INTERNATIONAL POTATO CENTER: WORLD SWEETPOTATO ATLAS
NIGERIA
HISTORY AND OVERVIEW
The sweetpotato, Ipomoea batatas, preceded the potato, Solanum tuberosum, in its diffusion beyond the American tropics, where it was widely grown in pre-Columbian times. Columbus brought sweetpotato with him on his return to Spain from his first voyage in 1492, eighty years before the potato reached Europe (Purseglove 1968, p. 81). By the 1520s, Portuguese mariners were carrying sweetpotato to ports and territories throughout Africa and Asia, a diffusion later continued by other Europeans (Huntington). The subsequent historical record is somewhat unclear since the term "potato" was derived from "batata" (the Carib term for sweetpotato), so that documents from this period could be referring to either crop.
Whatever the circumstances of the sweetpotato's introduction(s) to Africa, the crop has become well established across a broad arc of the humid forest zone, roughly from the coast of Guinea Bissau across West Africa and the Congo Basin, to the highlands of East Africa and farther east and south, including Madagascar. Though long regarded as a minor root crop, sweetpotato is rapidly becoming more important due to its high yield relative to both land and labor, its capacity to grow in relatively poor soils, and its high content of carbohydrates and vitamins, especially vitamin A.
Although sweetpotato cultivation appears to be increasing in many countries, in some cases very rapidly, official production data have for many years underestimated the extent of the crop. Sweetpotato is usually grown and locally consumed by independent smallholders on small plots, widely distributed across larger regions, thus difficult to measure. In most countries, including Nigeria, sweetpotato cultivation is not locally concentrated (Huaccho and Hijmans, 2000). In 1994, the United Nations Food and Agriculture Organization (FAO) estimated the area of sweetpotato cultivation in Nigeria at 69,000 hectares. That estimate had increased to over 500,000 hectares by 2003 (FAOSTAT, 2004), though it remains uncertain to what extent the data reflect an actual expansion of sweetpotato cultivation or an increased awareness of its importance.
Sweetpotato is currently being developed as a means to address one of the most serious health and nutrition problems of sub-Saharan Africa, Vitamin A deficiency. Lack of Vitamin A can weaken the immune system, leaving an individual more susceptible to deadly diseases such as measles, malaria, and diarrhea. Vitamin A deficiency is also a major risk factor for pregnant and lactating women and a leading cause of visual impairments such as xerophthalmia, corneal scars, and corneal xerosis.
The International Potato Center (CIP) is a major partner of Vitamin A for Africa (VITAA), a project intended to develop and distribute to several countries of sub-Saharan Africa a series of orange-fleshed sweetpotatoes that are naturally rich in beta-carotene, a precursor of vitamin A. Nigeria, given its current production and potential for sweetpotato production, could likewise benefit from this initiative. The VITAA project is an example of biofortification, intended to develop crop varieties with increased mineral and vitamin content to enhance nutritional standards.
For more information, please see:
GEOGRAPHY AND PRODUCTION ZONES
Physical Geography and Climate
Nigeria is characterized by a gradual rise from the coastal plains to the northern savanna regions, reaching altitudes not generally higher than 600 to 700 meters above seal level (masl). Higher altitudes, reaching more than 1,200 masl, are found only in isolated areas of the Jos Plateau and in parts of the eastern highlands along the Cameroon border. The coastal plain is separated by the Niger Delta, which extends over an area of about 10,000 square kilometers. Much of the delta is swampland, separated by numerous islands. Two rivers, the Niger and Benue, extend all the way across Nigeria to the northwest and northeast, respectively, and create extensive areas of braided streams and seasonal floodplains (US LOC: Geography).
Nigeria's soil is generally limited in its longer-term productive capacity, though areas of recurrent floods can be rejuvenated by alluvial deposits. Acidic and sandy ultisols characterize some of the most densely populated areas of the southern region, such as Imo State (Goldman 1996, p. 201). Of Nigeria's total land area of 91 million hectares, approximately 42 percent was being cultivated, as of a 1990 estimate (US LOC: Agriculture). Much of that land is under recurrent bush fallow, whereby an area is left uncultivated for varying periods of time to allow for natural regeneration of soil fertility, but fallow periods have been reduced under growing population pressure (Goldman 1996).
- Several maps of more specific classifications — for example by soil types, agro-ecological zones, and land use — are available from the European Digital Archive of Soil Maps, Nigeria
. Most images are digitized from paper maps, of variable clarity.
As in most of West Africa, Nigeria's precipitation is characterized by strong latitudinal zones, generally becoming more arid with greater distance to the north, away from the coast. There are distinct rainy and dry seasons, controlled by the interactions of moist northward-moving maritime air from the Atlantic Ocean, and dry continental air from the African savannas and deserts farther north. Rains begin as early as February and are generally heaviest May through June. The driest and coolest season occurs around December and January. Except for the isolated areas of higher altitude noted above, topographic relief plays an insignificant role in local climate. Although rainfall in the south is generally heavy, drought has become occasionally severe in the drier regions in the north (US LOC: Climate).
Regional Distribution of Sweetpotato Production
Distribution of sweetpotato in Nigeria, and generally elsewhere in Africa, is not known with any precision via geo-referenced survey data. Huaccho and Hijmans (2000) have estimated the global distribution of area devoted to sweetpotato cultivation, by assigning national data from various sources to subnational units. The Tuber and Root Information System (TRIS), developed by the International Institute of Tropical Agriculture (IITA), has taken a different approach for sweetpotato, as well as yams and cassava, by modeling the physical parameters which estimate an ordinal suitability ranking for each crop across Africa. Rankings are determined by a combination of factors including soil classes, annual rainfall, temperature regimes and length of growing period (Tewe et. al. 2001). The result indicates the likely potential for sweetpotato, but not necessarily actual distribution.
The TRIS model indicates that most potential sweetpotato cultivation in Nigeria occurs in the "middle belt," (Tewe et. al. 2001, p. 65), a broad range of the country occurring from approximately 7° to 11° north. Tewe et. al. (2001, p. 66) indicate that sweetpotato has traditionally been grown in a few restricted areas of the central and southeastern zones, which might have been centers of concentration as recently as 1971, but that the crop has become much more widespread in recent years. (But again, it also possible that it is being more widely noticed.)
Preliminary survey estimates indicate average yields in Nigeria of 7 tons per hectare (T/HA) in the southeast, declining to 3.5 T/HA to the north. However, an exception to this trend seems to occur in the centrally located Plateau and Bauchi states, where yields of 7-8 T/HA are common (Tewe et. al. 2001, p. 68).
Consistent with these general estimates, Eneji et. al. (1997) cite several states of the south and central regions of Nigeria (Oyo, Benue, Plateau, Taraba, Kogi, and Cross Rivers) as areas where sweetpotato (as well as yam and cassava) is widely grown. Cultivation of sweetpotato extends by their estimate as far north as Kaduna and Bauchi States.
(Please note: Given the inconsistency of data on this topic, a map of sweetpotato distribution is not included for Nigeria.)
PRODUCTION SYSTEMS AND CONSTRAINTS
Agricultural Calendar and Cropping Patterns
Sweetpotato production is determined primarily by seasonal rains. The best crop yields generally occur in areas of 750 to 1,000 millimeters (mm) annual precipitation, with at least 500 mm falling during the growing season (Tewe et. al. 2001, p. 68). Very generally, planting takes place from February through July in the central to southern regions, where rainfall is heavier. However, planting along riverbanks in the central zone, or in swampy areas (fadama ) in the north can extend the season to permit planting from September to December (ibid.).
Seasonal patterns by zone and states are as follows (Tewe et. al. 2001, p. 67):
- Northwest (Sokoto, Kano, Kebbi, Katsina, Kaduna)
- Planting July to August, possibly a second crop irrigated from November to December, where feasible;
- Northeast (Jigawa, Yobe, Borno, Adamawa, Bauchi)
- Central (Niger, Kwara, Kogi, Benue, Plateau, Taraba)
- Ridges are made from May to July, and vines are planted July through August
- Southwest (Oyo, Osun, Ondo, Ogun, Lagos, Delta, Edo)
- Planting is as early as March, as late as August in Anambra State.
As elsewhere in Africa, sweetpotato is generally cultivated by independent smallholders, on plots of less than one hectare. It is often intercropped, in the southern and central areas generally with other root crops (yam, cassava, and cocoyam), and in the north with cereals (maize and millet). In either case, sweetpotato can provide soil cover and leave a large quantity of vegetative residue to be incorporated into the soil after harvest (Eneji et. al. 1997, p. 64).
Where sweetpotato is intercropped with maize, it is often established in July, following the harvest of melons. Research has indicated that the sweetpotato and maize intercropping pattern can be efficient, as measured by the Land Equivalency Ratio (the sum of the value of both crops divided by the value of one crop, in this case maize, if grown as a monoculture). Such research has been carried out with the use of commercial NPK fertilizers and farmyard manure, suggesting that a mix of the two is most efficient (Eneji et. al. 1997). However, estimates of typical use of fertilizer (of any kind) by farmers growing sweetpotato is largely speculative. In Imo State, despite relatively unfertile soils and high population density, the primary root crops of cassava, yam, and cocoyam are grown with minimal fertilizer in most areas (Goldman 1996, p. 201).
The rapid growth and/or growing recognition of sweetpotato in Nigeria can be appreciated only in reference to rapidly shifting cropping patterns over the past several decades, reflecting several factors, including biological constraints of major crops, government policies, and shifting market preferences.
Examples include (Goldman 1996):
- Near elimination of groundnut, the main export crop of the northern savanna region through the 1960s, due to price reductions of the early 1970s, the drought of 1972-73, and epidemics of viral rosette through the 1980s;
- Losses of cocoyam in southeastern Nigeria due to a root blight complex caused by several pathogens, though some resistant varieties have been identified;
- Threats to banana and plantain from black sigatoka disease (Mycosphaerella fijiensis ), for which resistant varieties are being developed, and likewise threats to cassava from the cassava mealy bug (Phenacoccus manihoti ) and green spider mite (Mononychellus tanajoa ), with varying degrees of success at control.
Constraints such as these are not a new story to Nigeria, and the results have not always been catastrophic. However, the capacity of farmers to survive such challenges, by expanding and/or diversifying production as necessary, is becoming more limited as both the amount and quality of available land declines.
Land Use and Land Tenure
Nigeria, with a 2002 population estimate of more than 120 million growing at an annual rate of approximately 3.3 percent (FAOSTAT), faces a serious challenge of population pressure resulting in unsustainable agricultural intensification and other forms of natural resource depletion, for example loss of forest cover estimated at 5.2 percent annually (Arene 1996, p. 76). Population density is generally highest in the south, for example in the state of Anambra, where density is estimated at 920 persons per square kilometer, one of the highest in Nigeria or anywhere in Africa (Arene 1996, p. 78). Here, agricultural systems have been primarily based on cassava and yam, but the relatively rapid production of sweetpotato might enhance its role, especially in areas where soil quality has suffered due to reduced fallow periods.
Land tenure in Nigeria has traditionally been one of usufructure, whereby an individual or household could maintain the right to use land for agricultural or other specified purposes, but could not sell the land or dispose of it to anyone beyond the local community. The system was under strong pressure by the 1970s. In the north, farmers lacking formal title to land were sometimes dispossessed with little compensation by more powerful interests. In the south, where British colonial authorities had introduced stronger concepts of individual ownership, farmers often could not afford land under the rising prices of the real estate boom then underway in more densely populated areas (US LOC: Agriculture).
In response to a potential crisis in land distribution, the federal government in 1978 promulgated the Land Use Decree, establishing a uniform tenure system for all of Nigeria. The decree effectively nationalized all land by requiring certificates of occupancy from the government for land held under customary and statutory rights, but with the exception that anyone in a rural or urban area who normally occupied land and developed it could continue to do so. The law has provided a means for government authorities to redistribute land not being utilized or "improved." (US LOC: Agriculture).
Occurrence and Control of Diseases and Pests
The Sweetpotato Weevil (Cylas puncticolis and C. brunneus ) is a major pest in Nigeria, causing losses up to eighty percent. The International Institute of Tropical Agriculture (IITA) has identified several clones demonstrating some resistance to weevils: TIS 3053, TIS 3030, TIS 3017, TIS 2532, TIS 8524, TIS 8266, TIS 9172. Unfortunately, orange-fleshed clones have generally been found to be more susceptible. At the National Root Crop Research Institute (NRCRI), varieties found to resist C. puncticolis included TIB, JKO, and Anambra (Tewe et. al. 2001, p. 67).
Viral Infections. Sweetpotato virus disease (SPVD) is caused by the co-infection of two separate viral strains: Sweetpotato Mild Mottle Virus (SPMMV), genus Ipomovirus, family Potyiridae, transmitted by whiteflies; and Sweetpotato Chlorotic Fleck Virus (SPCFV), a Carlavirus transmitted by a vector still unknown. SPVD is apparently a serious constraint to sweetpotato production in Nigeria. Symptoms include small, distorted leaves which are often narrow and crinkled, and general stunting of plants (Kreuze et. al. 2000). Research in Nigeria, comparing local strains with those of Madagascar, has supported the consensus that there are separate strains of SPCFV in East and West Africa (Fenby et. al. 2000).
Recent research indicates the transmission of SPCSV via two species of whiteflies, the sweetpotato whitefly (Bemisia tabaci biotype B) and the banded winged whitefly (Trialeurodes abutilonea), at least in laboratory experiments conducted in Louisiana, USA (Sim et. al. 2000).
Knot Nematode (Meloidogyne spp.) can attack sweetpotato root and cause damage resulting in crop losses of potentially as much as twenty to thirty percent, but some clones (55 reported) have been found to demonstrate some resistance (Tewe et. al. 2001). Ipomoea trifida, a wild progenitor of the cultivated sweetpotato, has been the subject of research to further develop inheritable resistance (Shiotani et. al. 2000). Sharmi et. al. (1997) summarize research in the field of nematology, pertaining to sweetpotato and many other crops.
VARIETIES AND SEED SYSTEMS
Since sweetpotato in Nigeria has not long been a major topic of research, very little is known of the origins or characteristics of local varieties, most of which can be assumed to be derived from varieties introduced by Europeans several centuries ago. Please refer to this section of the Uganda chapter of the atlas, generally relevant to Nigeria as well.
Of recently released improved varieties, TIS 2498, released by the International Institute of Tropical Agriculture (IITA), was reported as highly productive and stable from on-farm trials across a wide range of environments, with average yields above ten tons per hectare (T/HA). Stability would seem to be a major consideration, as the reported yields of improved varieties are extremely variable, from less than one T/HA to over 70 T/HA under high input management. Advantages offered by TIS 2498 include resistance to viral infection, moderate resistance to weevils, high storability, and a relatively high dry matter content of 33 percent (Tewe et. al. 2001, p. 69).
CONSUMPTION, STORAGE, AND MARKETING
Consumption
Since sweetpotato is generally consumed close to where it is grown, consumption is well estimated by production data, however uncertain they may be. By FAO data, Nigeria's total sweetpotato production is approximately 2,150,000 metric tons. For a population of 120,911,000, per capita annual production, hence consumption, is approximately 17.5 kilograms. This figure is moderately high by world standards, though elsewhere in Africa where sweetpotato is a well-established major crop ([e.g] Uganda, Burundi, and Rwanda), annual per capita production and consumption can be around 100 kilograms. There is still much potential for increased sweetpotato cultivation in Nigeria.
Sweetpotato is usually consumed in boiled form, peeled either before or after boiling. Roots are sometimes prepared in a porridge that might also include cowpea, lima bean, sesame (better known locally as benniseed), and/or millet. Sweetpotato can also be peeled, sun-dried, and milled into a flour used for sweetening local dishes or even to prepare a fermented drink called kunu in the northern and central zones. Sweetpotato dough can also be used as an ingredient, with other root crop ingredients, of a West African staple dish, fufu, or to prepare a paste locally known as amala. There has been some development in recent years of sweetpotato snacks, such as chips sold in urban markets. Sweetpotato leaf is a popular vegetable, usually as an ingredient to soups (Tewe et. al. 2001, p. 69).
Storage
Since sweetpotato can be cultivated throughout most of the year and fresh roots are consistently available, longer-term storage is not commonly practiced. Storage is also constrained by the sweetpotato weevil and black rot damage. Leaving the crop unharvested for any length of time is generally not an option, again primarily due to weevil damage. However, when farmers want to take advantage of higher prices or to maintain a famine reserve, sweetpotato can be stored for two to three months in pits lined with dried grass and topsoil to protect against weevils, a practice more common in the more arid north (Tewe et. al. 2001, p. 69). However, other accounts of the potential for sweetpotato storage in Nigeria are less sanguine, including an assessment (Osagie 1991, p. 71) that roots become suberized (infiltrated by suberin, resulting in a corky texture) at high temperatures and humidity, again suggesting higher likelihood of success in the north.
Marketing
Data pertaining to the marketing of fresh sweetpotato in Nigeria are not readily available.
Nigeria might provide opportunities for agro-processing of sweetpotato, for example as livestock feed (especially for broiler chickens) and for alcohol, which currently utilizes cassava. A large and consistent supply of sweetpotato would have to be available at prices considerably below the current market (Tewe et. al. 2001, p. 73). However, given the size of the country and the potential for greater sweetpotato production if the "yield gap" between optimal and current practices can be narrowed, a sweetpotato industry might be feasible in Nigeria.
The International Institute for Tropical Agriculture (IITA) is a member of the Consultative Group for International Agricultural Research (CGIAR), headquartered in Nigeria. Information about sweetpotato can be obtained via a site search (for "sweetpotato" as one word, or "sweet potato" as two) from the homepage, or via "Research for Development" / "Root and Tuber Systems" (as of August 2006).
Contact (as of 2000) for the National Root Crop Research Institute (NRCRI) is:
Umudike, Umahia
[P.M.B] 7600
Umudike
Nigeria
Phone: 234-69-220188
CONTRIBUTORS
Kelly Theisen is the principal contributor to the initial (2006) Nigeria chapter of the Sweetpotato Atlas.
REFERENCES
Arene, [C.J] Population Pressure, Agricultural Intensification, and the Sustainability of Cropping Systems in Anambra State of Nigeria. 1996. Rivista di Agricoltura Subtropicale e Tropicale (Italy) 90 (1): 75-84.
Egrinya Eneji, A.; [A.A] Agboola; [B.E] Ubi. 1997. Effect of Farmyard Manure and NPK Fertilizers on Growth and Yield of Intercropped Maize (Zea mays L.) and Sweet Potato (Ipomoea batatas L.) in Southwestern Nigeria. Rivista di Agricoltura Subtropicale e Tropicale (Italy) 91 (1): 63-78.
FAOSTAT. (Agriculture, Agricultural Production, Crops Primary, Sweet Potato; Population, Annual Time Series).
Fenby, [N.S]; G. D. Foster; [R.W] Gibson; [S.E] Seal. 2002. Partial Sequence of HSP70 Homologue Gene Shows Diversity Between West African and East African Isolates of Sweetpotato Chlorotic Stunt Virus. Tropical Agriculture 79 (1): 26-31.
Goldman, Abe. 1996. Pest and Disease Hazards and Sustainability in African Agriculture. Experimental Agriculture (UK) 32: 199-211.
Huaccho, Luisa and Robert Hijmans. 2000. A Geo-Referenced Database of Global Sweetpotato Distribution. International Potato Center. Lima, Peru. (This article is not currently available. A subsequent article, of 2002, is available via subscription to ISHS/ Acta Horticulturae).
Huntington Library. Plant Trivia Timeline.
Kreuze, [J.F], [R.F] Karyeija, [R.F] Gibson, [J.P.T] Valkonen. 2000. Comparisons of coat protein gene sequences show that East African isolates of Sweet Potato Feathery Mottle Virus form a genetically distinct group. Archives of Virology 145:567-574.
Osagie, [A.U] 1991. Post Harvest Biotechnology of Root and Tuber Crops. In: Tropical Postharvest: Proceedings of the Short Course on Reduction of Postharvest Losses of Perishable Crops. University o Benin, Benin City, Nigeria, December 1-14, 1991.
Purseglove, [J.W] 1968. Tropical Crops: Dicotyledons. Longman Group Limited. Essex, England.
Shiotani, Itaru; Shuichi Oyagi; Saori Okumura. 2000. Inheritance of Resistance to the Root-Knot Nematode in the Tetraploid Cytotype of Ipomoea trifida ([H.B.K]) Proceedings of the Twelfth Symposium of the International Society for Tropical Root Crops: Potential of Root Crops for Food and Industrial Resources. Editors: Makoto Nakatani and Katsumi Komaki. ISTRC
Sharma, Shashi B.; Nigel S. Price; John Bridge. 1997. Nematology Abstracts. Volume 66, No. 3. CAB International. (Full access requires an on-line subscription to CABI Publishing.
Sim, J.; [R.A] Valverde; [C.A] Clark. 2000. Whitefly Transmission of Sweetpotato Chlorotic Stunt Virus. Plant Disease 84 (11): 1250. Published on-line as D-2000-0915-01N, 2000.
Tewe, [O.O]; [O.A] Abu; [E.F] Ojeniyi; [N.H] Nwokocha. 2001. Sweetpotato Production, Utilization, and Marketing in Nigeria. In: Akoroda, [M.O] and [J.M] Ngeve, eds. Root Crops in the Twenty-first Century. Proceedings of the Seventh Triennial Symposium of the International Society for Tropical Root Crops - Africa Branch, Cotonou, Benin. October 11-17, 1998.
US LOC (United States Library of Congress). Country Studies: Nigeria