manihot esculenta research starch crops 41 investigación 35 productivity 27 cultivation 25 field experiments 22 development research 21 development 20 root productivity 20 investigacion para el desarrollo 19 economics 18 cultivars 16 variedades 16 production 15 producción 14 marketing 12 productividad de raices 12 pests 11 cultivation systems 10 fitomejoramiento 10 uses 10 agricultores 9 farmers 9 mercadeo 9 planting 9 technology 9 timing 9 cassava products 8 composition 8