Effect of stubble height and harvesting intervals on yield and nutritive value of three sweet potato (ipomea batatas (l) lam) cultivars
Lusweti, C M
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Three sweet potato varieties namely, Kemb-30, Kemb-9 and Kemb- 8 were evaluated in two trials conducted at the Field station of the Faculty of Agriculture, University of Nairobi in the first cropping (1991 May planted crop) and second cropping (1991 October planted crop) seasons, respectively. The main objective of the two trials was to evaluate the effect of cutting fixtures and stubble height on dry matter yield of both tubers and vines, nutri tive quality, regrowth and varietal response. The trials of both seasons were laid out in a split-split plot randomized block design replicated three times. The three varieties of sweet potatoes constituted the main plots and two stubble heights (i.e. 25cm and 50cm) constituted the sub-plots. Four levels of cutting fixtures (i.e. 4-weekly, 6-weekly, 8-weekly and control) superimposed on stubble heights constituted sub- sub plots. In both seasons Kemb-8 tuber yields were significantly lower (P .:5 0.05) than those of Kemb-30 and Kemb-9. However, no significant differences in tuber yield was observed in the latter two. The tuber yields of the 25cm stubble were significantly lower (P .:5 0.05) than the less cut stubble of 50cm for Kemb-30 and Kemb-9. However, Kemb-8 was least affected by either stubble height. Cutting fixtures of 4-weekly, 6-weekly and 8-weekly were not significantly different in their tuber yields. However, the tuber yields of the control were significantly higher than the latter three. In the first cropping season Kemb-9 yielded a significantly better grade quality (marketable yield) (P ~ 0.05) than Kemb-30 and Kemb-8. However, in the second cropping season no significant differences were observed between Kemb-9 and Kemb-30. The stubble height and cutting treatments did not elicite any significant differences in grade quality (marketable yield) amongst the cultivars. In the first cropping season the 50cm stubble yielded significantly higher (P ~ 0.05) tuber number per plant than the 25cm stubble. In the second cropping season no significant difference in tuber number per plant occurred. Results have however, suggested that less cutting of vines (i.e. at 50cm stubble and at 8-weekly or control) generally tended to increase tuber yield, tuber grade and tuber number per plant. Kemb-8 vine yields were significantly higher (P ~ 0.05) than Kemb-30, but no significant differences (P ~ 0.05) were observed between Kernb-8 and Kemb-9; and also between Kemb-9 and Kemb-30 in both seasons. The response of Kemb-30 was however, consistently low across the cutting fixtures. The cuts at 8-weekly and the control, on the other hand, produced higher vine yields. In both seasons, highly significant positive association with tuber yield was exhibited by higher average number of tubers per plant, a better marketable yield (grade quality) and a less severe stubble height for the three varieties of sweet potatoes. Leaf area did not show as much association as there was a lot of variation among varieties and the association of the latter with tuber yield across all varieties and cuts in terms of leaf area in both seasons was actually negative. vines across all varieties had significantly (P ~ 0.05) a higher content of CP than napier grass and appropriate for complimenting napier grass in Zero grazing. The tuber fraction in terms of its possible utility value as a feed appears to rest in its inherently high hemicellulose content (53%) compared to napier grass (30%) and vines (9%).