cultivation manihot esculenta starch crops 66 cultivo 47 productivity 34 cultivars 29 root productivity 27 research 25 planting 23 development 22 cultivation systems 21 economics 21 variedades 21 cuttings 17 pests 17 production 16 productividad de raices 16 weeding 16 propagation materials 15 diseases and pathogens 14 harvesting 14 marketing 14 propagation 13 soil requirements 13 spacing 13 timing 13 costs 12 estacas 12 field experiments 12 fitomejoramiento 12