Introduction

Yams (Dioscorea spp.) are herbaceous food crops having twine and belong to the family Dioscoreaceae. It is considered the second most important root and tuber crop globally grown behind cassava based on the daily calorie supply and its production (Nweke et al., 1991). Yams are the fourth of the top 10 major crops cultivated in Nigeria, which follows guinea corn, cassava, and maize (Statista, 2019). Yams are described as climber plants that are tuberbearing and can be classified as a monocotyledonous annual or perennial crop with more than 600 species worldwide (IITA, 2009). It is grown on about 5 million hectares in 47 countries in the world mainly within the continent of Africa, Asia, South America, and North America (Acquaah, 2005;Vincent et al., 2015). The West African region contributes about 94.63 percent of global production (FAOSTAT 2022) mostly from Nigeria, Ghana, Benin, Cote D’Ivoire, and Togo.

In Nigeria, only a few yam species are considered edible and widely cultivated. Nigeria is regarded as the largest producer of yam both in Africa and globally with a production of 45.41 million MT which is 68.4% and 66.9% respectively as seen in Figure 1 (FAOSTAT 2022). It is predominantly grown in Benue, Taraba, and Nasarawa states where it is mostly exported to other countries. Production of yam in Nigeria is across regions with sufficient sunshine, rainfall, and soil fertility to support growth which includes the southern guinea savanna, forest, and derived savanna ecological zones as seen in Figure 3 (Nweke et al., 1991). Due to the ecological requirement of the different cultivars, it possesses the ability to thrive in these different ecological zones. The annual rainfall requirement for yam production is between 1168.4 mm and 2500 mm (Adegbite et al., 2006). The peculiar rainfall duration of lesser than 6 months in the Sahel and Sudan savanna of Nigeria, makes the region not suitable for yam production (Fig. 3). Yam is an important staple crop of Nigerians which supplies about 258 dietary calories per day, therefore, is believed to be key to food security (IITA, 2009). It chiefly supplies carbohydrates plus a good source of essential amino acids, minerals (phosphorus, calcium, and magnesium), and vitamins as food crops (Mudiope et al., 2012). However, in East Asia, yam is cultivated solely for medicine (Kwon et al., 2016).

There are differences in yam species mostly cultivated in Nigeria and Korea based on their uses and origin. In Korea, grown yams are D. polystachya and D. japonica which are of East Asia origin and important in herbal medicine (Hur et al., 2008;Cao et al., 2021). However, in Nigeria, yams mostly grown are D. alata and D. rotundata from Southeast Asia and West Africa respectively, and important for food while D. alata sometimes is used as herbal medicine (Otegbayo et al., 2005, Sugihara et al., 2020).

Although Nigeria’s yam production has increased over the years, is yet to attain optimum production as the achieved yield is significantly lower than the potential yield of grown varieties. Most yields from West African countries exceed Nigeria’s yield in farmers’ plots. The average yield in Ghana is 18.22 t/ha, in Benin is 13.76 t/ha while Nigeria produced 7.94 t/ha in 2020 (FAOSTAT 2022). There are varying factors accounting for the yield gap experienced in Nigeria’s yam production.

This review extensively examines the importance of yam in Nigeria’s food security and identified contributing and limiting factors to optimum production. This will guide further studies on effective management strategies to attain sustainable yam production.

Cultivation Practices of Yam in Nigeria

Yam is best cultivated on sandy loam soil that possesses good water-holding ability and organic matter content. The propagation of yam is mostly through yam setts from earlier harvested tubers grown on ridges with mulching to cushion against desiccation and excessive heat. The size of the yam sett affects the overall yield during harvesting. There are two planting dates for yam in Nigeria: Early yam is between November and December, and Late yam is between March and April. Stakes of different sizes are provided to support vegetative growth, for better exposure to sunlight, and to increase tuber yield by over 30% when compared to non-staked yam. Weeding is labor-intensive and done thrice before harvesting. Harvesting is within 6 to 12 months depending on the cultivar. The indicator of maturity before harvesting includes cessation of vine growth, fully flowering plant, and basal leaf color change to yellow. During harvesting, tubers are dug out from the soil with the aid of diggers, hoes, spades, and cutlasses. Yam tubers are most times stored under ambient temperature for a duration of 4-6 months (Sahore et al., 2007) to achieve more profit during sales. A study recommended 300kg/ha of NPK fertilizer at 8-12 weeks after planting for optimum yam production (Law-Ogbomo and Remison, 2009). A pre-emergence herbicide recommended for weed control is atrazine at the active ingredient dose of 2-3 kg/ha (Korieocha, 2014).

Adoption of Yam as a major crop in Nigeria

Out of the 10 edible yam types globally grown, six are cultivated in Nigeria. In Nigeria, guinea (white (Dioscorea rotundata) and yellow (Dioscorea cayenensis)) yam and water yam (Dioscorea alata) are widely cultivated. The minorly grown yams are the Aerial (D. bulbifera), sweet yam (D. dumetorum), and lesser (D. esculenta). They all belong to four taxonomic groups which are Enantiophyllum , Opsophyton, Lasiophyton, and Combilium (Lebot and Dulloo, 2021). West Africa is believed to be the origin of D. rotundata before it went through the domestication process that resulted in many landraces within Nigeria (Tamiru et al., 2007). An earlier study identified Nigeria and Togo as countries with the highest genetic diversity of D. dumetorum hence, considered centers of diversity and origin (Sonibare et al., 2010). The guinea yam is believed to emerge from the domestication process of wild yams belonging to the Enantiophllum group.

Yam production is widely adopted across the middle belt, South-South, South-East, and South-West regions of Nigeria among the Tivs, Efik, Ibibios, Igbos, and Yoruba cultural groups (Obidiegwu and Akpabio, 2017). Farmers are encouraged to produce this crop due to its high demand as the “King of food” in most ethnic groups’ social functions such as traditional ceremonies, marriage, rituals, new yam festival, or burial (Obidiegwu and Akpabio, 2017). It is a source of income with a potential impact on foreign exchange based on income generated from exporting yam worth US$ 27.7 million to the United States in 2011 (Babatunde, 2012). Yam produced in Nigeria is mostly exported to United Arab Emirates, Canada, the United States, and India. Two different profit analysis study of yam production per hectare in Nigeria in 2013 reveals a 202.44 % profit with a gross margin of ₦463,039.88K, and another was ₦450,000 which was US$2,000 equivalent (Simpa and Nmadu 2014;Asala and Ebukiba 2016).

The sociocultural preference for yam food products and increase in human population, and the size of land devoted to yam production account for the doubled production quantity between 1999 and 2020 (Figure 1).

Constraints on Yam Production in Nigeria

Biotic Stress

The activities of weeds, pathogens, insects, and pests on yam farmland limit average yield. Insect pests and diseases constitute a major constraint to yam production in Nigeria. Every stage of development of yam (from seedling to harvesting or storage) is susceptible to disease infection. Diseases affecting yam production are classified mainly into field and storage diseases. Earlier reports established that about 50% of harvested yam tubers are lost during storage (Onayemi, 1983) due to microbial attacks permitted through earlier wounds before or during harvesting (Amusa et al., 2003). That is, the postharvest losses are due to insect attack, sprouting, respiration, and bacterial and fungal infestation leading to rotting, (Aboagye-Nuamah et al., 2005;Ajayi and Olorundare, 2014). Viruses affecting yam production belong to about 3 genera which include Potyvirus, Cucumovirus, and Badnavirus while others remain unclassified (Amusa et al., 2003;Asala et al., 2012;Diouf et al., 2022). A very high incidence rate and severity are associated with Yam mosaic virus (YMV) across yamgrowing regions of Nigeria. Aphids transmit potyvirus that infects different yam cultivars. Symptom expressions are green spotting, chlorotic spotting, mosaic, vein banding, mottling, curling, and symptomless (in cases of latent infection) (Amusa et al., 2003). Activities of viruses on yam limit its exchange as germplasm internationally (Asala et al., 2012). Prevalent plant parasitic nematode (PPN) in yam-growing regions of Nigeria belongs to about 14 genera. However, Pratylenchus spp, Scutellonema spp, and Meloidogyne spp remain the most widely distributed across Nigeria (Kingsley, 2021). They affect yam both on the field and in storage. They cause root galling, root lesions, and dry, and soft rots depending on the type of PPN. The Xiphinema spp is a typical PPN that acts as a vector of viruses that pose secondary infection in the field. In severe cases of PPN, foliar symptom expression involves early yellowing and leaf senescence cessation of vine growth (Amusa et al., 2003). The most devastating diseases causing agent in storage is the dry rot of yam while the most widely spread disease-causing agent in the yam field is Anthracnose. Anthracnose diseases are mostly caused by Colletotrichum gloeosporiodes (fungus). Rots caused by Aspergillus flavus, Aspergillus oryzae, Aspergillus niger, Fusarium moniliforme, Fusarium oxysporium, and Fusarium culmorum affect yam in storage (Amusa et al., 2003;Ajayi and Olorundare, 2014). They account for a postharvest loss of about 20-60% (Okigbo and Ikediugwu, 2000). This rot can be dry, wet, or soft rot. In Southwest Nigeria, Glomerela angulate causes field anthracnose disease also called ‘scorch disease’. Symptoms include leaf lesions that are small black or dark brown and could be large-scale necrosis or die-back of the stem and withered leaves in severe cases. Concentric leaf spot diseases are caused by Sclerotium rolfsii a folial yam disease with circular leaf spots forming a concentric ring (Amusa, 2001). Bacteria belonging to about 4 genera which include Micrococcus spp, Bacillus spp, Staphylococcus spp, and Enterococcus spp (Ajayi and Olorundare, 2014) affect yam production. Infected yam tubers when pressed ooze out whitish fluid with is unique to wet rot. This wet rot is associated with Erwinia carotovora pv (Amusa and Baiyewu, 1999). Summarily, yam disease agents adversely affect yam production by reducing both the quality and quantity of yam produce and rendering them unappealing to consumers with possible economic loss to farmers.

Different forms of insect pests such as tuber and leaf beetles, mealy bugs, scale insects, termites, and crickets affect both on the field and in storage (Ogbonna, 2019). They create holes into tubers which enhance the activities of other pathogens such as fungal infestation that results in tuber rots. Grass cutters and bush rats also affect yam both on the field and in storage (Ogbonna, 2019). They dig into heaps to bring out planted yam sett or uproot growing tubers. Parasitic weeds such as Striga hermonthica and other non-parasitic weeds with rapid growth under staking limit yam production. The severity of weed effect on yam production occurs in cases of prolonged weed interference over a period of 3-4 months with a 54-90% yield reduction (Akobundu, 2006).

Abiotic Stress

Climate change significantly creates a varying pattern of precipitation and increased temperature across the different agroecology of Nigeria. This poses a great challenge for Nigerian farmers in compensating for the changes as they practice rain-fed farming. There have been frequent occurrences of flooding in yam producing region most especially in North Central which result in massive loss of farmland and displacement of farmers. A series of drought events in Nigeria resulted in the shift of agroecology with the physiological stress of crops grown (Shiru et al., 2020). Nigerian soils are characterized by diminishing soil fertility due to intense farming activities and less supply of fertilizer (Aniekwe and Mbah, 2014). Most of the soil under yam cultivation in the North-central and Southeast regions of Nigeria is found to have reduced Nitrogen, soil organic matter, and cation content which assures drastic yield decline with continuous yam cultivation (Neina, 2021). The record of yam yield over the years shows a decline from 9.4 t/ha in 1988 to 7.94 t/ ha in 2020 (FAOSATAT 2022), mostly attributed to decrease in soil fertility and biotic stress. A decline of at least 10% and 12% in white yam and yellow yam respectively are observed in the subsequent year of production (Neina, 2021). All of these suggests the need for augmentation of nutrient through fertilizer application. A study on the importance of fertilizer application on yam production presented an 80% increase in yield per hectare with NPK fertilizer application at the rate of 300 kg/ha when compared to zero fertilizer application (Law- Ogbomo and Remison, 2009).

Other Limiting Factor

Poor government policies such as the removal of government aid on the purchase of fertilizer lead to the inflation of prices which makes it unaffordable to Nigerian farmers (Aniekwe and Mbah, 2014). The frequent armed conflict in Northern Nigeria also militates against sustainable yam production due to threats to life. Agricultural inputs and resources are insufficient to close on yield gap. Most farmers lose a great percentage of harvested yam due to the inability to provide ambient conditions at the recommended storage temperature of about 12°-16 °C during storage. Farming activities in yam farms are purely labor intensive in the absence of farm machinery. The high cost of labor, staking, and improved yam cultivars limit production. Farmers lack access to funds to support farming activities. They recycle earlier harvested tubers which are easily perpetuated by pests and pathogens. Consumer demand for a few selected yam cultivars resulted in massive production of these preferred cultivars with biodiversity loss of other potentially important yam cultivars in Nigeria.

Yam Breeding and Improvement activities in Nigeria

The different constraints of yam production enhanced a concerted effort by the International Institute of Tropical Agriculture (IITA) and the National Root Crops Research Institute (NRCRI), Umudike, Nigeria to produce highyielding and resilient varieties. In achieving genetic improvement in yam, several studies were carried out to understand the complexity of the yam genome. This involves the characterization of germplasm and advancement in molecular markers designated in genome analysis. Also, linkage maps were constructed to identify quantitative trait loci in genes conferring resistance to Guinea and water yam against anthracnose and Yam mosaic virus (YMV) (Mignouna et al., 2003). Presently, IITA has about 5,839 germplasm accessions used in the yam improvement program. These germplasms include Nigeria’s 1,881 accessions (1,535 D. rotundata, 239 D. alata, and 40 D. cayenensis and others).

The institute IITA has extensively selected and bred Guinea yams since the 1970s with more focus on genetically improving D. rotundata and D. alata (Mignouna et al., 2003). The clones and populations of both were developed by IITA and subjected to further evaluation and selection across different agroecological zones of Nigeria. The NRCRI further evaluated the IITA-derived breeding line before the final release of about 21 varieties, some of which are shown in Table 1. Wild yam is regarded as crucial in future breeding, hence conserved. Landrace cultivars that possess resistance against viruses of yam were also identified.

Summarily, the goal of the breeding activities is to provide yam varieties that have stable and high tuber yield, resilient against pest and diseases, conform with consumers’ preference for tuber characteristics (culinary quality and size), and has a higher dry matter (Mignouna et al., 2007). A study projected at least a 30% yield increase if the improved varieties were adopted, and farmers utilizes sustainable technologies (Nwogha, 2022).

Future Directions and Conclusion

Although Nigeria’s yam production is thriving, there is a need to harness its potential to achieve food security. The indigenous practice of Nigerian yam farmers in managing the biotic stress of yam through roughing, timely planting and harvesting, weeding, burning, crop rotation, intercropping, and the use of botanical pesticides such as extracts from trees, wood ashes, and tree bark should be complemented with the adoption of released improved varieties. The build-up of this biotic stress is associated with the recycling of susceptible harvested yam tubers which makes farming unprofitable. The institutes IITA and NRCRI need to breed more improved yam cultivars that are resilient under drought, flooding, and other conditions that are linked with the current severity of climate change impact in Nigeria. Planting of disease-free yam setts assures tuber quality and quantity during harvest. Hence, the certification of planting material through rapid field tests is needed to limit yam infections (Asala et al., 2012). There is no updated information on the losses associated with biotic stress, therefore, the impact of weeds, pests, and diseases on yam production must be studied. Nigeria needs to fully explore more in utilizing active compounds in yam constituents to achieve therapeutic treatment of illnesses just like the Asians. There is a need for further studies on D. batatas and D. japonica grown in Korea for a better understanding of their nutritional benefits to humans and for genetic improvement.

In conclusion, this review showcases the importance of yam production and its product in Nigeria’s food security with the identification of both determinants and constraints. The socioeconomic relevance of yam in Nigerian society places a high demand on its production. The constant increase in yam production in Nigeria is due to an increase in available farmland dedicated to yam cultivation. Lack of finance, low soil fertility, pest and disease pressure, and high cost of labor and staking remains the major constraint to yam production in Nigeria. To boost productivity, government support should be encouraged through access to loans, provision of storage facilities for yam-producing communities, and subsidies on farm inputs such as fertilizers and planting materials. Nigeria farmers regard yam quality as tubers sustainably generating income and suitable for chief yam products while disregarding other indicators such as nutritional value or bio- mass of other yam parts except for tubers, hence limiting the production of some important yam cultivars (Otegbayo et al.2010). The Nigerian government, however, is supporting yam production through breeding programs for improved varieties, minisett technology development, and training of farmers via on-farm demonstrations on soil nutrient management approaches, and the adoption of released yam varieties.

적 요

마(yam)는 나이지리아에서 영양학적 가치 뿐만 아니라 사회 문화적 및 농민의 소득창출원으로 중요한 역할을 하는 덩이줄 기 작물이다. 이러한 국내외적 수요에 따라 현재 나이지리아 는 4,541만톤의 마를 생산하여 세계 마 생산량의 67%를 차지 하는 마의 주요 생산국이다. 본 논문에서는 나이지리아에서 마 생산과정의 문제점과 원인을 구명하고 생산성 증대를 위한 방 안을 제시하고자 한다.