Yams (Dioscorea spp) are starchy staples and a primary source of income in West Africa, which produces 94% of the world’s yams.
Nigeria alone accounts for 70% (Food and Agriculture Organization [FAO], 2016) of global yam production, equivalent to 50 million metric tonnes.
On a per-capita basis in West Africa, consumption is highest in Cote d’Ivoire, followed by Benin, Ghana, Nigeria and Togo.
Yam production is constrained by the unavailability of seeds and yam’s slow rate of propagation (less than 1:6), which is vegetative and accumulates diseases.
The seeds alone account for up to 63% of production costs, and approximately 30% of harvested yams are used in subsequent plantings as seed tubers, thus impacting yield for ware yams.
The use of unclean planting materials, preinfected by viruses, anthracnose and nematodes, significantly reduces yield, which directly affects smallholder farmers who grow yams for their livelihood.
Research breakthroughs from the Bill & Melinda Gates Foundation-funded project, Yam Improvement for Incomes and Food Security in West Africa, Phase II (YIIFSWA II), have demonstrated increased propagation rates of seed yams up to a thousandfold.
In the traditional method, one tuber is cut into pieces to generate a maximum of six setts (tuber pieces); for example, in three consecutive years, the traditional method (six by six by six) would produce 216 setts.
To overcome the challenge of inadequate availability of clean planting materials, researchers under YIIFSWA II used meristem culture combined with standardized heat therapy, disease diagnostics and quality management protocols to generate clean stocks of breeder seed yam plantlets.
These were further propagated using the Plantform Temporary Immersion Bioreactor System (TIBS), which also reduced the cost of producing the clean seed.
A perforated Vivipak was used for hardening the planting materials, which allows gradual acclimatization to the outside environment.
The aeroponics system (AS) is another method using TIBS plantlets in screen houses to generate, on average, 300 single-node vine cuttings per plant.
In a maximum of 45 days in nursery, the single vines grow into seedlings that are transplanted in the field and produced different categories of tubers.
Details of single-node vine seedlings multiplication rate and productivity can be found in the International Institute of Tropical Agriculture (IITA)/CIGAR presentation on single-node vines.
The hydroponics system (HS) is like aeroponics, but generates, on average, 100 single-node vines in two months.
The hydroponics combined with aeroponics generate a minimum of 100 seedlings per TIBS plantlet every three months, with 95% rooting success in field, to produce six different sizes of tubers with three years’ combined averages respective of 1,420 grams, 680 grams, 375 grams, 207 grams, 80 grams and 20 grams.
The Adaptive Yam Minisett Technology (AYMT) was also improved and recommends the use of 50-gram minisetts from single-node seedling tubers harvested from the field at six months after planting.
In combination, these technologies have increased the propagation rate of seed yams. In the first year, one tuber is cut into six portions each, giving 30 nodes, which are cleaned using conventional tissue culture (CTC) to produce 180 plantlets.
These are then propagated in TIBS during the second year every three months using a minimum propagation ratio of 1:3.
Two cycles of propagation yielded 1,620 planting materials from 32 units of TIBS. Upon hardening using the Vivipak system, 3,240 plantlets were produced, each of which gave at least 100 seedlings, making 324,000 tubers in the AS or HS.
In the third year, applying AYMT to the 324,000 tubers at a ratio of 1:3 gave at least 972,000 certified seed yams compared to the 216 using traditional propagation.
This calculation did not take into consideration the seed yam tuber produced in AS/HS that is, on average, three tubers per plant.
With the thousandfold increase in planting materials because of using TIBS, YIIFSWA out-scaled TIBS to more national agricultural research institutions (NARIs), reaching 288 in total in the year 2020.
These can potentially produce 28,800 in two cycles, or 57,600 plantlets generating 5.76 million single-node vine seedlings when grown in AS/HS.
The resulting foundation seed yam production is estimated at 17.28 million seed yam tubers of 50 grams.
Public research institutions that received the support for breeder seed production were Crops Research Institute and Savannah Agricultural Research Institute in Ghana, and National Center for Genetic Resources and Biotechnology and National Root Crops Research Institute in Nigeria.
AS and HS had been scaled out to private seed companies for foundation seed production. These included equipment support, human resource capacity building and provision of initial clean stock of planting materials.
It is now necessary to time seed yam production cycles in a cropping calendar that fits within the context of agro-ecologies to avoid a glut along the seed value chain.
Production research that will preserve vegetative propagules in an easily recoverable manner and achieve adequate control of seed yam tuber dormancy is also needed.
Increased sensitization among relevant stakeholders and end users is now being undertaken to enhance the uptake of these technologies and ensure smooth interaction between the public and the private sector players.