higher, about 15 to 25 metric tons per hectare. Since the tubers 

 are about the same size, yield differences may be due to differences 

 in plant spacing. The highest yield reported in Puerto Rico for 

 D. rotundata is 67.3 metric tons per hectare, a yield of healthy 

 tubers grown in a friable clay soil without plowing or construction 

 of mounds (6). Most reported yields are considerably less than 

 should be expected under improved agronomic conditions, especial- 

 ly the elimination of diseased plants and the use of high-quality 

 seed tubers. 



The distribution of yields among early and late harvests 

 varies. In some cultivars the early harvest yields more than the 

 late harvest, but this depends chiefly on variety and time of 

 harvest. 



Interesting differences can be seen when D. esculenta, D. 

 alata, and D. rotundata are grown together. The yield of D. alata 

 cultivars is often double that of D. rotundata and D, cayenensis. 

 D. esculenta yields are variable, but under good conditions are 

 better than those of any other species. 



STORAGE 



The best data on yam storage concerns D. rotundata. This 

 yam, and to a lesser extent D. cayenensis, is stored more frequently 

 for long periods and constitutes an important staple for a large 

 number of people. 



Good storage practice begins in the field. During the dry 

 season, tubers can be left unharvested in the ground without 

 appreciable loss of quality. Although they will lose weight, mainly 

 from respiration and to a lesser extent from dehydration, the 

 advantages of field storage outweigh such losses. However, tubers 

 stored in the field are subject to damage by beetles, rats, and 

 pigs, and of course, the depredations of human beings. It is per- 

 haps because of the latter that tubers are normally harvested for 

 storage. After harvest, tubers should never be left in the sun or 

 rain as both increase susceptibility to rot. 



Storage facilities vary among farmers and throughout the 

 yam-growing region. It is common enough to find tubers stored in 

 pits, in piles, or on the ground under houses, but more sophisticated 

 structures, yam barns, are also used (fig. 11). A yam barn is a 

 shed with woven-stick walls and a thatched roof. In the barns 

 the individual tubers are tied to walls. The tubers being separated 

 and surrounded by air, storage rot is minimized. 



Coursey (7) has shown that D. rotundata tubers store better 

 than those of D. cayenensis, as would be expected from the differ- 

 ing lengths of their growing seasons. The principal causes of loss 

 are respiration, desiccation, and rotting. 



The most effective protection against storage loss is selection 



29 



