Identification of the main constraints for upland rice crop in direct-seeding mulch-based cropping systems under the high altitude conditions of the Madagascar highlands

In the Madagascar highlands, increasing demand for rice combined with increasing land pressure in the lowlands led to the development of upland rice. To tackle the sustainability problem of upland crop production systems, Centre de coopération internationale en recherche agronomique pour le développement (CIRAD, an international governmental organization) and TAFA (an NGO) have developed direct-seeding mulch-based cropping systems (DMC), which not only decrease soil erosion but also increase soil fertility. To understand the mechanisms underlying the performance of upland rice DMC, an experiment was set up in 2003. The yield components of upland rice were studied under high-altitudinal conditions during six rainy seasons from 2003/04 to 2008/09. Treatments compared were two soil management techniques: conventional tillage with removal of most of the crop residues, associated with plowing ('plowing'); and a no-till system with direct seeding under mulch made of crop residues ('no-till'). The rice yields obtained were often better from plowing than from notill, except in the last season. This difference was due to blast disease, which was significantly lower in no-till with low fertilization (best percentage of full grains and better weight of grain). The differences in yields obtained between no-till and plowing were mainly explained by problems of crop installation. The plant densities and plant growth were lower in no-till. This was particularly linked to slower root development in notill. Overall, the biomass production of rotations of rice seemed too low in the highland conditions (low temperature) for the no-till system to be successful in the early years (low soil protection and slower restructuring of the soil). (Résumé d'auteur