African Crop Science Journal, Vol., 9, No., 1, March 2001, pp. 25-31
J. A. OTOO, A. MISSAH and A.G. CARSON1
Code Number: CS01031
Multilocational trials were conducted in the coastal savanna, forest and forest/savanna agro-ecologies of Ghana in 1996 and 1997 to identify early-maturing sweetpotato varieties. Seven genotypes (including a local check) were evaluated in five locations. Plants were sampled for tuber yields and other characteristics at 3, 4, 5 and 6 months after planting (MAP). Sweetpotato is normally harvested at 4 MAP, so the fresh and dry tuber yields and biomass production of the genotypes at 3 MAP gave an indication of their early -maturity potential especially when compared with the yields of the local variety (the check) at 4 MAP. Using these criteria, TIS 8266, TIS 84/0320, TIS 86/0350, TIS 3017, Sauti and Dugbadza were identified as early-maturing in the coastal savanna agro-ecology. In the forest agro-ecology, all the genotypes except TIS 86/0350 were also early maturing. In the forest/savanna transition zone, there were no significant tuber yield differences between the genotypes at 3 MAP and the local variety (Local Red) at 4 MAP. Similar results were obtained with the dry tuber yields and biomass production in the three agro-ecologies. They could, therefore, be considered as early maturing. Percent dry matter in all genotypes were high from the earliest to the latest harvesting (three to six months after planting) in all agro-ecologies.
Key Words: Ghana, harvest index, Ipomoea batatus, maturity period, tuber yield
Des essais multilocationaux ont été menés dans les zones agro-écologiques dans la forêt et des savanes côtières et forestières du Ghana en 1996 et 1997 pour identifier des variétés précoces de patate douce. Sept génotypes (y inclut un témoin local) étaient évalués dans 5 localités. Des plants étaient échantillonnés pour leurs rendements en tubercules et autres caractéristiques à 3,4,5 et 6 mois après la plantation (MAP). La patate douce est normalement récoltée à 4 MAP, ainsi les rendements en tubercules frais et secs et la production de biomasse des génotypes à 3 MAP ont donné une indication de leur précocité potentielle surtout par comparaison aux rendements de la variété locale (le témoin) à 4 MAP. Utilisant ces critères, le TIS 8266, TIS 84/0320, TIS 86/0350, TIS 3017, Sauti et Dugbaza ont été identifiées comme variétés précoces dans la zone agro-écologique de la savane côtière. Dans la zone forestière, tous les génotypes à l'exception de TIS 86/0350 ont été révélés aussi à maturité précoce. Dans la zone de transition entre la savane et la forêt, il n'y a pas eu de différence significative pour les rendements en tubercules entre les génotypes à 3 MAP et la variété locale (la Locale Rouge) à 4 MAP. Des résultats similaires ont été obtenus avec les rendements en tubercules secs et la production de biomasse dans les 3 zones agro-écologiques. On peut alors les considérer comme des variétés précoces. Les pourcentages de matière sèche des génotypes étaient élevés depuis la récolte précoce jusqu' à la derniére des récoltes (3 à 6 mois après plantation) dans toutes les zones agro-écologiques.
Mots Clés: Ghana, indice de la récolte, Ipomoea batatus, période de maturité, rendement en tubercule
Sweetpotato, Ipomoea batatus, is an important food crop in many parts of the world. In Ghana, cassava, cocoyam and yam have been the principal root and tuber crops over the years. As such, sweetpotato has been neglected by research in the past, but with the inception of the National Root and Tuber Crops Improvement Project (NRTCIP), some attention has been devoted to increasing the production of sweetpotato as a source of dietary energy, vitamins, minerals and proteins and its use as animal feed. Although it is cultivated mainly for the carbohydrate-rich tubers, the foliage has the potential for use as vegetable and animal feed.
Sweetpotato germplasm was imported from the International Institute of Tropical Agriculture, (IITA), Ibadan, Nigeria, and from Malawi. The accessions were evaluated, together with local collections, in different locations in the coastal savanna, forest, and forest /transition agro-ecological zones of Ghana (Missah et al. , 1995, 1996; Otoo et al. , 1995). From an initial 25 improved genotypes from IITA and several local accessions, seven were selected for further testing multilocationally with the aim of ascertaining their general performance, potential tuber yield, tolerance to diseases and pests, and quality characters preferred by consumers. Sweetpotato is usually harvested four months after planting (MAP). But we were also interested in varieties that could yield well at 3 MAP. Their yield performance at 3 MAP compared to the performance of the local variety (the check) at 4 MAP, would be a good indication of their early-maturing potential. Early - maturing varieties could help combat the hunger period before the season's crops are harvested. Early planting and harvesting of early maturing varieties could also attract good price for the farmer. This paper presents the results obtained for the selected genotypes in three agro-ecological zones in 1996 and 1997, with emphasis on their early-maturity potential at 3 MAP.
MATERIALS AND METHODS
The seven genotypes evaluated were grown in three agro-ecological zones in Ghana. The coastal savanna was represented by sites at Cape Coast and Pokuase; the forest by Fumesua and Akomadan, and the forest/savanna transition by Wenchi. The experiments were planted in the main cropping season (May to June) at all locations in 1996 and 1997, except in Cape Coast, where planting was done only in 1996.
The seven genotypes evaluated were TIS 8266, TIS 86/0350, TIS 84/0320 obtained from IITA; TIS 3017 and Sauti from Malawi; Local Red and Dugbadza selected from local accessions. These were planted in a split plot design, where the main plots were the sweetpotato genotypes while the sub- plots were the time of harvesting - 3, 4, 5, 6 MAP. Each treatment was replicated four times. Individual plots consisted of four 20 m long ridges 1 m apart. Vine cuttings measuring 30 cm were planted on the ridges at a spacing of 30 cm. Manual weeding was done as and when necessary. Plants within a 10 m2 area were dug with a hoe at each harvest. Records taken included the fresh weight of tubers and foliage (including vines). In 1996, percent dry matter of tubers was assessed; two 100-g composite samples were made from chips of several tubers and oven-dried at 80°C until constant weights were attained. Harvest index was calculated as the ratio of tuber weight to total biomass (the sum of tuber, foliage and vine yields) expressed as percentage. Data were then subjected to statistical analysis.
RESULTS AND DISCUSSION
Fresh tuber yield. Mean fresh tuber yield of the sweetpotato genotypes in the various agro-ecological zones are presented in Table 1. Fresh yields increased significantly at each successive harvest for most genotypes until 5 MAP. Although some genotypes yielded the highest at 6 MAP, the percent increase over 5 MAP was not significant. This is an indication of the possibility of harvesting sweetpotato piece-meal between 3 and 5 MAP. Yield levels were generally higher in the forest zone for all genotypes and harvest regimes. The differences in rainfall, soil physical and chemical properties associated with the different locations in the agroecological zones may have influenced the performance of all the genotypes. Genotype X environment interaction is significant for sweetpotato in Ghana (Otoo et al. , 1998). Local Red, however, remained the lowest-yielding in all agro-ecologies while TIS 8266, TIS 86/0350 and TIS 84/0320 were among the high-yielding ones. The highest percent increases in yield were recorded between 3 and 4 MAP, with values ranging from 46.6 percent in Sauti in the coastal savanna to 200 percent in Local Red in the transition zone. However, tuber sizes were marketable at 3 MAP; tuber yields at 3 MAP were also good for most of the genotypes. Sensory evaluations also showed that their cooking qualities were equally good at 3 MAP (Otoo et al. , 2000). Sweetpotato is normally harvested for tuber production at 4 MAP in Ghana, though sometimes, harvesting is delayed beyond 4 MAP because of market circumstances. Any genotypes that give tuber yields in 3 MAP which are as good as yields of the local variety (check) in 4 MAP could be considered as early-maturing. In the coastal savanna and forest/savanna transition agro-ecologies, there were no significant differences in fresh tuber yield of the six selections at 3 MAP and the local variety (Local Red) at 4 MAP. It should also be pointed out that harvesting sweetpotato 5 or 6 MAP may not be beneficial as damage to tubers by the sweetpotato weevil, Cylas spp. may be pronounced (Missah and Kissiedu, 1994).
Total biomass. Total biomass generally increased from 3 MAP, peaked at 5 MAP and depressed at 6 MAP in all agro-ecologies except for TIS 8266 in the forest zone (Table 2). Overall, yield levels were highest in the forest zone compared to the two other agro-ecologies. In all locations, Sauti, Dugbadza, TIS 86/0350 were among the highest biomass producers. Local Red recorded the lowest biomass production in most harvesting times in the agro-ecologies. Compared with fresh tuber yield, the increases in total biomass production from 3 to 4 MAP were not high. TIS 8266 out-yielded the other genotypes in tuber production at all locations at 3 MAP. But it was not among the highest biomass producers. This could be attributed to its relatively low vine production. Being good biomass and tuber producers, Sauti, Dugbadza, and TIS 86/0350 would be useful as dual-purpose varieties for the production of tuber and foliage for animal feed.
In the coastal savanna zone, all the genotypes produced either significantly high biomass or no significant differences in biomass at 3 MAP when compared with the local variety (Local Red) at 4 MAP. There were also no significant differences in the forest zone. In the forest/savanna transition, the genotypes either produced higher or no significant differences in biomass when compared with the local variety. The results indicate that the genotypes are early-maturing in biomass production.
Harvest index. Harvest indices for the various times of harvesting showed successive increases for all genotypes in all agro-ecologies except for TIS 84/0320 at 6 MAP in the coastal savanna (Table 3). TIS 8266 had the highest harvest index at every harvest in all locations; this could be an indication of the efficiency of its tubers to attract assimilates. The greatest increases in harvest indices between the third and fourth harvests were associated with Local Red, Sauti, Dugbadza and TIS 86/0350. The pattern of increases generally followed that of fresh tuber and total biomass yields, with the highest increases recorded between 3 and 4 MAP, and the least between 5 and 6 MAP for all entries in all agro-ecologies.
Dry matter and dry tuber yield. Percent dry matter of tubers and dry tuber yields are presented in Tables 4 and 5, respectively. The least percent dry matter (27.4) was recorded for Dugbadza at 3 MAP in the coastal savanna. Most genotypes had dry matter exceeding 30 percent in all agro-ecologies irrespective of the time of harvesting. So the early harvesting at 3 MAP gave acceptable levels of dry matter.
Dry tuber yields ranged from 0.6 t ha-1 in Local Red 3 MAP in the forest/savanna transition zone to 6.9 t ha-1 in TIS 8266, 6 MAP in the forest zone. As observed with other parameters measured, there were increases in dry tuber yield with delayed harvesting. The highest positive change was always found between 3 and 4 MAP. Between these months, most clones had dry yield increases of over 50 percent, but Local Red registering the highest increase of 200 percent in the forest/savanna transition zone. This perhaps explains why sweetpotato harvesting has traditionally been carried out at 4 MAP. The peak dry tuber yield appeared to be at 5 to 6 MAP, but some genotypes showed negative change between 5 and 6 MAP in the forest, coastal and transition zones. Dry tuber yields and harvest indices in the forest/savanna transition zone were lower than in the coastal savanna and forest zones. Erratic rainfall during the early growth phase after planting in that zone delayed canopy formation and was largely responsible for the low tuber yields and harvest indices. In the forest/savanna transition zone, harvest indices for TIS 8266, TIS 84/0320 and TIS 3017 were higher than for the other genotypes (Table 3). The genotypes that produced the high dry tuber yields at 3 MAP in that zone (TIS 8266 and TIS 84/0320) had the highest harvest indices. In the forest zone, the dry tuber yields of all the genotypes at 3 MAP (except TIS 86/0350) were not significantly different from the Local Red (check) at 4 MAP (Table 5). Similar results were obtained in the forest/savanna transition agro-ecology except for TIS 86/0350 and Sauti. Since a highly significant genotype X environment interaction exists in sweetpotato (Otoo et al. , 1998), it is, therefore, expected that all the genotypes which were early maturing in the forest and coastal savanna zones would not exhibit the same characteristics in the forest/savanna transition zone.
Considering the fresh and dry tuber yields (especially at 3 MAP) and biomass production of the genotypes, early maturing varieties could be identified from the sweetpotato genotypes evaluated in this study. TIS 8266, TIS 84/0320, TIS 86/0350, TIS 3017, Sauti and Dugbadza could all be regarded as early varieties in the forest, coastal savanna and forest/savanna transition agro-ecologies. Percent tuber dry matter in all genotypes was high, from the earliest harvesting to the latest (three to six months after planting) in all agro-ecologies. In coastal and forest/savanna transition agro-ecologies, TIS 86/0350, TIS 3017, Sauti and Dugbadza produced the highest biomass. In the forest agro-ecology, all the clones produced high biomass.
TABLE 1. Mean fresh tuber yield (t/ha) of sweetpotato in three agroecological zonesin Ghana (1996, 1997).
TABLE 2. Total biomass of sweetpotato in three agroecological zones in Ghana (1996, 1997).
TABLE 3. Harvest index of sweetpotato in three agroecolological zones in Ghana (1996, 1997).
TABLE 4. Mean percent dry matter of tubers of sweetpotato genotypes in three agroecological zones in Ghana (1996)
TABLE 5. Dry tuber yield of sweetpotato in three agro-ecological zones in Ghana (1997)