The influence of agro-ecological zones on growth, yield and accumulation of cyanogenic compounds in cassava (Manihot esculenta Crantz)
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The influence of agro-ecological zones on growth, yield and accumulation of cyanogenic compounds in cassava (Manihot esculenta Crantz) was studied in Nigeria during 1994/95 and 1995/96 cropping seasons. Five cassava genotypes were evaluated at the Sudan savanna (Minjibir), southern Guinea savanna (Mokwa) and forest-savanna transition (lbadan) agro-ecological zones. Plants were analyzed for several physiological, growth and development, and root quality parameters at 4, 6, 8, 10 and 12 months after planting. Cassava at the wetter agro-ecological zones, Mokwa and Ibadan had significantly higher crop and relative growth rates, leaf area, number and yield of tuberous roots, starch yield and bulking rate than those at Minjibir, the drier zone. In contrast, plants at Minjibir had higher cyanogenic potential than those at Ibadan. The agro-ecological zone effect seems to be an important factor in determining these parameters. Therefore, cassava production should be promoted in the wetter zones where the least toxic and high yielding roots are likely to be produced. Crop and relative growth rates, number of tuberous roots, bulking rate, starch yield and numbers of leaves per plant were highly positively correlated to root yields and negatively correlated to root cyanogenic potential. Leaf production and leaf cyanogenic potential were positively correlated to root cyanogenic potential. That would mean, genotypes with low leaf cyanogenic potential and high growth rates could be bred and selected for low root cyanogenic potential and high yields at the drier agroecological zone. Roots sugar content and cyanogemc potential were positively correlated, negating the association of sweet cassava roots with low cyanogenic potential. Selection for safe genotypes should thus be based on cyanide analysis and not on the taste or sugar content as that could be misleading and possibly fatal upon consumption. The genotypes under study matured at 8 months after planting. It is therefore recommended that a crop of these genotypes be harvested during this period when the highest yields and least toxic roots were produced. lITA seems to have been developing high yielding cassava genotypes for adaptation to dry agro-ecological zones. On the other hand, farmers appear to have been selecting genotypes with lo,w root cyanogenic potential and high root dry matter content for wetter zones. Therefore, in addition to high yields and drought tolerance, cassava genotypes with low root cyanogenic potential, high dry matter content and other root qualities desired by farmers should be developed. This study has demonstrated that a safe cassava genotype can change to be a toxic genotype with the change in agro-ecological zone. In that case, agro-ecological zone trials are recommended in making agro-ecological zone-specific classification and recommendation of cassava genotypes with particular levels of root cyanogenic potential.