@ARTICLE {VAN EK:1997, title = {Quantitative assessment of traditional cropping systems in the Sudan savanna of northern Nigeria II. Management and productivity of major cropping systems}, author = {van Ek, G. A. and Henriet, J and Blade, S. and Singh, B. }, journal = {Samaru Journal of Agricultural Research}, year = {1997}, volume = {14}, pages = {47--60}, }