M. Ngendahayo and A.G.O. Dixon
Sets of 35 and 25 earlier cassava cultivars of the IITAs breeding program were planted at Ibadan and Onne in Nigeria at the beginning of the rainy season (May/June) in 1992 and harvested at 3-monthly intervals from 6 to 24 months after planting (MAP). The aim was to assess the effect of harvesting date on the root yield, dry matter %, starch %, dry matter yield, starch yield and harvest index. Results showed that average cassava fresh root yield increased over time up to 24 MAP at both locations Fresh root yield of cultivars increased linearly by different amounts up to 12 MAP and beyond which, different cultivars attained their optimum yield at different times. The trend for dry matter (%) was more closely related to rainfall pattern between 6 and 18 MAP. When harvesting time coincided with the early part of the rainy season following a dry season, usually at 12MAP, dry matter % is generally lower. Dry matter % reached its first peak at six MAP and the second peak between the 15 and 18 MAP, beyond which the dry matter % decreased sharply. The trend for starch % and starch yield was closely related to those of dry matter % and dry matter yield respectively throughout plant growth. For most of the cultivars evaluated the maximun or near maximum starch % was already obtained at 6 MAP. Thereafter, it was dependent on harvesting time, genotype an rainfall pattern. Harvest index (HI) showed very little difference as the crop aged. There was no maturity time for cassava because the optimum harvest time of genotypes for all traits depended on the cultivar and rainfall distribution pattern. Thus, optimum harvest time of cassava genotypes has to be determined at the target agroecologies where they will be grown.