EFFECTS OF EDAPHIC FACTORS AND AGRONOMIC PRACTICES ON CYANIDE CONTENT (BITTERNESS) IN IMPROVED CASSAVA VARIETIES IN COASTAL KENYA

Abstract

assava is a staple food for over 500 million people in the tropics, Latin America and Asia. The crop is among the major security food crops in coastal Kenya. Kenya Agricultural Research Institute (KARI) researchers bred cassava for high yields and disease tolerance. The varieties were tested in Kwale, Malindi, Kilifi and Lamu between the years 2006 and 2008. However, the adoption of the improved cassava varieties was threatened by bitter taste in the roots and their potential toxicity. A study was set up in Kwale County between the years 2015 and 2018 involving a survey and field experimentations. The survey was conducted in Matuga and Msambweni sub-counties to assess farmers’ perception on improved cassava varieties. A total of 300 farmers were selected through purposive randomized sampling procedure and interviewed using a structured questionnaire. The objectives of field experimentation were to evaluate the effects of agronomic practices and soil-related variables on bitterness in cassava roots. The field experimentations were conducted in Tsimba and Waa locations using Randomized Complete Block Design (RCBD) with four replications. The agronomic practices studied were weeding, age of cassava and cassava varieties while physical and chemical soil characteristics were soil temperatures, moisture, sodium, potassium and pH. Cassava varieties planted were Kibandameno (Control), KME-08-06 (Nzalauka), KME-08-05 (Karembo) and KME-08-02 (Tajirika). Samples of cassava root tissues were collected from plots for analysis at Government Chemist laboratories in Mombasa. Weeding levels were: no weeding, moderate weeding and intensive weeding while age levels were: 6, 8, 10 and12 months after planting. The data was analyzed using Statistical Package for the Social Sciences (SPSS) and GenStat software. Results revealed that 81% of cassava farms were in the coconut-cassava agro-ecological zone (CL3), 21% of farmers chose a variety based on sweetness, 43% choice a variety based on early maturity, 76% preferred Kibandameno while 60% were subsistence and commercial farms. Weeding, soil temperature, moisture, sodium, potassium, variety, age of cassava after planting were significant (P ≤ 0.05). Cyanide in cassava decreased with increased soil temperature, moisture and weeding frequency. The interaction effects of cassava variety by xxi weeding, variety by plant age, soil texture by plant age, soil texture by weeding regime were significant (P ≤ 0.05). Effects of weeding on cassava root cyanide were variety dependent. Regression and correlation analyses revealed that increasing soil potassium significantly decreased cyanide in cassava roots. Cassava variety KME-08-05 (Karembo) had lower root cyanide content at 6 months after planting while variety KME-08-06 (Nzalauka) had higher root cyanide content at the same age. It was important to establish the right age of harvesting, weeding frequency and suitable agronomic practices for good quality cassava. Farmers need be sensitized on cassava processing techniques to minimize bitterness prior to consumption. Further research was necessary to evaluate the effects of a wider range of soil textures on cyanide in cassava roots. Breeders need to develop cassava varieties that have favorably low root cyanide content irrespective of plant age. Key wards: Cassava; Cyanogens; Linamarin; Lotaustralin; Linamarase; Bitterness