the technique, a lower wire is also used, and strings are passed 

 between the two wires. 



Vines should normally be grown in full sunshine; indeed, their 

 climbing habit tends to assure them a place in the sun. For best 

 yields yams should not be planted under trees or in shade. 



Harvests and Yields 



Perhaps the most important characteristic of D. rotundata 

 is not the one which West Africans appreciate, the sticky quality 

 of the cooked tuber that makes the preparation of fufu possible, 

 but the suitability of most cultivars for double harvesting. Unique 

 among yam species in this property, D. rotundata bears twice 

 because of its adaptation to short rainy seasons. 



Whereas the onset of the first harvest is determined by dig- 

 ging around the tuber and deciding when it is large enough, that 

 of the second harvest is shown by senescence of the foliage. In the 

 Northern Hemisphere this occurs from October until January 

 and is dependent on rainfall. Early senescence is sometimes associ- 

 ated with leaf-spot diseases or stem wilt, but it appears that 

 dieback normally begins before disease conditions become severe. 



As soon as dieback occurs, the tuber can be harvested. How- 

 ever, if the tuber has been growing rapidly, the growing tip will 

 not be well protected with bark. If left in the soil several weeks, 

 the bark will develop (at the expense of the flesh, of course) and 

 the tuber can then be stored with less risk. 



Harvest is usually managed with digging sticks, knives, or 

 spades. It is easy to open the mound structure and remove tubers. 

 Vines, stakes, and possibly other plants intercropped with the 

 yams and on the same mounds are left undisturbed. Later, the 

 stakes are sometimes removed for use in new plantings. Once 

 harvested, it is important to remove tubers from the sun, as 

 exposure predisposes them to rot in storage. 



The components of yield are tuber size, tuber number, number 

 of plants per unit of area, and in certain varieties the ability to 

 regrow after a first harvest. Complete crop failures are unusual, 

 even when virus or leaf-spot diseases are present. 



The size of the individual tubers varies considerably, but 

 few exceed 5 kilograms. In contrast, D. alata varieties have 

 produced 50-kilogram tubers. Although D. rotundata has not been 

 studied in detail with respect to the production of giant yams, its 

 potential seems to be limited. 



Most reported African yam yields are low when compared 

 to those of the chief competing species, cassava, and those of 

 Asian yams. Coursey (8) summarized yields for Africa as 7.5 to 

 18.0 metric tons per hectare. In the West Indies yields are usually 



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