Collective management of cropping systems is common in tropical countries when one production factor (e.g. machinery or an irrigated scheme) is shared by a number of farmers. To achieve their technical objectives in these contexts, farmers have to co-ordinate decision-making processes among themselves and with their economic partners. A 3-year study was carried out on two irrigated schemes in the Senegal river delta, its aim was to understand (1) the problems farmers' organizations managing schemes faced when carrying out annual double cropping of rice and (2) how they managed to co-ordinate the different actors (individual farmers, contractors and collective organizations) interacting on these schemes. The results presented here relate mainly to the harvest part of the problem. They show double cropping success varies from one site to another and from one year to another, depending on different starting dates and global harvest performances. The comprehensive model proposed to explain this diversity includes (1) analysing individual farmers' and contractors' decision-making processes indicating the uncertainty of their behaviour and its effects on plot conditions (maturity and trafficability) and machiner), performances and (2) classifying collective co-ordination processes under three main strategies of contractualizing relations with local contractors by granting harvest monopolies, simplifying complex decisions such as choosing a harvest starting date at scheme level and adjusting to unforseen Events during task operation, mainly by looking for extra combine harvesters. The efficiency of these strategies is analysed in relation to the structural characteristics of the two schemes. In our discussion we propose a general framework to explain the co-ordination problems met by farmers in this context including lack of experience, diversity of individual behaviour and uncertainty. Some suggestions are put forward to improve and accelerate the organizational learning processes already acquired by farmers, in terms of technical references, scheme design and modelling. (C) 1998 Elsevier Science Ltd. All rights reserved.