimum standard of living. These farmers manage complex farming systems, cultivating rice and other food crops based on its degree of importance as food and as a cash crop, unlike farmers in Asia, where rice is a crop mainly grown in lowland and irrigated agro-ecosystems. The major constrains of global rice production are drought, flood, heat, pathogens, pests, declining productivity in intensive rice production systems, low grain yield in some areas of the developing world, increasing production costs in the industrialized world, and rising public concern regarding sustainability of rice farming. In addition, mechanization, the high cost of irrigated rice production, as well as the poor management of uplands and rainfed lowlands are among the main challenges for producing rice in Africa [3
Advances in rice production in Africa are at various developmental stages due to its relative importance to respective local economy. Hybrid rice seed technology is key for increasing its production and maintaining self-sufficiency and food security. Hybrid rice has been used in rice production for more than 40 years in Asia and North America, and more recently in Egypt because of its high grain yield potential. This yield advantage plus water and nitrogen use efficiency, and host plant resistance to pathogens and pests are the main determining factors for adapting hybrid seed technology in rice production. Heterosis improves grain yield and quality for many crops especially when facing a limited area for farming. Hybrid cultivars have been developed to take advantage of heterosis in the production of many field crops such as cotton, maize, oilseed rape, rice, sorghum, sunflower, and vegetables [5
]. African farmers may boost rice production by using hybrids particularly in the largely unutilized lowland rice areas. Mali, Nigeria, and Senegal are willing to adopt this seed technology to increase their rice production.
Rice breeders in China led the developing and commercializing of rice hybrids that had 15 to 20% yield advantage [7
] or at least 1 t ha−1
] over inbred cultivars. The new set of Green Super Rice (GSR) hybrids need less chemical inputs to increase grain yields than the old rice hybrids. Likewise, many of these GSR hybrids show adaptation to drought, thereby requiring less water and could be grown in rainfed agro-ecosystems. Nonetheless, their adaptability to African farming systems needs to be assessed across target areas, as well as the relative inputs they require vis-à-vis local cultivars. Farmers need to invest in both seeds and inputs for getting high grain yields when using rice hybrids. The increased grain yield of rice hybrids should pay for this investment and bring profitability to those using this seed technology.
There are various pathogens and pests affecting rice production in Africa. The African rice gall midge (AfRGM; Orseolia oryzivora
; Diptera: Cecidomyiidae
) is an important insect pest in irrigated lowland rice areas, causing 25 to 80% grain yield loss in West Africa [9
]. AfRGM is an endemic pest to Africa, where was first reported in Sudan, and currently spreading throughout the continent. It can be found in 12 West African, two Central African, and five East and Southern African countries [10
]. The insect pest causes 20 to 100% grain yield loss in the worst-affected areas. There are 16 quantitative trait loci (QTL) associated with host plant resistance to AfRGM, of which three are in [ITA306 × BW348-1], five in [ITA306 × TOS14519], and eight in [ITA306 × TOG7106] breeding populations. The major effect genomic region for AfRGM resistance was in the [ITA306 × TOS14519] population, which was at 111cM on chromosome 4 (qAfrGM4), had a LOD score of 60 and accounted for 34.1% of the total phenotypic variance [11
]. Likewise, Rice Yellow Mottle Virus
(RYMV, a Sobemo virus) is another major constraint to rice production in the continent [12
] because it causes 17 to 100% grain yield loss according to both the infection date and time, and the cultivar host [13
]). RYMV is highly infectious to rice, especially to Asian indica
cultivars in lowland and irrigated agro-ecosystem. RYMV is prevalent in all major rice growing ecosystems of Africa [15
]. Rice blast, caused by Magnaporthe oryzae
(anamorph: Pyricularia oryzae
], is another serious disease affecting rice in temperate and tropical regions, including Africa [17
The main purpose of this research was to determine the suitability of GSR hybrid cultivars bred in China at the irrigated lowland rice areas of Mali, Nigeria, and Senegal. We assessed their grain yield across suitable West African rice growing areas.
Hybrid rice seed technology was developed in China more than 35 years ago. Many of the released rice hybrids have shown between 15% and 20% grain yield advantage over inbred cultivars in Asia, South America, and Egypt [26
]. These results encouraged some to test hybrid rice in sub-Saharan Africa. Our previous research indicated that the grain yield of IRRI-bred hybrids was similar to that of check cultivars in Senegal [28
]. This research shows, however, that there was a high yield potential of hybrid rice bred in China after testing them in Mali, Nigeria, and Senegal. The grain yield of the most promising hybrids was higher than that of the best local inbred check cultivar. The hybrids exhibited a wide range grain yield, which was affected by the testing location and growing season.
The genotype × environment interactions (G × E) were highly significant for both sets of the hybrids and their respective cultivar checks across the three countries; i.e., their performances were different across sites from country to country. There was not the same hybrid or check cultivar showing superiority over others across the sites used in these three countries. This result may be due the soil characteristics, fertilizer applications, and weather conditions in the sites used for testing. The results of this study exhibited the high potential of these hybrids under the optimum conditions (Mali and Senegal), while the yield affected by RYMV and AfRGM stresses (in Nigeria). This finding suggests developing different hybrids for each region. In West Africa, high rice grain yields are often associated with high solar radiation, high maximum temperature, intermediate air humidity, multiple split nitrogen (N) fertilizer applications, high frequency of weeding operations, the use of certified seeds, and well-leveled fields in the irrigated lowland system [29
]. Local cultivars with host plant resistance to pathogens and pest may be used to develop new hybrids with high adaptability to stress-prone African sites.
The most promising hybrids across the testing locations had 20% more grain yield than the best check cultivars. This additional grain yield advantage may encourage some African farmers to grow hybrid rice cultivars. Testing sites in Mali and Senegal have irrigated areas with higher solar radiation than the Nigerian testing site [30
], which could be optimum environments for rice hybrids to show their heterosis for grain yield. The most promising hybrids also showed a good plant type and other interesting attributes. For example, most of them had medium or slender grain shape and higher milling recovery than the preferred check cultivar. Likewise, two hybrids had low GT, thereby needing less energy inputs for their cooking.
A major shortcoming of most of the hybrids bred in China is their susceptibility to both AfRGM and RYMV. Host plant resistance to both of them needs to be bred or incorporated into the parental lines of the hybrid rice germplasm before their release to West African farmers. In order words, indigenous parental lines such as traditional African (O. glaberrima
) and Asian (O. sativa
) rice-derived cultivars grown already in West Africa are proven useful sources of resistance to AfRGM and RYMV. They should be used in developing hybrids for farmers in West Africa. The levels of resistance shown in O. glaberrima
gene pool are better than those of found in O. sativa
. TOG7106 and TOS14519 were rated highly stable and highly resistant to AfRGM [31
]. Crossing should be made with donors that possess multiple tolerance to biophysical stresses, and off-sites should be used to select appropriate lines from segregating generations [32
]. Recently the major QTL of AfRGM were located and validated [11
], being the QTL with major effect (qAfRGM4) on chromosome 4. They could be easily incorporated into the parental lines of the promising hybrids through marker-aided backcrossing [33
]. Knowledge on the distribution of virulent RYMV populations across different target sites will also help to deploy safely rice hybrids in sub-Saharan Africa. It was interesting to note that in excess of 60% of the hybrids were either partially resistant (with slight symptoms) or highly resistant (without any symptom) to blast. Moreover, multi-environment testing will be still necessary to assess whether their host plant resistance will remain across space and time. Microsatellite-aided screening for fertility restoration genes Rf
may further facilitate hybrid development with high adaptability in West Africa [34
]. Preliminary results are promising: some adapted breeding lines and cultivars had high grain yield per plant with cytoplasmic male sterility lines and may be used as fertility restorers.