Conservation agriculture is defined by three main principles: minimum soil disturbance, permanent soil cover and crop rotations. CA is promoted as a promising technology for Africa, but to date, only a small area under CA fully complies with the above three principles. CA has both short and long term effects on crop productivity and sustainability through the modification of various agroecological functions. These functions are related to the quantity of crop and cover crop biomass produced and kept as mulch. One of the main challenges in designing CA for smallholder farming systems in developing countries is the competing uses for biomass, in particular for feeding livestock. The main difficulties are linking the efficiency of agroecological functions to varying degrees of biomass export, and evaluating the performance of cropping systems at farm level, which is where the decisions are made. In North Cameroon the quantity of biomass produced in the field has been doubled by associating a cover crop with a cereal crop. Part of the biomass was consumed by cattle during the dry season but the quantity of mulch that remained on the ground had a positive impact on the cotton water balance in the driest part of North Cameroon. In the Lake Alaotra region of Madagascar, the soil cover in rice fields under CA can vary, from 30% to 84% even in the same type of field depending on the plant used as cover crop, the quantity of biomass produced and management of the residues. The range is even greater when different kinds of fields are taken into consideration. Of course, the different agroecological functions can be fulfilled to a greater or lesser extent depending on the amount of available biomass and the resulting soil cover. The relationship between the quantity of biomass and soil cover has been calculated for different kinds of residues. We used these relationships to explore the variability of soil cover that could be generated in farmers' fields, and to estimate how muc