Enhancing the nutritional quality of cassava roots to improve the livelihoods of farmers in marginal agriculture land : Case: Vitamin A - Morillo Coronado, Yacenia; Chávez, Alba L.; Sánchez, T.; Morante, N.; Domínguez García, M.; Tohme, Joseph M.; Fukuda, Wania Maria Gonçalves; Ceballos, H. (International Center for Tropical Agriculture, 2007)