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Evaluation of yam cultivars for seed yam production, using the minisett technique
Field Crops Research, 19 (1988) 81-89
81
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Evaluation of Yam Cultivars for Seed Yam Production, using the Minisett Technique NDUBISI IGWILO ~and OBINANI O. OKOLI
National Root Crops Research Institute, Umudike P.M.B. 7006, Umudike, Imo State (Nigeria) (Accepted 18 March 1988)
ABSTRACT Igwilo, N. and Okoli, O.0., 1988. Evaluation of yam cultivars for seed yam production, using the minisett technique. Field Crops Res., 19: 81-89. Nine yam cultivars were evaluated over three seasons for their capacity to yield saleable seed yams ( > 200 g) from 25-g yam pieces called minisetts. Five of the cultivars belong to Dioscorea rotundata, two to D. alata and one each to D. cayenensis and D. dumetorum. All the cultivars responded well to the 'minisett' technique. All, at one time or another within the three years, gave sprouting percentages between 65 and 100. Under good management, about 1 t of planting material gave yields of 8-24 t/ha depending on the cultivar; of the tubers yielded, 50-83% weighed above 200 g, with average multiplication ratios of 8-25. The best performance came from cultivars of D. alata, which often yielded some ware yams ( > 1 kg). Minisett plants were, however, very sensitive to weed competition, the cultivars of D. rotundata being the most vulnerable.
INTRODUCTION
Seed yams, small tubers used as planting material, constitute the major cost item in the production of ware yams, i.e. those usually larger than 1 kg, accounting for 30-50 % of the total outlay (Oyolu, 1978 ). The average size of seed yams used by farmers in West Africa is 200-300 g (Onwueme, 1978). Traditionally, farmers either use whole small tubers which arise during the process of ware-yam production, or cut large tubers into smaller pieces of the desired size, which are called 'seed setts' (Wilson, 1978). Planting materials can also arise from the practice of double-harvesting, whereby tubers are first harvested for food 2-3 months before maturity. Careful opening of the ground causes minimum damage to the plant's root system; the tuber is then excised, leaving the head (the corm) still attached to the vine, which is covered again with ~Present Address: Institute of Agricultural Research and Development, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Nigeria.
0378-4290/88/$03.50
© 1988 Elsevier Science Publishers B.V.
82 earth. The cluster of tubers developing thereafter is harvested (second harvest) at the end of the season, to be used as planting material the following season (Onwueme, 1977a,b). Seed yams have low multiplication ratios (ratio of fresh weight harvested:fresh weight planted) of less than 5 as against 15-80 obtainable from legumes and cereals (Onwueme, 1978); thus, the recommended plant population of 10 000 stands/ha for ware yam production, using 250-g seed yams, requires 2.5 t / h a of planting material for an expected yield of 12.5 t/ha. In the search for a cheap method of rapidly multiplying improved cultivars of Dioscorea spp. for evaluation and distribution to farmers, studies at the National Root Crops Research Institute {NRCRI), Umudike, revealed that the smaller the yam piece planted, the higher the multiplication ratio (Okoli et al., 1982 ). Thus, 25-g minisetts sown at 40 000 stands/ha yielded tubers of which 51-87% were heavier than 200 g, and therefore saleable as seed yams. The potential yield of this 1 t / h a of minisett planting material was estimated to be 7.8-21.1 t/ha, depending on variety and environmental factors. The average yield of ware yams using the standard 2.5-t/ha of planting materials, therefore, falls within this range. A direct comparison between the two types of planting material using two cultivars, Nwopoko (D. rotundata) and Um 680 (D. alata), showed that plants grown from minisetts of Um 680, under good management, such as early staking and efficient weed control, significantly out-yielded those from the standard seed yams, while Nwopoko yields of the two seed sizes were similar, with multiplication ratios of 16-39 for the minisetts and 7-14 for the seed yams. The efficient performance of the minisett plants was obtained through earlier foliation (see Figs. 1 and 2) and the development of leaf area index (LAI) equal to or greater than that of the plants grown from seed yams (Igwilo, 1984 ) with corresponding effects on crop growth rates and bulking rates of tubers (Enyi, 1972 ). This suggests that minisetts might usefully replace seed yams or provide a cheap method of improving upon the productivity of the standard setts, thereby reducing the cost of yam production. In these earlier studies on minisetts, only two cultivars were used, Nwopoko and Um 680. In this present study, carried out between 1982 and 1984, nine selected cultivars belonging to four species were used to investigate their responses to the minisett technique. These cultivars and species vary in prominence in the farming systems of the various yam-growing areas of the tropics. In West Africa, the major species grown is the white yam (D. rotundata) followed by the yellow yam (D. cayenensis) while in tropical Asia, the Caribbean and the Pacific Islands, the water yam (D. alata) is predominant (Ayensu and Coursey, 1972). In some communities of the rainforest areas of West Africa, especially Nigeria and the Cameroons, the trifoliate yam (D. dumetorum) is the most important edible yam (Lyonga and Ambe, 1983).
83
Fig. 1. Vine of seed yam (D. rotundata) at the early growthstage. Note the absence of true leaves. The vine measuredabout 2.6 m (coiledin the picture for convenience). Fig. 2. Vine of minisett (D. rotundata) at the earlygrowthstage. Note the presence of true leaves. The vine measuredabout 0.6 m. (Both the seed yam and the minisett were planted on the same date ). MATERIALSAND METHODS Nine yam cultivars - Ekpe, Obiaoturugo, Okwocha; Nwopoko, Abi (all belonging to Dioscorea rotundata), Ominelu, Um 680 (both D. alata), Okumanu (D. cayenensis ) and Ona (D. dumetorum) - were used. Each tuber was cut into discs 2 cm thick, discarding the end pieces. Each disc was then cut into, usually, 4 minisetts of approximately 25 g, although it could give more or less t h a n 4 minisetts depending on its surface diameter. T w e n t y minisetts from the head, middle and tail regions of the tuber were mixed, giving 60 minisetts of which 56 were planted per 14-m 2 plot. The minisetts were spaced at 25-cm intervals, along ridges i m apart, equivalent to a seed rate of i t (40 000 stands) per ha. The minisetts were dusted with Aldrex T, a protective insecticide/fungicide before planting. The cultivars were arranged in a randomised complete block with 4 replicates. The experiment was planted early in the rainy season, on 1 April 1982, 4 May 1983, and 8 May 1984. Sprout counts were taken weekly. After sprouting,
84 the vines were staked at the rate of 4 vines per stake instead of the usual 1216 stands per stake, in order to facilitate sampling. Fertilizers were applied at the rate of 60 kg N, 15 kg P and 70 kg K per ha. Plots were weeded 3 times in 1982, 4 times in 1983, and twice in 1984, after heavy weed infestation. On 10 October 1983 and 12 October 1984 after the growth of vines had ceased, 2 plants were sampled at random from each plot for dry-matter and leaf-area determination. For dry matter, plant parts were dried in ventilated ovens at 70°C for 72 h. Area:dry-weight ratios of leaf discs cut with a cork borer of 1.6 cm dia were used to determine leaf area. Mature "tubers were harvested in December of each year, and multiplication ratio was calculated. The experiment was sited at the Eastern Farm of NRCRI, Umudike (05 ° 29' N, 07 °33'E; 122 m a.s.1.). The soil was a sandy loam.
RESULTS AND DISCUSSION
Sprouting: Ominelu (D. alata) was always the first cultivar to sprout, usually 8-14 days after planting (PAP) while the others sprouted 23-35 DAP except Ona (D. dumetorum) which took 25-50 days (Fig. 3). Some cultivars completed sprouting faster than others, the time interval being between 20 and 40 days from first sprout emerged to complete sprouting. Maximum percentage sprouting varied among cultivars (Table 1 ). The two cultivars ofD. alata gave the highest percentage, often as much as 100%, followed closely by Abi and Ekpe, both D. rotundata. The only cultivar of D. rotundata which tended to give a low percentage was Nwopoko. The sprouting of Okumanu (D. cayenensis) was similar to those of most cultivars of D. rotundata. Ona (D. dumetorum) gave the lowest sprouting, except in 1984. These data show that minisetts from the head, middle and tail portions of the tuber sprouted. Sprouting was therefore not a limitation to yield under favourable conditions, as most cultivars had 80-100% sprouting in one season or another. There was an indication that the low percentage sprouting of Ona (D. dumetorum), which increased to 66% in 1984, can be improved by the use of healthy tubers as planting materials; tubers to be chosen for this technique should be free from diseases and pests (Plumbley and Rees, 1983 ) especially nematodes, which feed below the skin (periderm) of the tuber. It is known that the yam tuber has no dormant buds and that bud initials are located just below the periderm (Onwueme, 1973). Any damage to this meristematic tissue would result in poor sprouting.
85 100
•~
50
0
~o
~°
I
I
I
I
I
I
I
I
I
I
I
I
I
8
15
22
29
36
43
50
57
64
71
78
85
92
Days
after
planting
Fig. 3. Time course of sprouting of minisetts, 1982 data. Vertical bars are LSD (P=0.05) at two stages of sprouting2 (Fig. 3, although presenting only 1982 data, is representative of 1983 and 1984 experiments). Symbols o--o Ominelu D. alata • --e Um 680 [::]--[~ Abi m--M Nwopoko D. rotundata A--A Okwocha O--~ Obiaoturugo 0--# Ekpe D. cayenensis A--A Okumenu D. dumetorum V--V Ona
Tuber yield: The two cultivars of D. alata significantly outyielded the other cultivars (Table 1 ), probably because D. alata sprouted faster (Fig. 3) and had higher percentage sprouting (Table 1) than most of the others. Dioscorea alata also had greater leaf area indices (LAI) than the other cultivars (Table 2); the resulting greater leaf-area duration (LAD)gave greater tuber yields (Enyi, 1972). Maximum LAI (Table 2 ) was significantly correlated with tuber yield per ha (Table 1) in both 1983 (r = 0.72, P < 0.05) and 1984 (r = 0.80, P < 0.01). Nwopoko yielded least of all cultivars ofD. rotundata, probably because of its low establishment (Table 1). Similarly, Ona (D. dumetorum) increased its yield from
86 TABLE 1 Sprouting of minisetts, tuber yieldand tuber size distribution of yam cultivarsaveragedoverthree cropping seasons, 1982-1984 Species/ Cultivar
Sprouting ( %)
Tuberyield
Size distribution (%)
t/ha
'000/ha
200 g
200500 g
5001000 g
> 1000 g
85.6 81.5
22.5 16.1
35.6 43.2
27.9 45.4
27.9 30.1
34.0 19.0
10.2 5.5
81.0 74.9 69.9 70.5 51.7
11.2 7.3 9.3 10.2 5.3
34.1 23.0 22.8 23.1 16.1
38.7 65.1 53.8 48.0 63.2
36.6 21.4 28.4 34.4 23.7
22.1 13.5 13.3 15.0 12.1
2.5 0.0 4.0 2.6 0.9
67.1
8.9
21.7
55.3
27.6
13.9
3.1
37.8
7.7
11.7
39.3
33.0
26.3
1.4
4.5
4.1
D. alata
Ominelu Um 680 D. r o t u n d a t a Abi Ekpe Obiaoturugo Okwocha Nwopoko
D. vayenensis
Okumanu D, durnetorurn
Ona LSD (0.05) between cultivars:
an average of 6.0 t / h a in 1982 and 1983 to 11.1 t / h a in 1984, when its percentage sprouting increased from 24% to 66%. O k u m a n u (D. cayenensis) had, on average, similar yields to those of most cultivars of D. rotundata. The multiplication ratios in 1982 and 1983 ranged between 7.1 and 29.9, which fall within the range found among cereals and legumes {Onwueme, 1978). In terms of consistency, D. alata gave the highest multiplication ratios. It is remarkable that, in 1983, Ona (D. dumetorum ) gave the highest multiplication ratio although its establishment was poor. All four species used in this study, therefore, responded well to the minisett technique. Water yams, D. alata, responded better t h a n the other species in terms of both yield and multiplication ratios. The response of D. alata to this technique can be exploited to increase production for local consumption, for export, and as a replacement for grains in supplying the energy component in livestock feeds. Yields of 40-50 t / h a (2530% dry matter) have been recorded in experimental plots at NRCRI, Umudike, higher t h a n yields obtainable from corn (Phillips, 1964) and the best available varieties of cassava (Ene and Okeke, 1983) in Nigeria. Tuber yields in 1984 were lower than in 1982 and 1983 (Table 1 ), even though 1984 had more favourable weather. In 1984, sunshine hours and rainfall were
87
TABLE2 Leafareaindex(LAI)•tubersizeandmultiplicati•nrati•s•fyamsinthreecr•ppingseas•ns••982-•984 Maximum LA!
Average tubersize (g)
Multiplication ratio
1983
1984
1982
1983
1984
1982
1983
1984
6.2 5.1
4.7 1.8
529 429
703 566
269 220
25.1 10.4
24.1 20.2
10.0 7.9
2.8 1.4 1.7 1.9 0.7
1.6 0.9 1.2 1.2 0.6
245 203 267 245 189
487 274 463 560 374
154 84 75 145 115
8.5 8.6 10.2 11.1 7.1
8.3 13.9 18.4 16.8 14.5
5.9 2.8 1.8 5.4 4.2
Okumanu
2.0
1.3
262
370
149
10.8
19.4
5.5
D. d u m e t o r u m Ona
3.3
1.5
294
479
259
12.4
29.9
10.4
Seasonal mean
2.8
1.6
295
475
163
11.6
18.3
6.0
LSD (0.05) between two varieties in one season
1.4
1.0
75
91
49
5.3
6.4
2.2
Species/cultivar
D. alata Ominelu Um 680
D. rotundata Abi Ekpe Obiaoturugo Okwocha Nwopoko
D. eayenensis
LSD (0.05) between two seasonal means
0.7
56
3A
LSD (0.05) between two seasons for one variety
0.6
97
5.!)
higher by 2.8% and 34.1%, respectively, than the 1982 and 1983 averages. This low yield was due to inadequate weed control (Unamma and Melifonwu, 1986). Weed interference did not have a significant effect on number of tubers/ha within cultivars, its main effect being in the reduction of tuber size (Table 2). The reduction in yield might have been mediated through reduction in leaf area (Table 2 ), two variables that were closely correlated. In 1984, average LAI was reduced by 43% when compared with the 1983 cropping season, whereas tuber yield was reduced by 58%. This might suggest that weed competition also had a direct effect on tuber yield, probably by increasing soil physical impedance to tuber growth (Lal and Hahn, 1973; Kang and Wilson, 1981). In 1982, the percentage of saleable seed yams ( > 200 g) ranged between 45 and 81%, in 1983 between 58 and 83%, and in 1984 between 6 and 55%. The cultivars varied in their response to weed competition, the most tolerant being Ona (D. dumetorum) followed by the cultivars of D. alata; D. rotundata was the most sensitive. It is noteworthy that, in 1983, all cultivars except Ekpe yielded some ware yams ( > 1 kg), suggesting that the minisetts of most cultivars have the
88 p o t e n t i a l of giving a m u l t i p l i c a t i o n r a t i o g r e a t e r t h a n 40. I n t h e y a m - p r o d u c i n g a r e a s of W e s t Africa, seed y a m s are sold b y n u m b e r , so t h e m o r e saleable t u b e r s t h a t are h a r v e s t e d , t h e g r e a t e r t h e e c o n o m i c r e t u r n s . T h e r e f o r e , to m a x i m i s e r e t u r n s in y a m p r o d u c t i o n u s i n g t h e m i n i s e t t t e c h nique, h e a l t h y p l a n t i n g m a t e r i a l s s h o u l d be u s e d in o r d e r to o b t a i n high s p r o u t ing p e r c e n t a g e a n d high p o p u l a t i o n s , this, followed b y good m a n a g e m e n t (especially e a r l y s t a k i n g a n d efficient c o n t r o l of w e e d s ) s h o u l d e n s u r e a p r e p o n d e r a n c e o f saleable t u b e r s . ACKNOWLEDGEMENT T h e a u t h o r s are g r a t e f u l to t h e D i r e c t o r , N a t i o n a l R o o t C r o p s R e s e a r c h I n s t i t u t e ( N R C R I ) , U m u d i k e for p e r m i s s i o n to p u b l i s h t h e s e d a t a , a n d to t h e s t a f f o f t h e Y a m P r o g r a m m e a n d P l a n t P h y s i o l o g y D i v i s i o n for t e c h n i c a l assistance.
REFERENCES Ayensu, E.S. and Coursey, D.G., 1972. Guinea yams. The botany, ethnobotany, use and possible future of yams in West Africa. Econ. Bot., 26:301-308. Ene, L.S.O. and Okeke, J.E., 1983. Evaluation of ten top selections of cassava in five ecological zones of Nigeria. Annual Report 1983, National Root Crops Research Institute, Umudike, pp. 2-3. Enyi, B.A.C., 1972. Effect of staking, nitrogen and potassium on growth and development in lesser yam: Dioscorea esculenta. Ann. Appl. Biol., 72: 211-219. Igwilo, N., 1984. Comparison between the yield and growth patterns of yams grown from minisetts and normal seed yams. Paper presented at the 20th Annual Conference of the Agricultural Society of Nigeria, 19-24 August, 1984, at the Rivers State University of Science and Technology, Port Harcourt (unpublished). Kang, B.T. and Wilson, J.E., 1981. Effect of mound size and fertilizer on white guinea yam (Dioscorea rotundata) in Southern Nigeria. Plant Soil, 61: 319-327. Lal, R. and Hahn, S.K., 1973. Effect of method of seed bed preparation, mulching and time of planting of yam in Western Nigeria. In: C.R.A. Leakey (Editor), Proc. Third Symp., International Society for Tropical Root Crops, 2-9 December 1973, Ibadan, Nigeria. IDRC, Ottawa, pp. 293-306. Lyonga, S.N. and Ambe, J.T., 1983. Effect of staking on tuber yield of three cultivars of trifoliate yams. In: E.R. Terry, E.V. Doku, O.B. Arene and N.H. Mahungu (Editors), Proc. Second Triennial Symp., International Society for Tropical Root Crops, Africa Branch, 14-19 August 1983, Douala, Cameroons, pp. 138-139. Okoli, O.0., Igbokwe, M.C., Ene, L.S.O. and Nwokoye, J.U., 1982. Rapid multiplication of yam by minisett technique. National Root Crops Research Institute, Umudike, Nigeria, Res. Bull. No.2, 12 pp. Onwueme, I.C., 1973. The sprouting process in yam (Dioscorea spp.) tuber pieces. J. Agric. Sci., 81: 375-379. Onwueme, I.C., 1977a. Field comparison of West African planting and harvesting pactices in yam {Dioscorea rotundata): pre-sprouting, dry season planting, and double harvesting. J. Agric. Sci., 88: 311-318.
89 Onwueme, I.C., 1977b. Effect of varying the time of the first-harvest, and of late planting on double-harvest yield of yam (Dioscorea rotundata) in field plots. J. Agric. Sci., 88: 737-741. Onwuene, I.C., 1978. The Tropical Tuber Crops. John Wiley, Chichester, 95 pp. Oyolu, E., 1978. Inherent constraints to high productivity and low production cost in yam {Dioscorea spp.) with special reference to Discorea rotundata Poir. In: Proc. Seminar on Yams, 1 7 October, 1978, Buea, Cameroons. International Foundation for Science, Stockholm, Provis. Rep. No. 3,, pp. 183-192. Phillips, T.A., 1964. An Agricultural Notebook. Longmans, Nigeria, 240 pp. Plumbley, R.A. and Rees, D.P., 1983. An infestation by Aracerus [asciculatus (Degeer) (Coleoptera: Anthribidae) and Decadarchis minuscula {Walsingham) {Lepidoptera: Tineidae) on stored fresh yam tubers in S.E. Nigeria. J. Stored Prod. Res., 19: 93-95. Unamma, R.P.A. and Melifonwu, A.A., 1986. Herbicides for 'seed' yam production from "minisetts" in the rainforest zone of Nigeria. Weed Res., 26: 115-120. Wilson, J.E., 1978. Recent developments in the propagation of yam (Dioscorea spp). In: Proc. Seminar on Yams, 1-7 October, 1978, Buea, Cameroons. International Foundation for Science, Stockholm, Provis. Rep. No. 3, pp. 87-92.
81
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Evaluation of Yam Cultivars for Seed Yam Production, using the Minisett Technique NDUBISI IGWILO ~and OBINANI O. OKOLI
National Root Crops Research Institute, Umudike P.M.B. 7006, Umudike, Imo State (Nigeria) (Accepted 18 March 1988)
ABSTRACT Igwilo, N. and Okoli, O.0., 1988. Evaluation of yam cultivars for seed yam production, using the minisett technique. Field Crops Res., 19: 81-89. Nine yam cultivars were evaluated over three seasons for their capacity to yield saleable seed yams ( > 200 g) from 25-g yam pieces called minisetts. Five of the cultivars belong to Dioscorea rotundata, two to D. alata and one each to D. cayenensis and D. dumetorum. All the cultivars responded well to the 'minisett' technique. All, at one time or another within the three years, gave sprouting percentages between 65 and 100. Under good management, about 1 t of planting material gave yields of 8-24 t/ha depending on the cultivar; of the tubers yielded, 50-83% weighed above 200 g, with average multiplication ratios of 8-25. The best performance came from cultivars of D. alata, which often yielded some ware yams ( > 1 kg). Minisett plants were, however, very sensitive to weed competition, the cultivars of D. rotundata being the most vulnerable.
INTRODUCTION
Seed yams, small tubers used as planting material, constitute the major cost item in the production of ware yams, i.e. those usually larger than 1 kg, accounting for 30-50 % of the total outlay (Oyolu, 1978 ). The average size of seed yams used by farmers in West Africa is 200-300 g (Onwueme, 1978). Traditionally, farmers either use whole small tubers which arise during the process of ware-yam production, or cut large tubers into smaller pieces of the desired size, which are called 'seed setts' (Wilson, 1978). Planting materials can also arise from the practice of double-harvesting, whereby tubers are first harvested for food 2-3 months before maturity. Careful opening of the ground causes minimum damage to the plant's root system; the tuber is then excised, leaving the head (the corm) still attached to the vine, which is covered again with ~Present Address: Institute of Agricultural Research and Development, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Nigeria.
0378-4290/88/$03.50
© 1988 Elsevier Science Publishers B.V.
82 earth. The cluster of tubers developing thereafter is harvested (second harvest) at the end of the season, to be used as planting material the following season (Onwueme, 1977a,b). Seed yams have low multiplication ratios (ratio of fresh weight harvested:fresh weight planted) of less than 5 as against 15-80 obtainable from legumes and cereals (Onwueme, 1978); thus, the recommended plant population of 10 000 stands/ha for ware yam production, using 250-g seed yams, requires 2.5 t / h a of planting material for an expected yield of 12.5 t/ha. In the search for a cheap method of rapidly multiplying improved cultivars of Dioscorea spp. for evaluation and distribution to farmers, studies at the National Root Crops Research Institute {NRCRI), Umudike, revealed that the smaller the yam piece planted, the higher the multiplication ratio (Okoli et al., 1982 ). Thus, 25-g minisetts sown at 40 000 stands/ha yielded tubers of which 51-87% were heavier than 200 g, and therefore saleable as seed yams. The potential yield of this 1 t / h a of minisett planting material was estimated to be 7.8-21.1 t/ha, depending on variety and environmental factors. The average yield of ware yams using the standard 2.5-t/ha of planting materials, therefore, falls within this range. A direct comparison between the two types of planting material using two cultivars, Nwopoko (D. rotundata) and Um 680 (D. alata), showed that plants grown from minisetts of Um 680, under good management, such as early staking and efficient weed control, significantly out-yielded those from the standard seed yams, while Nwopoko yields of the two seed sizes were similar, with multiplication ratios of 16-39 for the minisetts and 7-14 for the seed yams. The efficient performance of the minisett plants was obtained through earlier foliation (see Figs. 1 and 2) and the development of leaf area index (LAI) equal to or greater than that of the plants grown from seed yams (Igwilo, 1984 ) with corresponding effects on crop growth rates and bulking rates of tubers (Enyi, 1972 ). This suggests that minisetts might usefully replace seed yams or provide a cheap method of improving upon the productivity of the standard setts, thereby reducing the cost of yam production. In these earlier studies on minisetts, only two cultivars were used, Nwopoko and Um 680. In this present study, carried out between 1982 and 1984, nine selected cultivars belonging to four species were used to investigate their responses to the minisett technique. These cultivars and species vary in prominence in the farming systems of the various yam-growing areas of the tropics. In West Africa, the major species grown is the white yam (D. rotundata) followed by the yellow yam (D. cayenensis) while in tropical Asia, the Caribbean and the Pacific Islands, the water yam (D. alata) is predominant (Ayensu and Coursey, 1972). In some communities of the rainforest areas of West Africa, especially Nigeria and the Cameroons, the trifoliate yam (D. dumetorum) is the most important edible yam (Lyonga and Ambe, 1983).
83
Fig. 1. Vine of seed yam (D. rotundata) at the early growthstage. Note the absence of true leaves. The vine measuredabout 2.6 m (coiledin the picture for convenience). Fig. 2. Vine of minisett (D. rotundata) at the earlygrowthstage. Note the presence of true leaves. The vine measuredabout 0.6 m. (Both the seed yam and the minisett were planted on the same date ). MATERIALSAND METHODS Nine yam cultivars - Ekpe, Obiaoturugo, Okwocha; Nwopoko, Abi (all belonging to Dioscorea rotundata), Ominelu, Um 680 (both D. alata), Okumanu (D. cayenensis ) and Ona (D. dumetorum) - were used. Each tuber was cut into discs 2 cm thick, discarding the end pieces. Each disc was then cut into, usually, 4 minisetts of approximately 25 g, although it could give more or less t h a n 4 minisetts depending on its surface diameter. T w e n t y minisetts from the head, middle and tail regions of the tuber were mixed, giving 60 minisetts of which 56 were planted per 14-m 2 plot. The minisetts were spaced at 25-cm intervals, along ridges i m apart, equivalent to a seed rate of i t (40 000 stands) per ha. The minisetts were dusted with Aldrex T, a protective insecticide/fungicide before planting. The cultivars were arranged in a randomised complete block with 4 replicates. The experiment was planted early in the rainy season, on 1 April 1982, 4 May 1983, and 8 May 1984. Sprout counts were taken weekly. After sprouting,
84 the vines were staked at the rate of 4 vines per stake instead of the usual 1216 stands per stake, in order to facilitate sampling. Fertilizers were applied at the rate of 60 kg N, 15 kg P and 70 kg K per ha. Plots were weeded 3 times in 1982, 4 times in 1983, and twice in 1984, after heavy weed infestation. On 10 October 1983 and 12 October 1984 after the growth of vines had ceased, 2 plants were sampled at random from each plot for dry-matter and leaf-area determination. For dry matter, plant parts were dried in ventilated ovens at 70°C for 72 h. Area:dry-weight ratios of leaf discs cut with a cork borer of 1.6 cm dia were used to determine leaf area. Mature "tubers were harvested in December of each year, and multiplication ratio was calculated. The experiment was sited at the Eastern Farm of NRCRI, Umudike (05 ° 29' N, 07 °33'E; 122 m a.s.1.). The soil was a sandy loam.
RESULTS AND DISCUSSION
Sprouting: Ominelu (D. alata) was always the first cultivar to sprout, usually 8-14 days after planting (PAP) while the others sprouted 23-35 DAP except Ona (D. dumetorum) which took 25-50 days (Fig. 3). Some cultivars completed sprouting faster than others, the time interval being between 20 and 40 days from first sprout emerged to complete sprouting. Maximum percentage sprouting varied among cultivars (Table 1 ). The two cultivars ofD. alata gave the highest percentage, often as much as 100%, followed closely by Abi and Ekpe, both D. rotundata. The only cultivar of D. rotundata which tended to give a low percentage was Nwopoko. The sprouting of Okumanu (D. cayenensis) was similar to those of most cultivars of D. rotundata. Ona (D. dumetorum) gave the lowest sprouting, except in 1984. These data show that minisetts from the head, middle and tail portions of the tuber sprouted. Sprouting was therefore not a limitation to yield under favourable conditions, as most cultivars had 80-100% sprouting in one season or another. There was an indication that the low percentage sprouting of Ona (D. dumetorum), which increased to 66% in 1984, can be improved by the use of healthy tubers as planting materials; tubers to be chosen for this technique should be free from diseases and pests (Plumbley and Rees, 1983 ) especially nematodes, which feed below the skin (periderm) of the tuber. It is known that the yam tuber has no dormant buds and that bud initials are located just below the periderm (Onwueme, 1973). Any damage to this meristematic tissue would result in poor sprouting.
85 100
•~
50
0
~o
~°
I
I
I
I
I
I
I
I
I
I
I
I
I
8
15
22
29
36
43
50
57
64
71
78
85
92
Days
after
planting
Fig. 3. Time course of sprouting of minisetts, 1982 data. Vertical bars are LSD (P=0.05) at two stages of sprouting2 (Fig. 3, although presenting only 1982 data, is representative of 1983 and 1984 experiments). Symbols o--o Ominelu D. alata • --e Um 680 [::]--[~ Abi m--M Nwopoko D. rotundata A--A Okwocha O--~ Obiaoturugo 0--# Ekpe D. cayenensis A--A Okumenu D. dumetorum V--V Ona
Tuber yield: The two cultivars of D. alata significantly outyielded the other cultivars (Table 1 ), probably because D. alata sprouted faster (Fig. 3) and had higher percentage sprouting (Table 1) than most of the others. Dioscorea alata also had greater leaf area indices (LAI) than the other cultivars (Table 2); the resulting greater leaf-area duration (LAD)gave greater tuber yields (Enyi, 1972). Maximum LAI (Table 2 ) was significantly correlated with tuber yield per ha (Table 1) in both 1983 (r = 0.72, P < 0.05) and 1984 (r = 0.80, P < 0.01). Nwopoko yielded least of all cultivars ofD. rotundata, probably because of its low establishment (Table 1). Similarly, Ona (D. dumetorum) increased its yield from
86 TABLE 1 Sprouting of minisetts, tuber yieldand tuber size distribution of yam cultivarsaveragedoverthree cropping seasons, 1982-1984 Species/ Cultivar
Sprouting ( %)
Tuberyield
Size distribution (%)
t/ha
'000/ha
200 g
200500 g
5001000 g
> 1000 g
85.6 81.5
22.5 16.1
35.6 43.2
27.9 45.4
27.9 30.1
34.0 19.0
10.2 5.5
81.0 74.9 69.9 70.5 51.7
11.2 7.3 9.3 10.2 5.3
34.1 23.0 22.8 23.1 16.1
38.7 65.1 53.8 48.0 63.2
36.6 21.4 28.4 34.4 23.7
22.1 13.5 13.3 15.0 12.1
2.5 0.0 4.0 2.6 0.9
67.1
8.9
21.7
55.3
27.6
13.9
3.1
37.8
7.7
11.7
39.3
33.0
26.3
1.4
4.5
4.1
D. alata
Ominelu Um 680 D. r o t u n d a t a Abi Ekpe Obiaoturugo Okwocha Nwopoko
D. vayenensis
Okumanu D, durnetorurn
Ona LSD (0.05) between cultivars:
an average of 6.0 t / h a in 1982 and 1983 to 11.1 t / h a in 1984, when its percentage sprouting increased from 24% to 66%. O k u m a n u (D. cayenensis) had, on average, similar yields to those of most cultivars of D. rotundata. The multiplication ratios in 1982 and 1983 ranged between 7.1 and 29.9, which fall within the range found among cereals and legumes {Onwueme, 1978). In terms of consistency, D. alata gave the highest multiplication ratios. It is remarkable that, in 1983, Ona (D. dumetorum ) gave the highest multiplication ratio although its establishment was poor. All four species used in this study, therefore, responded well to the minisett technique. Water yams, D. alata, responded better t h a n the other species in terms of both yield and multiplication ratios. The response of D. alata to this technique can be exploited to increase production for local consumption, for export, and as a replacement for grains in supplying the energy component in livestock feeds. Yields of 40-50 t / h a (2530% dry matter) have been recorded in experimental plots at NRCRI, Umudike, higher t h a n yields obtainable from corn (Phillips, 1964) and the best available varieties of cassava (Ene and Okeke, 1983) in Nigeria. Tuber yields in 1984 were lower than in 1982 and 1983 (Table 1 ), even though 1984 had more favourable weather. In 1984, sunshine hours and rainfall were
87
TABLE2 Leafareaindex(LAI)•tubersizeandmultiplicati•nrati•s•fyamsinthreecr•ppingseas•ns••982-•984 Maximum LA!
Average tubersize (g)
Multiplication ratio
1983
1984
1982
1983
1984
1982
1983
1984
6.2 5.1
4.7 1.8
529 429
703 566
269 220
25.1 10.4
24.1 20.2
10.0 7.9
2.8 1.4 1.7 1.9 0.7
1.6 0.9 1.2 1.2 0.6
245 203 267 245 189
487 274 463 560 374
154 84 75 145 115
8.5 8.6 10.2 11.1 7.1
8.3 13.9 18.4 16.8 14.5
5.9 2.8 1.8 5.4 4.2
Okumanu
2.0
1.3
262
370
149
10.8
19.4
5.5
D. d u m e t o r u m Ona
3.3
1.5
294
479
259
12.4
29.9
10.4
Seasonal mean
2.8
1.6
295
475
163
11.6
18.3
6.0
LSD (0.05) between two varieties in one season
1.4
1.0
75
91
49
5.3
6.4
2.2
Species/cultivar
D. alata Ominelu Um 680
D. rotundata Abi Ekpe Obiaoturugo Okwocha Nwopoko
D. eayenensis
LSD (0.05) between two seasonal means
0.7
56
3A
LSD (0.05) between two seasons for one variety
0.6
97
5.!)
higher by 2.8% and 34.1%, respectively, than the 1982 and 1983 averages. This low yield was due to inadequate weed control (Unamma and Melifonwu, 1986). Weed interference did not have a significant effect on number of tubers/ha within cultivars, its main effect being in the reduction of tuber size (Table 2). The reduction in yield might have been mediated through reduction in leaf area (Table 2 ), two variables that were closely correlated. In 1984, average LAI was reduced by 43% when compared with the 1983 cropping season, whereas tuber yield was reduced by 58%. This might suggest that weed competition also had a direct effect on tuber yield, probably by increasing soil physical impedance to tuber growth (Lal and Hahn, 1973; Kang and Wilson, 1981). In 1982, the percentage of saleable seed yams ( > 200 g) ranged between 45 and 81%, in 1983 between 58 and 83%, and in 1984 between 6 and 55%. The cultivars varied in their response to weed competition, the most tolerant being Ona (D. dumetorum) followed by the cultivars of D. alata; D. rotundata was the most sensitive. It is noteworthy that, in 1983, all cultivars except Ekpe yielded some ware yams ( > 1 kg), suggesting that the minisetts of most cultivars have the
88 p o t e n t i a l of giving a m u l t i p l i c a t i o n r a t i o g r e a t e r t h a n 40. I n t h e y a m - p r o d u c i n g a r e a s of W e s t Africa, seed y a m s are sold b y n u m b e r , so t h e m o r e saleable t u b e r s t h a t are h a r v e s t e d , t h e g r e a t e r t h e e c o n o m i c r e t u r n s . T h e r e f o r e , to m a x i m i s e r e t u r n s in y a m p r o d u c t i o n u s i n g t h e m i n i s e t t t e c h nique, h e a l t h y p l a n t i n g m a t e r i a l s s h o u l d be u s e d in o r d e r to o b t a i n high s p r o u t ing p e r c e n t a g e a n d high p o p u l a t i o n s , this, followed b y good m a n a g e m e n t (especially e a r l y s t a k i n g a n d efficient c o n t r o l of w e e d s ) s h o u l d e n s u r e a p r e p o n d e r a n c e o f saleable t u b e r s . ACKNOWLEDGEMENT T h e a u t h o r s are g r a t e f u l to t h e D i r e c t o r , N a t i o n a l R o o t C r o p s R e s e a r c h I n s t i t u t e ( N R C R I ) , U m u d i k e for p e r m i s s i o n to p u b l i s h t h e s e d a t a , a n d to t h e s t a f f o f t h e Y a m P r o g r a m m e a n d P l a n t P h y s i o l o g y D i v i s i o n for t e c h n i c a l assistance.
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89 Onwueme, I.C., 1977b. Effect of varying the time of the first-harvest, and of late planting on double-harvest yield of yam (Dioscorea rotundata) in field plots. J. Agric. Sci., 88: 737-741. Onwuene, I.C., 1978. The Tropical Tuber Crops. John Wiley, Chichester, 95 pp. Oyolu, E., 1978. Inherent constraints to high productivity and low production cost in yam {Dioscorea spp.) with special reference to Discorea rotundata Poir. In: Proc. Seminar on Yams, 1 7 October, 1978, Buea, Cameroons. International Foundation for Science, Stockholm, Provis. Rep. No. 3,, pp. 183-192. Phillips, T.A., 1964. An Agricultural Notebook. Longmans, Nigeria, 240 pp. Plumbley, R.A. and Rees, D.P., 1983. An infestation by Aracerus [asciculatus (Degeer) (Coleoptera: Anthribidae) and Decadarchis minuscula {Walsingham) {Lepidoptera: Tineidae) on stored fresh yam tubers in S.E. Nigeria. J. Stored Prod. Res., 19: 93-95. Unamma, R.P.A. and Melifonwu, A.A., 1986. Herbicides for 'seed' yam production from "minisetts" in the rainforest zone of Nigeria. Weed Res., 26: 115-120. Wilson, J.E., 1978. Recent developments in the propagation of yam (Dioscorea spp). In: Proc. Seminar on Yams, 1-7 October, 1978, Buea, Cameroons. International Foundation for Science, Stockholm, Provis. Rep. No. 3, pp. 87-92.