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African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 97-101
H. C. Kiozya, K. Mtunda, R. Kapinga1, B. Chirimi2 and E. Rwiza2
Code Number: CS01039 ABSTRACT The objectives of this study were to assess the effects of leaf harvesting frequency on growth and yield of sweetpotato (Ipomoea batatus L.); to determine the suitable varietal types for use as source of leaf vegetable and to determine the suitable leaf harvesting frequency(ies) for reduced detrimental effects on growth and yield of sweetpotato. Two narrow leafed varieties Budagala and Mwananjemu and two broad leafed varieties SPN/O and Biganana were used in a randomised complete block design with 3 replications. Plot size was 8.4 m2 . Tip cuttings of 20 cm long were used as planting materials. The fully expanded top and tender leaves were plucked, counted and weighed. Tender leaves were harvested once, twice and three times during the growing period. Results showed that there were small differences between varieties and harvesting frequencies, however, harvesting at one month intervals for three times gave the highest leaf vegetable production although the yield was affected tremendously. Harvesting twice in a growing period proved the best in leaf production as well as root yields. Overall, there were significant differences between varieties and harvesting frequencies. Key Words: Ipomoea batatus, leaf harvesting, Tanzania, tuber and vine yields RÉSUMÉ Les objectifs de cette étude étaient d' évaluer les effets de la fréquence de récolte de feuilles sur la croissance et le rendement de la patate douce (Ipomoea batatus L.), de déterminer des types de variétés aproppriées comme source de légumes végétales et de déterminer les fréquences aproppriées de récolte des feuilles pour minimiser des effets néfastes sur la croissance et le rendement de la patate douce. Deux variétés à petites feuilles Budagala et Mwananjemu et deux variétés à grandes feuilles SPN/O et Biganana ont été utilisées dans un blocs aléatoires complètement randomisés evec trois répétitions. Les dimensions des parcelles étaient de 8.4 m2 . Des boutures de sommets de 20cm de long ont été utilisées pour la plantation. Les feuilles tendres complètement développées du sommet de la tige ont été coupées, contées et mesurées. Les feuilles tendres ont été recoltées une fois, deux fois et trois fois durant le période de croissance. Les résultats on montré qu' il y avaient de petites différences entre les variétés et les fréquences de récoltes, cependant la récolte à un mois d' intervalle pour trois fois a donné une bonne production de légumes végétales bien que le rendement a été affecté très gravement. La récolte de deux fois durant la période de croissance a été trouvée meilleure pour la production foliaire et le rendement des tubercules. Générallement il y avaient des différences significatives entre les variétés et les fréquences de récolte. Mots Clés: Ipomoea batatus, la récolte des feuilles, Tanzanie, productions des tubercules et des cordes de tige INTRODUCTION Sweetpotato (Ipomoea batatus) is an important secondary food crop in Tanzania and is grown mainly for household food consumption (Kapinga et al., 1995). Storage roots are the most important part of the crop and are consumed in various ways, boiled, roasted, masted, etc. However, in some areas tender leaves are utilised as relish particularly during the dry season, when there are few leaf vegetables. A country-wide study by Kapinga et al. (1995) reported that sweetpotato leaves are mainly used as vegetable in the Lake, Eastern and Central zones. The Western zone, not covered in the survey, is also well known for using sweetpotato leaves as a relish. Leaf harvesting at household level involves plucking the fully expanded tender leaves. The same field can be visited frequently if it is the only available sweetpotato field. The effect of this practice on the overall performance of sweetpotato plants is, however, not known. Studies conducted in Tanzania indicated that in all places where the use of sweetpotato leaves as a vegetable is common, narrow leafed varieties are preferred to broad leafed ones. Reasons given were good processing qualities and reduced elasticity (slimeness) of vegetable soup made out of leafed varieties (Kapinga et al., 1995). Of recent farmers were invited to Ukiriguru and Kibaha institutes to assess postharvest qualities of some advanced sweetpotato varieties from the 1996 Advanced Yield Trials. Among the attributes recommended for consideration by the breeding programme, was suitability for leaf vegetable production. The varieties assessed were a mixture of narrow as well as broad leaved types. The farmers recommended broad leaved varieties such as SPN/O and Biganana for leaf relish production. These new findings posed a challenge to researchers on which type of variety could be recommended for leaf vegetable. Secondly, although leaves are harvested frequently by women farmers, no study had been conducted in the country to establish the effect of this practice on overall performance of sweetpotato. Therefore, the objectives of this study were to: (i) investigate the suitability of different types of varieties for leaf vegetable production, (ii) determine the effect of harvesting frequency (ies) of sweetpotato leaves on growth and yield of sweetpotato, and (iii) assess the processing qualities of different sweetpotato varieties for leaf vegetable production during the dry season. MATERIALS AND METHODS The experiment was planted at ARI- Ukiriguru in three seasons starting during the1996/97 season. The trial was repeated at SRI- Kibaha in two seasons from 1997 to 1998. The varieties used were Budagala and Mwananjemu with narrow leaves, SPN/O and Biganana with broad leaves. At each location, the experiment was laid out in a 2 x 3 factorial in a RCBD with three replications. The gross plot size was 6.0 m x 4.0 m and sampling plots each measured 4.2 m x 2.0 m. Spacing used was 30 cm between plants and 1.0 m between rows. Planting was done in December of each season at Ukiriguru and in March for each season in Kibaha. The first five tender and fully expanded leaves were plucked at one month after planting and this was continued at monthly intervals for three months. For the second treatment plucking was stopped after two months. The last treatment involved plucking leaves only once at one month after planting. The control plot was left unplucked throughout the growing season. Each season the experiment was harvested 5 months after planting. RESULTS AND DISCUSSION The total weights of leaves harvested indicated non significant difference between varieties and harvesting frequencies in both locations (Table 1) . However, harvesting three times at one month intervals gave relatively the highest leaf vegetable production. Also, the mean total leaf weight at Ukiriguru was higher than at Kibaha for all varieties. In terms of leaf production, broad leafed varieties generally performed better than narrow leafed - varieties although the differences were statistically not significant Table 1). The effect of leaf harvesting on tuber yield depended on cultivar and frequency of leaf harvesting (Table 2). At Ukiriguru cv. Budagala was the highest tuber yielder and no effect of frequency of leaf harvest was apparent. The impact of leaf harvesting was greater on Biganana: it had the highest tuber yield without leaf harvest, but yield was drastically curtailed by leaf harvesting, imperative of the frequency of leaf harvest. There was also slight tuber yield decline for Mwananjemu following leaf harvesting, while the converse seemed to be the case with SPN/O, except with leaf harvesting twice. Tuber yields were generally lower at Kibaha, but leaf harvesting generally caused only slight reduction in tuber yield. At this site differences were not statistically significant, except for the markedly low yield of cv. Budagala in comparison to cvs. Mwananjemu and SPN/O. The findings at Ukiriguru indicated that harvesting twice at one month intervals might be a suitable frequency for reduced effects on root and foliage yield. This was not the case at Kibaha. Harvesting twice at one month interval gave the relatively higher storage root yield than the other two harvesting frequencies. The relative low reduction in tuberous root during the seasons could be attributed to the method of harvest which left enough leaves unharvested. The remaining leaves could photosynthesise and store enough food in the tubers. It is also possible that harvesting twice at one month interval might have promoted branching which provides room for more leaves. The differences in architecture probably account for superior leaf yield of the improved varieties like SPN/O and local best varieties like Mwananjemu over the local varieties Budagala. The effect of leaf harvesting frequency on the vine yield are shown in Table 3. At Kibaha no significant differences were detected. However, the cv. Mwananjemu had the lowest vine yield at both Ukungiri and Kibaha. At Ukingiri, leaf harvesting twice or thrice promoted vine yield, but harvesting once had no impact on vine yield. Contrastingly, leaf harvesting tended to supress vine yield for cv. Biganana at Ukungiri but not at Kibaha. The small differences in vine yield at Kibaha could be due to abundant moisture throughout the growing season. The findings of this study suggest that leaf harvesting had limited impact on tuberous yield of sweetpotato. A similar finding was reported in Uganda (Lugojja et al., 2001). However, the impact of leaf harvesting will likely depend on weather conditions, being likely adversely affected in drought periods. Generally, broad-leaved varieties appear better producers of leaf vegetables in terms of yield. REFERENCES
TABLE 1. Effects of leaf harvesting on sweetpotato leaf vegetable production at Ukiriguru, Mwanza (average of two seasons 1997/98,1998/99) and at Kibaha (1998/99 season).
TABLE 2. Effects of leaf harvesting on sweetpotato storage root yield at Ukiriguru and Kibaha, Tanzania
TABLE 3. Effects of leaf harvesting on sweetpotato vine yield production at Ukiriguru in the 1997/98
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