TROPICAL YAMS AND THEIR POTENTIAL, PART 3 



27 



Harvesting 



Near the end of the growing sea- 

 son, which generally coincides 

 with the beginning of the dry sea- 

 son and shorter days and cooler 

 temperatures, vegetative growth 

 ceases in most varieties of wing- 

 stemmed yams. Flowers and aerial 

 tubers are sometimes produced. At 

 this time the underground tubers 

 are also produced. These develop 

 rapidly until the foliage dies back. 

 It is not known what triggers sea- 

 sonal dieback, but once begun, 

 fungi often kill the foliage rapidly. 

 This terminal fungal infection of 

 the leaves, not invariable, should 

 not be confused with premature 

 dieback associated with Colleto- 

 trichum. Tuber growth cannot con- 

 tinue after the foliage has dried 

 up, but maturation of the tissue 

 may continue, and in particular 

 the cork cap over the tuber will 

 continue to develop. Therefore, it 

 may be advantageous to harvest 

 several weeks after foliage die- 

 back. 



Tubers are frequently dug with 

 special sticks that look somewhat 

 like large spoons. Conventional 

 spading forks and shovels are 

 more useful. 



Most varieties of D. alata pro- 

 duce large, deep tubers that must 

 be dug with some care. On the 

 other hand, a few varieties produce 

 compact, often multiple tubers 

 near the surface of the soil. Con- 

 ventional machinery for the har- 

 vest of potatoes and sweetpotatoes 

 can be adapted to such varieties. 

 In these cases it may be necessary 



to remove vines and staking sys- 

 tems first. Needless to say, me- 

 chanical harvesting is facilitated 

 by planning spacing in advance 

 and by using ridges of soil from 

 which tubers can easily be re- 

 moved. 



Poor care, bad weather, and dis- 

 ease can reduce yields to zero. On 

 the other hand, favorable combi- 

 nations of variety, cultural tech- 

 nique, and weather can produce 

 exceptional yields. Unfortunately, 

 high rather than average yield fig- 

 ures tend to be made available, so 

 it is difficult to know what typical 

 yields are. 



Exceptional weights of indi- 

 vidual tubers have been recorded. 

 Haynes and Coursey (16), in a 

 study of giant yam production in 

 Trinidad, mention tubers up to 81 

 kilograms. This weight compares 

 favorably to that of the largest 

 yams produced in Papua New 

 Guinea. These giant yams are pro- 

 duced by giving them individual 

 attention not compatible with com- 

 mercial production. 



An experiment station's yield 

 from small plots sometimes reaches 

 40 to 50 metric tons per hectare. 

 Normal yields under very good 

 agronomic conditions range from 

 15 to 20 metric tons. In Barbados 

 yields obtained without stakes 

 range up to about 18 metric tons 

 per hectare under the very best 

 conditions (13). In Fiji yields 

 range from 4 to 25 metric tons per 

 hectare, depending on staking and 

 size of seed piece (2). Coursey 

 (9) has summarized typical yields 



