agronomy
Article
Seed Yam Production Using High-Quality Minitubers Derived
from Plants Established with Vine Cuttings
Beatrice Aighewi 1,*, Norbert Maroya 2, P. Lava Kumar 3 , Morufat Balogun 3,4, Daniel Aihebhoria 1,
Djana Mignouna 2 and Robert Asiedu 3
1 International Institute of Tropical Agriculture (IITA), Kubwa, Abuja 901101, Nigeria; d.aihebhoria@cgiar.org
2 International Institute of Tropical Agriculture (IITA), 08 BP 0932 Trip Postal, Cotonou, Benin;
n.maroya@cgiar.org (N.M.); d.mignouna@cgiar.org (D.M.)
3 International Institute of Tropical Agriculture (IITA), PMB 5320, Oyo Road, Ibadan 200001, Nigeria;
l.kumar@cgiar.org (P.L.K.); m.balogun@cgiar.org (M.B.); r.asiedu@cgiar.org (R.A.)
4 Department of Environmental Biology, Faculty of Agriculture and Forestry, University of Ibadan,
Ibadan 900001, Nigeria
* Correspondence: b.aighewi@cgiar.org; Tel.: +234-706-600-3489
Abstract: Yam (Dioscorea spp.) is a valuable food security crop in West Africa, where 92% of the
world production occurs. The availability of quality seed tubers for increased productivity is a major
challenge. In this study, minitubers weighing 1, 3, and 5 g produced from virus-free single-node vine
cuttings of two improved yam varieties (Asiedu and Kpamyo) growing in an aeroponics system were
assessed for suitability in seed production at a population of 100,000 plants ha−1. A 3 × 2 factorial
experiment with randomized complete block design and three replications was set up during the



 cropping seasons of 2017 to 2019 at the International Institute of Tropical Agriculture Research Station


in Kubwa, Abuja, Nigeria. Results showed field establishments of 87%–97.8%. Yields differed with
Citation: Aighewi, B.; Maroya, N.; minituber size, variety, and cropping season; the highest was 31.2 t ha−1 in 2019 and the lowest,
Kumar, P.L.; Balogun, M.; Aihebhoria,
10 t ha−1 in 2018 from 5 and 1 g Kpamyo minitubers, respectively. The estimated number of tubers
D.; Mignouna, D.; Asiedu, R. Seed
produced per hectare by 1, 3, and 5 g minitubers was 101,296, 112,592, and 130,555, with mean
Yam Production Using High-Quality
Minitubers Derived from Plants weights per stand of 159.2, 187.3, and 249.4 g, respectively. We recommend using less than 6 g
Established with Vine Cuttings. minitubers for seed yam production due to their high multiplication rates.
Agronomy 2021, 11, 978. https://
doi.org/10.3390/agronomy11050978 Keywords: Dioscorea rotundata; minituber; seed yam; production
Academic Editor: Erin N. Rosskopf
Received: 23 April 2021 1. Introduction
Accepted: 11 May 2021 Yam is a tuber crop of the genus Dioscorea and the family Dioscoreaceae, of which
Published: 14 May 2021 the most important species used as food are D. rotundata Poir (white yam) and D. alata
L. (water yam). It is an important staple crop that is mainly grown in tropical regions
Publisher’s Note: MDPI stays neutral of West Africa, the first and Asia the latter. Yam is produced primarily in the savannah
with regard to jurisdictional claims in region of West Africa, which contributes 92% (66.8 million tons) of the world yam pro-
published maps and institutional affil- duction of 72.6 million tons [1]. Nigeria, Ghana, and Côte d’Ivoire together have 86% of
iations.
world production.
Yam plays a vital role in enhancing food security and alleviating hunger in many parts
of West Africa [2]. It contributes substantially to the human diet as a significant source
of energy. The yam value chain provides income-earning opportunities to its producers,
Copyright: © 2021 by the authors. processors, and sellers. It is also a source of pharmaceutical compounds, such as saponins
Licensee MDPI, Basel, Switzerland. and sapogenins [3], and its peels can be used as feed for livestock [4]. Yam is the only crop
This article is an open access article that is usually celebrated during and after harvest with traditional festivals [5–7] in the
distributed under the terms and West Africa region.
conditions of the Creative Commons Yam productivity has been on the decline despite the increasing demand for local
Attribution (CC BY) license (https://
consumption and export. This decline in productivity can be attributed to several problems:
creativecommons.org/licenses/by/
high labor demand, high cost of cultural operations, pests and diseases, declining soil
4.0/).
Agronomy 2021, 11, 978. https://doi.org/10.3390/agronomy11050978 https://www.mdpi.com/journal/agronomy
Agronomy 2021, 11, 978 2 of 13
fertility, unpredictable weather conditions, and the lack of quality planting material [8,9].
In Nigeria, the scarcity of quality planting material is critical, affecting both commercial
and subsistence production. The high cost of planting materials, which can be up to 62%
of the total production outlay [10], encourages the competition between edible and seed
tubers [11]. Hence, the need to seek alternative propagation methods to resolve issues
related to the availability of planting materials, especially on a commercial scale.
In a pre-project study of the “Yam Improvement for Income and Food Security in West
Africa (YIIFSWA)” project of the International Institute of Tropical Agriculture (IITA), it
was noted that in West Africa, out of the attainable yam yield of 22 t ha−1, there was a gap
of 11.2 t ha−1 due to several factors. The most important of these are poor quality seed, low
soil fertility and use efficiency, drought, weeds, and diseases, all contributing to 9.1 t ha−1
of the yield gap [12]. Virus infection specifically contributed to almost 10% of the yield
gap. With this background, the YIIFSWA project had an objective to establish viable and
sustainable yam seed systems in the two highest yam producing countries, Nigeria and
Ghana. The project started by cleaning available planting materials of viruses before the
investigations into various techniques of multiplying the crop commenced.
Conventionally, yam is propagated using tubers, which also serve as food. Large
chunks of tubers weighing up to 500 g are planted to harvest both ware and seed yam.
In this method, the seed is sorted from the ware yam, thus reserving about 150–1000 g
tubers to plant the next crop. In traditional systems, the tubers multiply slowly with
a multiplication ratio of 1:3; hence there is a perpetual under-supply of seed tubers at
planting time, especially for first-time yam producers and farmers seeking to expand
production. The yam minisett technique (YMT) of propagation was developed [13,14] to
improve multiplication and seed availability. It involves cutting yam tubers of less than
1000 g into small pieces of 25–30 g to produce seed tubers, which are then used for ware
yam production. This technique was the first attempt to separate the seed from ware yam
production, increasing the sett multiplication ratio to 1:30. Another benefit of this technique
was reducing the number of yam tubers used as planting material instead of food.
An attempt was made to produce minitubers using a progeny of microtubers from
in vitro derived plantlets [15]. Ahloowalia [16] similarly produced small whole tubers
by growing micro-propagated potato plants or micro-tubers in vivo in green- or screen
houses, or directly in the field. Seed yam production using minitubers was intended to
build a bridge between in vitro rapid multiplication and regular field production of seed
tubers. Producing seed yam using minitubers was developed in 2003 by the National Root
Crops Research Institute (NRCRI), Umudike, Nigeria, as a backup for the YMT [17]. The
NRCRI study summarized yam tuber size classifications by different authors as follows:
<50 g = microtubers; 50–99 g = mini seed yam (minitubers); 100–249 g = seed yam grade 2;
250–1000 g = seed yam grade 1; and >1000 g = ware yam. The minitubers were reported to
produce seed yams of up to 900 g when planted in good soil. It was also claimed that the
minituber production technique was to enhance uniformity in seed yam sizes for export
and to reduce the scarcity and high cost of seed yams often experienced by farmers [18].
However, research on increasing yam’s multiplication rate and preserving more tubers
for food has continued. Earlier attempts to use yam vines for propagation showed that
it was possible. Still, several challenges limited the degree of success related to the poor
establishment and the tiny tuber size produced [19,20]. Recent work at IITA has shown that
yam vine cuttings can be used successfully with novel techniques for large-scale seed yam
production. The plantlets from temporary immersion bioreactors (TIB) are planted in an
aeroponic system (AS) and other hydroponics systems to rapidly propagate the crop [21,22],
producing different sizes of tubers, as well as vines used for further multiplication as single-
node cuttings. These methods can increase the multiplication ratio to 1:300 and more. In
the AS, plant roots grow continuously or discontinuously in an environment saturated with
a nutrient solution mist [23]. Freshly cut two-node vine cuttings or pre-rooted single-node
vines planted in the AS produced minitubers ranging from 0.2 to 110 g depending on
the genotype, plant age at harvest, and composition of the nutrient solution [21]. The
Agronomy 2021, 11, 978 3 of 13
system allows for a rapid multiplication rate and enhances seed yam production in the
field, reducing the scarcity and high cost of both seed and food tubers [3].
This study’s objective was to investigate the yield potential of whole minitubers below
6 g, derived from virus-free vine cutting seedlings obtained from yam plants grown in an
AS. Unlike the classification given by NRCRI [17], the below 6 g tubers used in this study
are referred to as minitubers because microtubers are considered to signify products of
micropropagation in tissue culture.
2. Materials and Methods
The trials were conducted at the Kubwa station of IITA, Abuja, Nigeria, during
three cropping seasons from 2017 to 2019. The research station is in Nigeria’s “Yam Belt”
at 9◦09′51.6′′ N, 7◦20′44.6′′ E, and 424 m above sea level. During the experiments, the
weather data collected showed that rainfall started in March and stopped in October in
2017 and 2018. In contrast, rain fell from February to November in 2019 (Figure 1), and
the total rainfall during the cropping period was 874.2, 1214.7, and 1768.2 mm in 2017,
2018, and 2019, respectively. Monthly rain and temperatures during the cropping seasons
are presented in Figure 1. In all the years, the experimental fields were fallowed with
Aeschynomene histrix Poir., then Mucuna spp., before the experiment was planted.
Figure 1. Rainfall and temperature during the cropping seasons from 2017 to 2019 at International Institute of Tropical
Agriculture Station, Abuja, Nigeria.
Minitubers of less than 6 g of the yam varieties Kpamyo and Asiedu were used for the
experiments. The minitubers were obtained from vine-cutting-derived plants. During the
previous year of each trial, yam vines were harvested from virus-free plants in an AS. The
vines were cut into single-node cuttings, each with one leaf or two leaves, and planted in
black polythene nursery bags or nursery seed trays (Figure 2). Minitubers were harvested
three months after planting from the cuttings that rooted but did not produce any shoot.
The minitubers were stored in a cool and well-aerated room until dormancy was broken
after about three months, evidenced by the appearance of sprouts on the tubers. They were
Agronomy 2021, 11, x FOR PEER REVIEW 4 of 12 
 
in black polythene nursery bags or nursery seed trays (Figure 2). Minitubers were har-
Agronomyv20e2s1t,e1d1, 9t7h8ree months after planting from the cuttings that rooted but did not produce any 4 of 13
shoot. The minitubers were stored in a cool and well-aerated room until dormancy was 
broken after about three months, evidenced by the appearance of sprouts on the tubers. 
They were then sorttehde ninstoor ttehdreinet ocatthergeoerciaetse gboyr iseiszeb:y 0s.i1z–e1: .09.91 –g1,. 929.0g–,32.9.09– 3g.,9 9angd, a 4n.d0–45.0.9–95 .g99, g, averaged
averaged and referreandd tore ifner rtehdist opianptehri sapsa 1p,e 3r,a asn1d, 3 5,  agn,d re5sgp, erecstpiveecltyiv.e Tlyh. eT hmeimniitnuitbuebresr swweerree treated with
treated with a chemiaccahl emiixc aclomnitxaicnoinntgai n1i0n0g g1 0o0f gMoafnMcaonzceobz eabndan 4d04 m0 mL Lofo fcycyppeerrmetthriin  iin 1 0 L of water
10 L of water for 10 mfoirn1 t0o mprinotteocpt raogtaeicnt satg raoint satnrdo tpaensdt dpaemst adgaem. aTghee.yT wheeyrew aeirre-dairri-eddr iuednduenrd er shade for
shade for about 24 ha boefuotr2e4 phlabneftoinregp ilna nthtien gfiienldth. e field.
 
Figure 2. Production of minitubers. 1 = Yam plants in an aeroponics system; 2 = Nodal yam cuttings planted in a polystyrene
nurFseigryutrrea y2.( CPurottdinugcstiwointh oofu mt sihnoitoutsbweresr.e 1a l=l oYwaemd tpolapnrotsd uince amn ianeitruobpeorsn,itchso ssyesttheamt f;o 2rm = eNdoshdoaol tysawmer ceutrt-ansplanted to
thetfiinelgds) ;p3la=nMteidn iitnu bae rpsoplyrosdtyurceendea fnteurrstherreye tmraoyn t(hCsuftrtoimngssi nwgilethloeauftc suhttoinogtss w(weirthe oaulltoswhoeodts t)o; 4 p=roSdpurocuet ed minitubers
usemd iinnitthuebeexrpse, rtihmoesnet .that formed shoots were transplanted to the field); 3 = Minitubers produced after 
three months from single leaf cuttings (without shoots); 4 = Sprouted minitubers used in the exper-
iment. At the beginning of each season, poultry litter was applied at the rate of 10 t ha−1 to
the experimental field before it was disc-harrowed and made into 3 m long ridges spaced
At the beginnin1g0 0ocfm eaacpha rst.ePaslaonnti,n pgowualtsrdyo lniteteorn wtoapso afpripdlgiesda at t3 –th4ec mradte potfh 1in0 sti nhgal−e1 rtow s and 10 cm
the experimental fielbde tbweefeonres tiat nwdasso dnis7c, -5h,a8rJruonweeidn a2n01d7 ,m2a0d18e, ianntdo 230 m19 ,lorensgp ercidtigveelsy s. pAac3emd single ridge
100 cm apart. Plantirnegp rwesaesn tdeodnae torena ttmopen ot/f prildotgaens datc o3n–t4a icnmed d3e0pmthin iintu sbienrgsl.eT rhoewsse eadnrda t1e0s for 1, 3, and−1
cm between stands o5ng 7m, 5in, i8t uJubenres iwn e2r0e−1 1
170,0 2,031080,, aanndd 5200019k,g rehsapec, trievseple×y
c.t iAve 3ly m, w siitnhgalep rlaidngt ep opulation of
100,000 plants ha . The experiment was laid out as 3 2 factorial, fitted into a randomized
represented a treatmceonmtp/plelotet balnodck cdoenstiaginn(eRdC 3B0D m) winitihtutbhereres.r eTphleic asetieodn sraanteds sfioxrt r1e,a 3tm, aenndts .5 g 
minitubers were 100, 300A, at nonde 5w00ee kkga hftaer−1p, lraenstpinegct(iWveAlPy), ,wthiethp lao ptslawnetr epsoppruaylaetdiowni tohfa 1h0e0r,b0i0c0id e mixture of
plants ha−1. The expePrriemeenxttr aw(a2s9 0lagidL/ oSu-Mt aesto 3la ×ch 2lo fra, 3c7to0rgia/lL, fAitttreadzi ninetaot 5a Lrahnad−o1)manizdedG rcaommox-one (200 g/L
plete block design (RPCarBaDqu) awt aitth5 tLhrheae− r1e),palfitceartwiohnisc hanadh asnixd -threaldtmhoeentws.a s used to remove weeds, earth-up
At one week afttheer pllannttsinangd (rWefAorPm),t htheer ipdgloetss.  Swtaekrien gsporfayyoeudn gwpiltahn tas hstearrbteidci6deW mAPixatnudrel asted for two
of Premextra (290 gw Le/eSk-Ms aestpollaanchtsloatrt,a 3in7e0d g4/–L6 Aoptreanzlienaev east. 5T hLe hstaa−k1)in agnwd aGs rdaomneouxosinneg (th20e0tr ellis system,
g/L Paraquat at 5 L whah−e1)r,e aroftpeers wwheircehti ead hbaentwd-eheenldtw hoosetr wonags buasmebdo toop roelmesopvlaec ewdeaetdtsh,e ebaergthin-ning and end
up the plants and refoofremac hthreid rgide,gaensd. Ssttraiknignsgu osefd ytooudnirge cptltahnetsp lsatnatrstetodt 6h eWroApPe. aFnedrt illaizsetr−1 ed
w afosra pplied at the
rate of 400 kg NPK (15:15:15) ha [24] at 8 WAP using the side placement method.
two weeks as plants attaFinroemd 4s–ix6d oapyesna flteearvpelsa.n Ttihnge (sDtaAkPin),gt hweansu dmobneer uofsienmge trhgeed trpellalnist ssywsa-s recorded at
tem, where ropes wfeoruer -tdieady ibnetetwrveaelsnu tnwtiol  3s8trDoAngP .bAatmeibgohot  wpoeelekss ,pslixacpeldan atst wtheer ebreagnidnonminlgy selected per
and end of each ridtgreea, tmanedn ts, tarnindgdsa tuasceodll etcot ediroencst tethme lepnlagnthts( mto) atnhde trhoepneu. mFebretriloifzleera vweassa nd vines per
applied at the rate opf l4an00t. kTgh eNlePaKf  a(r1e5a:1in5d:1e5x) (hLaA−I1) [w24a]s amt e8a sWurAedP auts1i2ngW tAhPe usisdineg ptlhaeceCmI-1e1n0t digital plant
method. canopy imager, capturing a 1500 fisheye image of the canopy (CID Bio-Science, Bozeman,
From six days aMftTe,r UpSlaAn).tiWngh e(Dn AviPn)e, sthenee nscuemncbeewr oasf ecommeprgleetde  aptlathnetse nwdaso frethceorsdeeadso ant, tubers were
four-day intervals uhnatrivl e3s8t eDdAonP1. 5A, 1t 9e,iagnhdt 1w2eDeekcse,m sibxe rpilnan20ts1 7w, 2e0r1e8 ,raannddo2m01l9yc oserrleescpteodnd pinegr to 27.3 WAP,
treatment, and data 2c8o.1lleWcAtePd,  aonnd s2t6e.m7 W leAnPg,trhe s(pmec)t iavnedly .thTeh ensuemedbmeru oltfip lleiacavteiosn arnadti ovi(nSMesR p) ewra s calculated,
plant. The leaf area ainsdsheoxw (LnAbeIl)o wwa: s measured at 12 WAP using the CI-110 digital plant 
canopy imager, capturing a 1500 fisheye image of thWee cigahSMR = ntoopfyh (aCrvIeDst Bediot-uSbceier n(sc)e, Bozeman, 
Weight of the planted material
MT, USA). When vine senescence was complete at the end of the season, tubers were har-
vested on 15, 19, and 12A Dlledceamtabweer riens u20b1je7c,t e2d01to8, thaneda n2a0ly19si scoorfrveasrpiaonncdein(Ag NtoO V27A.3)  uWsiAngP,t he statistical
28.1 WAP, and 26.7a WnaAlyPsi,s rseysspteemct(iSvAeSly, .2 0T1h6e). sWeehder emmuelatinpdliicffaetrieonnc ersawtioer e(SsMignRifi) cwanats, tchaelycuw-ere separated
with the least significant difference (LSD) at p ≤ 0.05.
lated, as shown below: 
 
Agronomy 2021, 11, 978 5 of 13
3. Results
3.1. The Effect of Seed Tuber Size and Variety on Crop Establishment
The planted minitubers started emerging from the soil within the first week of planting,
although the mean number of days to emergence differed with size. Figure 3 shows the
days to emergence, with the 5 g minitubers of Kpamyo and Asiedu being the earliest and
were significantly different from the 1 g minitubers of both varieties. Although the increase
in the proportion of emerged plants from 1 g and 3 g minitubers was gradual and reached
a peak in 38 and 34 days, respectively, that of 5 g was much faster between 18 and 30 days,
when Kpamyo peaked at 96.7%. The 1 and 3 g minitubers were similar in their time to
emergence (Figure 3). The period to 50% of the plant’s emergence from the soil from 5 g
minitubers was considerably earlier (p = ≤0.05) than plants from 1 and 3 g minitubers
Agronomy 2021, 11, x FOR PEER REV(IETWab le 1). There was generally no difference between the yam varieties regarding the6 toifm 1e2 
 to 50% emergence and final crop establishment (Table 1, Figure 3).
120
100
80
60
40
20
0
6 DAP 10 DAP 14 DAP 18 DAP 22 DAP 26 DAP 30 DAP 34 DAP 38 DAP
-20
Asiedu 1 g Asiedu 3 g Asiedu 5 g Kpamyo 1 g Kpamyo 3 g Kpamyo 5 g
 
FFiigguurree 33.. CCoommbbiinneedd nnuummbbeerr ooff ddaayyss ttoo eemmeerrggeennccee ooff mmiinniittuubbeerrss ooff yyaamm vvaarriieettiieess KKppaammyyoo aanndd AAssiieedduu oovveerr tthhrreeee yyeeaarrss 
((22001177––22001199)) iinn AAbbuujjaa,, NNiiggeerriiaa.. 
33..22.. TThhee EEffffeecctt ooff MMiinniittuubbeerr SSiizzee aanndd VVaarriieettyy oonn YYiieelldd aanndd RReellaatteedd TTrraaiittss 
TThhee 55 gg mmiinniittuubbeerrss hhaadd ssiiggnniiffiiccaannttllyy lloonnggeerr vviinneess,, hhiigghheerr LLAAII,, mmeeaann ttuubbeerr wweeiigghhtt ((gg)) 
ppeerr ssttaanndd,, yyiieelldd ((tt hhaa−−11),) ,aanndd mmoorree ttuubbeerrss ppeerr ppllaanntt aanndd ppeerr hheeccttaarree,, ffoolllloowweedd bbyy tthhee 33 gg 
aanndd tthheenn tthhee 11 gg mmiinniittuubbeerrss ((TTaabbllee 11)).. AAtt 33 MMAAPP,, tthhee LLAAII ooff 55 gg mmiinniittuubbeerrss--ddeerriivveedd ppllaannttss 
((44..22)) wwaass 3333%% aanndd 5500%%,, mmoorree tthhaann 33 gg aanndd 11 gg mmiinniittuubbeerrss ppllaannttss,, rreessppeeccttiivveellyy.. PPllaannttss ffrroomm 
tthhee llaarrggeesstt miinniittuubbeerrss hhaadd ssiiggnniiffiiccaannttllyy moorree lleeaavveess tthhaann tthhoossee ffrroom ssmaalllleerr miinniittuubbeerrss.. 
TThhee yyiieelldd ffrroom 55 gg miinniittuubbeerrss ((2233..66t th haa−−11)) waass 3399% aanndd 2255% moorree tthhaann tthhoossee ffrroom 11 gg aanndd 
33 gg miinniittuubbeerrss,, rreessppeeccttiivveellyy.. Onnllyy tthhee sseetttt muullttiipplliiccaattiioonn rraattiioo hhaadd aann iinnvveerrssee rreellaattiioonnsshhiipp,, 
wiitthh tthhee ssmaalllleesstt miinniittubbeerrss hhaavviinngg tthhee hhiigghheesstt vvaalluee ((113388..11)) ccoompaarreed ttoo tthhee 55 gg aannd 33 gg 
miiniittubeerrss ((48..1 and 58..6,, rreesspeeccttiiveelly)) ((Tabllee 11))..  
The analysis of variance for the yam varieties showed no significant differences for 
all the variables measured except for the LAI, where Kpamyo had a better performance 
than Asiedu. However, there were seasonal differences for some of the variables. For ex-
ample, the number of days to 50% emergence, length of vine, and LAI were not signifi-
cantly different across the years. In contrast, the crop establishment, yield, sett multipli-
cation ratio, and the mean weight and number of tubers were substantially higher in 2019, 
perhaps due to the higher and more extended rainfall period. The mean weights of tubers 
produced were quite different, between 140.8 g (1 g) and 158.2 g (3 g) for the smaller 
minitubers and 181.8 g for the biggest (5 g). The number of tubers per plant was signifi-
cantly more for the 5 g minitubers and least for the 1 g minitubers.  
Significant interactions (Table 2) were found between and within minituber sizes, 
varieties, and cropping seasons for some measured variables. The percentage of crop es-
tablishment ranged from 87% to 97.8% across the years, and significant differences were 
observed between tuber sizes. Irrespective of the variety, 1 g minitubers were not signifi-
cantly different in performance across the experiment years and had crop establishments 
ranging between 88.9% and 93.3%. The highest percentage of crop establishment was ob-
served for 3 and 5 g minitubers of Asiedu and Kpamyo, respectively, in 2019. For the LAI 
of Asiedu and Kpamyo, 5 g minitubers had the highest value of 4.8 and 4.3, respectively, 
which were significantly different from the 1 g minitubers of both varieties with mean 
values of 2.0 and 1.6, respectively.  
The yield parameters were observed to be significantly different among and between 
minituber sizes and varieties. The 5 g minitubers of Kpamyo obtained the highest yield of 
 
Agronomy 2021, 11, 978 6 of 13
Table 1. Mean seed multiplication ratio, growth, and yield response variables of two D. rotundata Poir. varieties planted with 1, 3, and 5 g minitubers in Abuja, Nigeria during the 2017,
2018, and 2019 cropping seasons.
Factors Days to 50%
Crop Estab- Seed Multi- Mean Tuber Tuber
Emergence lishment (%)
Vine Length Number of Leaf Area Yield Mean Number
at 8 WAP (cm) Leaves Index (t ha
−1) plication Weight (g) Tubers Plant−1 NumberRatio Stand−1 ha−1
Minituber
Size
1 g 23.8 b 90.4 a 135.2 a 68.3 a 2.1 a 14.4 a 138.1 b 159.2 a 1.1 a 101,296 a
3 g 21.7 b 94.1 b 158.6 b 77.6 a 2.8 b 17.7 b 58.6 a 187.3 b 1.2 b 112,593 b
5 g 18.8 a 94.6 b 186.9 c 105.4 b 4.2 c 23.6 c 48.1 a 249.4 c 1.4 c 130,556 c
LSD 2.16 3.32 13.68 7.19 0.36 2.26 11.27 25.32 0.07 7231
Variety
Asiedu 21.6 a 92.6 a 158.1 a 86.4 a 2.9 a 18.3 a 78.5 a 195.8 a 1.2 a 113,457 a
Kpamyo 21.4 a 93.5 a 162.3 a 81.2 a 3.2 b 18.9 a 84.7 a 201.5 a 1.2 a 116,173 a
LSD 1.77 2.71 11.17 5.87 0.29 1.84 9.20 20.67 0.06 5904.1
Year
2017 22.2 a 90.9 a 160.5 a 89.8 b 3.1 a 14.9 a 69.6 a 164.6 a 1.3 b 114,444 ab
2018 21.6 a 92.8 ab 159.0 a 82.8 ab 3.0 a 15.7 a 63.2 a 168.3 a 1.2 a 110,370 a
2019 20.6 a 95.4 b 161.1 a 78.8 a 3.0 a 25.1 b 112.0 b 263.1 b 1.3 b 119,630 b
LSD 2.16 3.32 13.68 7.19 0.36 2.26 11.27 25.32 0.07 7231
Means with the same letter along column are not significantly different at p ≤ 0.05.
Agronomy 2021, 11, 978 7 of 13
The analysis of variance for the yam varieties showed no significant differences for all
the variables measured except for the LAI, where Kpamyo had a better performance than
Asiedu. However, there were seasonal differences for some of the variables. For example,
the number of days to 50% emergence, length of vine, and LAI were not significantly
different across the years. In contrast, the crop establishment, yield, sett multiplication
ratio, and the mean weight and number of tubers were substantially higher in 2019, perhaps
due to the higher and more extended rainfall period. The mean weights of tubers produced
were quite different, between 140.8 g (1 g) and 158.2 g (3 g) for the smaller minitubers and
181.8 g for the biggest (5 g). The number of tubers per plant was significantly more for the
5 g minitubers and least for the 1 g minitubers.
Significant interactions (Table 2) were found between and within minituber sizes,
varieties, and cropping seasons for some measured variables. The percentage of crop
establishment ranged from 87% to 97.8% across the years, and significant differences were
observed between tuber sizes. Irrespective of the variety, 1 g minitubers were not signifi-
cantly different in performance across the experiment years and had crop establishments
ranging between 88.9% and 93.3%. The highest percentage of crop establishment was
observed for 3 and 5 g minitubers of Asiedu and Kpamyo, respectively, in 2019. For the LAI
of Asiedu and Kpamyo, 5 g minitubers had the highest value of 4.8 and 4.3, respectively,
which were significantly different from the 1 g minitubers of both varieties with mean
values of 2.0 and 1.6, respectively.
The yield parameters were observed to be significantly different among and between
minituber sizes and varieties. The 5 g minitubers of Kpamyo obtained the highest yield
of 31.2 t ha−1. This yield was similar to 29.3 t ha−1 of Asiedu in 2019. The lowest yields
were 10 and 10.2 t ha−1 from 1 g minitubers of Kpamyo and Asiedu, respectively, in 2018
(Table 2). Some 1 g minitubers gave comparable yields to 3 g and 5 g minitubers in 2019
when 1 g of Asiedu yielded 18.8 t ha−1. This yield was slightly higher but not significantly
different from Asiedu 3 g in 2017 (13.9 t ha−1) and 2018 (15 t ha−1), as well as 5 g Asiedu in
2017 (16.3 t ha−1).
The mean number of tubers per plant ranged from 1.1 to 1.5, with the largest minitu-
bers producing significantly more tubers than the smallest. The mean number of tubers
produced per hectare by 1, 3, and 5 g minitubers was 101,296, 112,592, and 130,555, respec-
tively. The highest mean tuber weight was obtained from 3 g (219 and 205.7 g) and 5 g
(238.5 and 210.6 g) minitubers of Kpamyo and Asiedu, respectively, in 2019 (Table 2). On
an individual tuber basis, the smallest tuber size produced ranged from 4 g obtained from
1 g minitubers of Asiedu to 14 g by 5 g minitubers of Kpamyo and Asiedu. In comparison,
the maximum tuber weights ranged from 488 g (by 1 g minitubers of Kpamyo) to 1054 g
(by 5 g minitubers of Asiedu). The SMR was between 35.1 and 213.8, being lowest for the
5 g minitubers and highest for 1 g minitubers. Irrespective of the variety and year, the
1 g minitubers had significantly higher SMR values (p = ≤0.05) than 3 and 5 g minitubers,
except for 3 g minitubers of Asiedu and Kpamyo in 2019 that was similar to 1 g of both
varieties in 2018.
Agronomy 2021, 11, 978 8 of 13
Table 2. All interactions for D. rotundata Poir. varieties planted in Abuja during the 2017, 2018, and 2019 seasons.
Crop Seed Mean
Sett Size Variety Year Days to 50% Vine LengthEmergence Establishment (m) No. Leaves
Leaf Area Yield
Index (t ha−1) Multiplication Weight/Tuber
Number of
−1
(%) Ratio (g) Tubers (t ha )
1 g Asiedu 2017 21.3 a–g 91.1 abc 155 a–d 78 bc 2.7 b–e 11.3 ab 106.3 e 114.5 ab 97,778 ab
2018 25.7 g 87.8 a 136.3 abc 69 b 2.1 abc 10.2 a 99.5 de 108.3 ab 94,444 a
2019 24.3 d–g 91.1 abc 134 abc 76.7 bc 2 ab 18.8 cde 180.7 g 179.5 cde 104,444 abc
Kpamyo 2017 23.7 c–g 88.9 ab 127 ab 71 b 2.2 abc 13.9 abc 134.7 f 131 abc 105,556 abc
2018 24.7 efg 91.1 abc 133 abc 65.3 ab 1.9 ab 10 a 93.8 de 104.2 a 96,667 ab
2019 23.3 b–g 92.2 abc 125.7 a 50 a 1.6 a 22.4 ef 213.8 h 207 def 108,889 abc
3 g Asiedu 2017 22.7 b–g 92.2 abc 163.3 cde 79.7 bcd 2.5 bcd 13.9 abc 47.4 a 127.9 ab 110,000 abc
2018 21.7 a–g 93.3 abc 151 a–d 91.7 cde 2.5 bcd 15 abc 49.8 ab 133.5 abc 111,111 abc
2019 21.7 a–g 97.8 c 161.7 cd 74.7 bc 2.3 abc 23.6 ef 75.5 bcd 205.7 def 115,556 cde
Kpamyo 2017 25.3 fg 88.9 ab 157 a–d 74.7 bc 2.9 c–f 15.8 bcd 56.6 abc 154.2 abc 106,667 abc
2018 20 a–f 94.5 abc 160 bcd 70 b 3.6 fgh 12.3 ab 40.6 a 109 ab 113,333 bcd
2019 19 abcd 97.8 c 158.3 a–d 75 bc 3.2 def 25.5 fg 81.5 cde 219.1 ef 118,889 c–f
5 g Asiedu 2017 19.7 a–e 90 abc 163.3 cde 121.3 f 3.7 f–i 16.3 cd 35.1 a 123.9 ab 132,222 efg
2018 18.7 abc 93.3 abc 177.7 def 95.7 de 3.5 efg 26 fgh 53.7 ab 223.1 ef 116,667 cde
2019 18.3 ab 96.7 bc 180.3 def 91 cde 4.3 g–j 29.3 gh 58.3 abc 210.6 def 138,889 g
Kpamyo 2017 20.7 a–g 94.5 abc 197.3 f 114.3 f 4.5 ij 18.2 cde 37.2 a 135.6 abc 134,444 fg
2018 19 a–d 96.7 bc 196 ef 105 ef 4.4 hij 20.8 def 41.7 a 159.3 bcd 130,000 d–g
2019 16.7 a 96.7 bc 206.7 f 105.3 ef 4.8 j 31.2 h 62.3 abc 238.5 f 131,111 efg
LSD 5.3 8.12 33.51 17.62 0.87 5.53 27.6 51.33 17,712
Means with the same letter along column are not significantly different at p ≤ 0.05.
Agronomy 2021, 11, 978 9 of 13
At harvest, the tubers were sorted out into four categories (Figure 4). It was observed
that 5 g minitubers of Kpamyo produced 18% and 40% of seed tubers in the range of 150 to
249 g and 250 to 500 g, respectively, while the 5 g minitubers of Asiedu produced 22% and
32% of these seed tuber sizes, respectively. The 1 g minitubers of both varieties and 3 g
minitubers of Asiedu produced the most proportion of tubers in the category of <50–249 g,
Agronomy 2021, 11, x FOR PEER REVIEW while the bigger minitubers produced more seed of more than 500 g. The 1 g m8 ionf i
 t1u2b ers of
Asiedu did not have any tuber bigger than 500 g.
40%
30%
20%
10%
0%
Asiedu Kpamyo Asiedu Kpamyo Asiedu Kpamyo
1 g 3 g 5 g
< 50 g 50—149 g 150—249 g 250—500 g > 500 g
 
FigureF4ig. Puerrec e4n.t aPgeerocfevnatraiogues ocaf tvegaorriioesuosf ctuabteegrsoprrioedsu ocef dtubbyedrisff eprernotdmuicneitdu bbeyr  sdiziefsfeorfeynatm mvainriiettuiebseKrp saimzyeos aonfd yAamsie du.
varieties Kpamyo and Asiedu. 
4. Discussion
4. Discussion 4.1. The Effect of Minituber Size and Variety on Yield and Related Traits
4.1. The Effect of MinitFuobrer2 S01iz9ew anhidc hVaprrieesteyn otend Ythieeldb aenstdy Rieelldast,edth Ter5aigtsm initubers had yields of up to
29.3 and 31.2 t ha−1 for Asiedu and Kpamyo, respectively. These yields are pretty high con-
For 2019 whsicidhe rpinregstehnatteind ttrhadei tbioensta lypirealcdtsic,e t,hfaer m5 egr smpilnanittusbeeedrst uhbaedrs yoife2l5d0s– 5o0f0 ugpa ntod 2o9b.t3ai n less
and 31.2 t ha−1 fort hAansi1e0dtuh a−n1da Kftepra7m–1y0om, ornetshpse, calttihvoeulygh. Tathleoswe pyliaenltdpso apruela ptiroentstyof h6i0g0h0  tcoo1n0s,0i0d0-plants
ering that in tradhitai−o1n.aIlf wpreaccotnicseid, efratrhmeemrse apnlavnaltu seeseodf  3t7u5bgerosf soefe d25tu0–b5er00p lagn atendda to8b0t0a0inp llaenstss ha−1,
−1 −1
than 10 t ha−1 afterit 7t–ra1n0s mlatoens tthosa, aseltehdoruagteho afta lboowu tp3l0a0n0t kpgophaula,ticoonmsp oafr e6d00to0 tthoe 1100,000–500 p0lkagnthsa of
ha−1. If we considmeri ntihtueb meresaunse vdailnutehsi sosftudy.when this is extrapolated t o37N5i gg
A
e 
no enriaf ’sse
ormo
eendtir t
uusbqeur apnltaitnyteodf t uabt e8r0s0could be save−d1 for foode yam-producing region0s porlathnatts ohfaWe,s ittA frica.
translates to a seed rate of about 3000 kg ha−1 −1In this study, three significant fa, cctoorms,paamroendg toot htehrse, c1o0u0l–d5b0e0 reksgp ohnasib leoff or the
minitubers used einxc tehpitsio sntaulldyyh. iAghn yeienlodrsmdoesupsi tqe uthaentsimtya lol fs itzueboefrsth ceopulladn tbineg smavateedri afolsr.  fFoiorsdtl y, the
when this is extratpyopleaotefdse ttot  pNlaignteerdia(’ws heonlteirteu byearmag-pairnosdt uthceincgut rseegttisonthsa ot rm tohsattf oarfm Weresstu Asef)riincflau. e nces
In this studyh,o twhrfeaset spilganntisfiecmanetr gfeadctfororsm, athme osonigl. oAtihgehresw, ic[o2u5]ldo bbsee rvreesdptohnatsipbllaen tfsofrr otmhew hole
exceptionally higthu byeiersldess tdabelsisphieted tfhaest esrmaanldl syiizeeld oefd thmeo prelathnatinntgu bmear tpeireicaelss.( mFiirnsitsleyt,t st)h. eT thyepqeu icker
of sett planted (wthheoplela tnutsbeemr aerggaei,ntshte tlhone gceurtt hseetctrso pthgarto wmtohspt efraiormd, earnsd uthsee)l oinngfleurepnhcoetosynthesis isperformed and its products are produced and stored in the tuber to increasse hyioewld . Yam
fast plants emergiesdp rferdoomm itnhaen tslyoiclu. lAtivigahteedwais [a2r5a]i nofebdsecrrvoped, a nthdaste npelascnetnsc efrsoemts  iwn 2h–o3lew etuekbsearfst er the
established fasterr ainsds ytoipelidrreedsp mectoivre otfhtahne ttubbeerr’s psiizeecoers m(matiunriitsye. tTtsh)i.s Tsihtuea tqiouniciskienr litnheew pilthanthtse study
emerge, the longeorf Cthoern certoept  agl.r[o2w6],thw hpoerniotded, athnadt tthheee laorlniegreprl apnhtsoteomseyrngteh, ethseish iigs hpeerrtfhoertmubeedr yield,
and its products arergea prdrloedssuocfewde aanthde rsctoonredditi oinn st.he tuber to increase yield. Yam is predomi-
nantly cultivated as aD reasipnifteedth ecrsompa,l lasnizde soefntuebsecresn(c0e.1 –s5e.t9s9 ign) ,2b–e3tw weeene8k7s. 8a%fteanr dth9e7 .8r%ainosf  psltaonpts were
irrespective of thef utullybeesrt’asb sliizshee odrb my a38tuDrAitPy.( TTahbilse s2i)t.uCaotniotrna riys tion tlhineee xwteinthde tdhep esrtiuoddyo foefm Ceorgrennecte from
et al. [26], who noted that the earlier plants emerge, the higher the tuber yield, regardless 
of weather conditions.  
Despite the small size of tubers (0.1–5.99 g), between 87.8% and 97.8% of plants were 
fully established by 38 DAP (Table 2). Contrary to the extended period of emergence from 
the soil observed with larger cut setts, crop emergence and establishment was fast for this 
category of minitubers (Figure 5). A few days after emergence, the vines from the 
minitubers started producing leaves compared to the situation with bigger setts. In an 
ongoing experiment comparing the growth and yield of various categories of seed tuber 
sizes (tubers ranging from 10 to 250 g), it was observed that up to 25 DAP, there was an 
increase in vine length with an increase in the seed size. For example, the 10 g minitubers 
had a length of 82.3 ± 14.9 cm, while vines from the 250 g seed were 120.6 ± 35.2 cm. How-
ever, the mean number of opened leaves per stand for the 10 g seed tubers was 12 ± 3, 
whereas that of 250 g setts was 2 ± 4 (data not presented). This phenomenon suggests that 
when there is sufficient nutrient in the planted sett (as in the 250 g setts) to support initial 
 
Precentage of tuber sizes
Agronomy 2021, 11, 978 10 of 13
the soil observed with larger cut setts, crop emergence and establishment was fast for
this category of minitubers (Figure 5). A few days after emergence, the vines from the
minitubers started producing leaves compared to the situation with bigger setts. In an
ongoing experiment comparing the growth and yield of various categories of seed tuber
sizes (tubers ranging from 10 to 250 g), it was observed that up to 25 DAP, there was an
increase in vine length with an increase in the seed size. For example, the 10 g minitubers
Agronomy 2021, 11, x FOR PEER REVIEWh ad a length of 82.3 ± 14.9 cm, while vines from the 250 g seed were 120.6 ± 35.2 cm9. of 12 
 However, the mean number of opened leaves per stand for the 10 g seed tubers was
12 ± 3, whereas that of 250 g setts was 2 ± 4 (data not presented). This phenomenon
suggests that when there is sufficient nutrient in the planted sett (as in the 250 g setts) to
plasnutp gprorwt itnhi,t ipalripolraintyt  girso ownt hth, pe rvioinriety’si eslon gthaetivoin.e ’Hs oelwonegvaetri,o wn.iHtho lwimeviteerd, w sittohrelidm nituedtrients 
(as sitno rtehde n1u0 tgri emnitns i(tausbienrsth),e th10e gplmanintsit buebceorsm),et haeuptolatrnotspbheic osmooenaeurt,o atnrodp lheiacfs poroonderu, catniodn and 
expleaanfspiorond aurcet iaocncaenledraextepda ntsoi osnusatraeianc cpellaenrat tgedrotwo sthu satanidn pdleavnet lgorpomwtehnat.n Tdhdee veealrolpym esetnatb. lish-
meTnht eweiatrhly caesntoabplyis hpmroednut cwtiiothn craensoupltyedpr iond uac ltoionngreers ucrltoepd dinuaraltoinogne, racnr oepxtdeunrdaetidon p, earniod of 
nuterxiteenntd aecdcpuemriuodlaotifonnu itnri etnhtea tcucbuemru, alantido nminorteh eytieulbde.r , and more yield.
 
FigurFei g5u. rFeie5l.dF ieplldanptlaendt ewd iwthit h1 1gg, ,3 g,, aanndd5 g5 mg inmitiunbiteursbaetrtsh aetI nttheern IantitoenranlaItnisotnitault eInofstTirtouptiec aolfA Tgriocpuilctuarle ,AAgbruicjau,lNtuigrer, iaA. buja, 
NigerPiala. nPtlsaantttsh aret ethwreek ws aefetekrsp alfatnetri npgla(WntAinPg) (lWefAt),P5) W(lAefPt)(,m 5 iWddAleP), (amndid1d6lWe)A, aPn(dri g1h6t )W. AP (right). 
FroFmro mthteh sepsrperaeadd ooff sseeeedd ttuubbeerr ssiizzeessa at th haravrevset,stth, ethgee ngeernaelrtarel ntdrewndas wthaast tthheatb tighgee brigger 
thet hsiezsei zoef omf miniintuitubberesr,s ,tthhee llaarrggeerr tthhees seeeeddt utbuebresrps rpodroudceudc.eMd.o Mstofasrtmers prefer to plantsmall whole tubers of 250–500 g, which were produced more by 3 g mi nfaitrumbeerrsso pf rKepfearm tyoo plant 
smaanlld w5hgomlei ntuitbuebresr soof f2b5o0t–h5v0a0r ige,t iweshuicsehd winerthee perxopdeuricmeedn mt. oSorem beyfa 3r mg emrsinmitauybuesress oeftt Ks opfamyo 
and15 50 gg fmorinsmituabllewrsa roef tbuobethrs ,vbauritesteitetss usmseadll eirnt hthaen ethxipsearriemmeanitn.l ySoumseed ftaorpmroedrsu cmeabyig ugseer  setts 
of 1s5ee0d gt ufobre rssmfoarltlh we anreex ttsuebaesorsn,, bour tth seeyttasr esmroasllterd tahnadne tahteins asrea msnaicnkl.yT hueseladr gteo sperttosdizuecse big-
geru steileizde dtubbyefrasr fmoerr tshdeo nneoxtt ysiealdsopnro, poor rtthioenya aterley rtoatshteesdi zaenpdl aenatteednd ause at osnpaocokr.q Tuhaeli tlya.rIgfe sett 
size5sg umtiilniziteudb ebrys cfaanrmyieerlsd duop ntoo2t 3y.6ietldh ap−r1oapfoterrtiaobnoauttefilyv etom tohnet hssizien pthlaenfiteeldd, dcoume ptoa rpeodotro qual-
ity.t hIfe 5av ge rmagienyitiuelbdeorfs 8c.4ant  hyai−e1ld[1 u] opf taob o2u3t.62 5t0 hgas−e1 eadftaefrte arbaobouut tfeivigeh tmmoonntthhss ,iint  sthhoew fsietlhda,t com-
partehde ptoo ttehnet iaavl oefrathgee cyleiealnds oeef d8.i4s th ihgah−.1 U[1s]i nogf vaibrouus-tf r2e5e0s ege sdesemd aayftreerq uabiroeumt uecighhstm malolenrths, it 
shopwlasn tthinagt mthaterials than the 250 g customarily used to produce average-sized yam tubers.Since the coes tppoetresneteidaly oafm thies mcleaiannly sedeedte irsm hinigehd.b Uysiitnsgsi vzeir, usms-afrlleeer sseeeeddstu mbearys rweiqllusiarvee much 
smcaollsetrf oprlathnitsinmgo smt aextepreinaslsiv tehiannp utht ein 2y5a0m g pcruosdtuocmtiaorni.ly used to produce average-sized yam 
tubers. SSeicnocned tlhy,et hceosset epdetru sbeeersdu yseadmi nist hmisaeixnplyer idmeetenrtmwienreedo fbgyo oitds qsuizaeli,t ys,mobatlalienre sdeferdom tubers 
wilvl israuvs-ef rceoespt lfaonrt sthoifsi mporsotv eexdpveanrsieivties ingrpouwt ninin yanma phroidd-uprcotioofns.c r een house. Diehl [27]
noStedcotnhdatlyth, ethqeu aselietyd atsupbeecrts oufsyeadm inh athveist wexopdeirmimenensito nws;ergee totifn gothoed mqousatlistuyi,t aobbletained 
frovma rvieirtyusf-ofrreae ppalratnictus loarf ipmropdruocvteiodn vsaitruieattiieosn garnodwenn sinu raing apthheidin-ptergoroitfy socrfeceunlt hivoaurssein. Diehl 
[27d] inseoatseedr athtiantg ,ththee qvuigaolritoyf aesstpaebclitssh omf eynat,msi zhea, vaned twtyop edoimf seentts. iTohnesv; igruetst-ifnrege twhheo mleotustb esrusitable 
variety for a particular production situation and ensuring the integrity of cultivars in dis-
ease rating, the vigor of establishment, size, and type of sett. The virus-free whole tubers 
used were better able to demonstrate their potential by having an early crop establishment 
that eventually produced lush vegetation and high yields. 
Lastly, planting was done at a high density of 100,000 plants ha−1 due to the seed 
tubers’ small size. Even at the high plant population, it was observed that the interrow 
spacing of 1 m was too much since plants were staked and large portions of land were not 
covered by the foliage, allowing weeds to populate the area. Otoo [28] also observed that 
microsett populations could be as high as 250,000–444,000 ha−1, and high densities could 
replace plastic mulch in the control of weeds. The yield from the pea-sized yam minitubers 
(Table 2) was a surprise. It far exceeded what was anticipated, especially in 2019, when 
the rainfall (Figure 1) was more than in the previous years (more than double the rainfall 
of 2017). Although there was not much difference in crop establishment between the 
 
Agronomy 2021, 11, 978 11 of 13
used were better able to demonstrate their potential by having an early crop establishment
that eventually produced lush vegetation and high yields.
Lastly, planting was done at a high density of 100,000 plants ha−1 due to the seed
tubers’ small size. Even at the high plant population, it was observed that the interrow
spacing of 1 m was too much since plants were staked and large portions of land were
not covered by the foliage, allowing weeds to populate the area. Otoo [28] also observed
that microsett populations could be as high as 250,000–444,000 ha−1, and high densities
could replace plastic mulch in the control of weeds. The yield from the pea-sized yam
minitubers (Table 2) was a surprise. It far exceeded what was anticipated, especially in 2019,
when the rainfall (Figure 1) was more than in the previous years (more than double the
rainfall of 2017). Although there was not much difference in crop establishment between
the minituber sizes, the largest seed (5 g) of both varieties had the longest vines, more
leaves, highest LAI values, and eventually the best seed yam yields.
4.2. The Benefits of Using Minitubers for Seed Yam Production
The 3 and 5 g minitubers produced a mean tuber weight of 158.2 and 181.8 g, respec-
tively, with an average of 112,592 and 130,555 seed tubers ha−1, respectively (Table 1).
Considering that the seed tubers produced are cleaner than farmer-saved seed in terms of
virus infection, the sizes of seed tubers obtained could be used for ware yam production.
With the current recommendation of planting 10,000 plants ha−1 for ware yam production,
the number of tubers estimated to be produced per ha by the 3 and 5 g minitubers can
plant 11 and 13 ha, respectively. In conventional practice, about one-third of the yield
from one hectare is reserved for planting an equivalent land area as the one harvested [29].
Consequently, from a yield of 10 t ha−1, about 3.3 tonnes will be reserved to plant another
one hectare during the subsequent cropping season. However, if a system to produce and
use minitubers for seed yam production is established, farmers may sell or use most of
their ware yam for food and acquire planting materials from seed producers. Yam farmers
mostly prefer the seed size produced by the 1, 3, and 5 g minitubers because they are
planted whole with minimal cutting. Crops grown with whole seeds are more uniform in
establishment and yield than those produced with a combination of whole and cut seed
tubers, which is the customary practice [25].
Other advantages of using minitubers in seed yam production include reduction
in bulk of the planting material, land preparation, and the possibility of mechanization.
Transporting such small seed tubers would be less cumbersome. Planting was done on
ridges, which are easier to prepare than the mounds used in conventional ware/seed yam
production systems. Additionally, there is the possibility of adapting grain planters (e.g.,
maize, peanut, or soya bean) to plant the minitubers, many of which were similar to these
grain sizes. The multiplication rate using minitubers at the onset of seed multiplication pro-
grams will be highly enhanced considering the SMR of 1:93.8–1:213.8 for the 1 g minitubers
(Table 2) compared to 1:3 or 1:30 obtained using ordinary setts or minisetts, respectively.
The technology to produce minitubers of less than 10 g is currently available [21,22,30],
so the use of such tubers by seed programs should be encouraged to get improved and
released varieties more readily available to yam farmers. The minitubers were found to
behave like larger seed tubers in storage. Those used in this experiment were stored in
brown paper envelopes to reduce dehydration and desiccation and stored in a well-aerated
room with temperatures not exceeding 26 ◦C. They were in good condition for up to four
months with about 3% loss, especially of the less than 1 g tubers. The storage loss was
primarily observed after two months when dormancy was broken and occurring more
within the fourth month of storage. This situation indicates that more extended storage as
may be required by commercial seed yam producers would be possible if done in a more
favorable environment such as in rooms with a temperature of 18 to 21 ◦C attainable with
domestic air conditioners.
This study is the first to investigate the performance of high-quality minitubers of
0.1–5.99 g under field conditions. Consequently, further research is needed to determine the
Agronomy 2021, 11, 978 12 of 13
optimum agronomic management practices (appropriate plant density, as well as fertilizer,
water, and storage requirements, among others) and production cost of this category of
planting material for seed yam production.
5. Conclusions
Yam is a valuable tuber crop that enhances food security in West Africa, a region that
produces 92% of the world crop. The crop is propagated conventionally with a portion
of the tubers that would have been used for food. High-quality minitubers of 0.1 g to
about 6 g (represented as 1, 3, and 5 g) of two improved and released yam varieties
produced from vine cuttings of plants in an aeroponics system were planted in the field at
100,000 plants ha−1 to assess their potential in seed yam production. Results showed that
for both varieties, the minitubers of 5 g emerged fastest from the soil (19 DAP) with crop
establishment of 94.6% compared to the minitubers of 1 g that emerged from the ground at
24 DAP and had 90.4% establishment. The yields varied with variety, size of minituber,
and cropping season, with the lowest yield of 10 t ha−1 obtained from 1 g minitubers of
variety Kpamyo in 2018 and the highest yields, 31.2 t ha−1 from 5 g of Kpamyo in 2019.
The very high SMR of up to 213.8 for the 1 g minitubers and the large number of sizeable
seed tubers produced from the minitubers makes them suitable for inclusion in seed yam
production programs.
Author Contributions: Conceptualization, B.A.; methodology, B.A.; formal analysis, D.A.; investi-
gation, B.A. and D.A.; writing—original draft preparation, B.A.; writing—review and editing, B.A.,
N.M., P.L.K., M.B., D.M., R.A. and D.A.; project administration, N.M. and R.A.; funding acquisition,
(IITA: R.A., N.M., P.L.K., M.B., D.M. and B.A.). All authors have read and agreed to the published
version of the manuscript.
Funding: This research was funded by the Bill and Melinda Gates Foundation, bearing Investment
Number OPP1159088. Grant to the International Institute of Tropical Agriculture, Ibadan, Nige-
ria, November 2016 to 31 December 2021. IITA receives funding for research on yams from the
CGIAR Research Program on Roots, Tubers, and Bananas (RTB) and is supported by CGIAR Trust
Fund contributors.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
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