cultivation systems manihot esculenta starch crops 38 productivity 30 intercropping 27 root productivity 25 economics 22 cultivation 21 production 21 cultivars 18 productividad de raices 18 producción 16 cultivos intercalados 15 marketing 15 technology 13 cereals 12 development 12 maize 12 planting 12 sistemas de cultivo 12 variedades 11 costs 10 research 10 income 9 maiz 9 spacing 9 trade 9 cultivo 8 siembra 8 uses 8