Effect of sett-size on the yield of two yam (Dioscorea sp.) varieties grown in the dry- and rainy-seasons
Nigeria Agricultural Journal
Traditionally yams ( .) are produced in one season only in a year i.e. during the rainy-season lasting from March/April to October/November in southern Nigeria. Thus yam tubers are planted and harvested at 12month intervals. Because of this long interval between harvests, a lot of the tubers get rotten in storage and in the markets between June and November. When yam tubers are harvested in November, they remain dormant for 3-4 months before they sprout in February/March. It is this spouting of
... yam tubers that triggers off the physiological changes that result in tuber deterioration (Coursey 1961, Passam and Noon 1977) . It has been shown that gibberellic acid (GA ) solution delays sprouting of yam tubers for 4-8 months depending on yam variety (Igwilo 1982 , Wickam 1983 , Igwilo . 1988). Because of this yam tuber response to GA treatment, yam tuber sprouting can be delayed to resume sprouting in August /September to be planted in October near the end of the rainy-season and the onset of the dry-season. Thus, yam can now be planted in the dry-season in addition to the traditional rainy-season planting. It has been shown that sprouting of yam tubers is controlled by an endogenous rhythm with annual periodicity (N. Igwilo, unpublished). Consequently, when yams sprout in the dry-season, they maintain a dry-season periodicity permanently. When sprouted tubers are therefore planted in October/ November, they mature in May/June and sprout again in the next October/November ready for another dry-season growth cycle. In other words, there can now be two crops of yam in a year-the traditional rainy-season cropping and the new dryseason cropping with irrigation during the dry period. Thus when the tubers that sprouted in February/ March, at the beginning of the rainy-season, begin to deteriorate in May/June in storage and markets, fresh mature tubers from dry-season planting begin to arrive in the markets and fresh mature tubers are available in the market all year round. The sprouting time of yam tubers had been changed permanently from rainy-season to dry-season period with Giberillic Acid (GA) solution. The yield performance of minisetts and peelsetts of two yam varieties were compared in both seasons (using irrigation during the dry periods). Peelsetts (6.25g), obtained from the periderm of yam tubers, and minisetts( 25g), which were cut pieces of the yam tubers, were planted in April for the rainy-season crop and in October for the dry-season crop, starting from April 2004 to October, 2005. The yam varieties used were Obiaoturugo (Dioscorea rotundata) and Um 680 (Dioscorea alata). The rainy-season crop matured in November and the dry-season crop matured in May for (D. rotundata) and in June for (D.alata). In the rainy-season harvests, the yields of minisetts and peelsetts per hectare were similar in each variety, whereas in the dry-season harvests, the minisetts of outyielded (P =0.05) the peelsetts while the peelsetts of Dialata, Um680, on the contrary, outyielded the minisetts. Um680 outyielded (P = 0.05) Obiaoturugo in both seasons. On the average, yields were higher in the dry-season than in the rainy-season due mainly to size of tubers rather than number of tubers. The implication of the new technology of growing yams in the dry-season is discussed.