Upland rice varieties for smallholder farming in the cold conditions in Madagascar's tropical highlands

Upland rice cropping has become a familiar part of the landscape in Madagascar's central highlands, a densely populated region mainly characterized by resource poor family farmers and cold climatic conditions. A survey of 485 farmers conducted in 2011-2012 revealed that 71% of them cultivated upland rice although the crop was absent in this region before the first cold tolerant varieties were released in 1995. It also revealed that a single variety, Chhomrong Dhan (CD) originating from Nepal and released in 2006 was by far the most widely cultivated variety and accounted for 82.5% of the total acreage of upland rice. Farmers appreciate CD for its relatively high yield under low input conditions. However, this dominance raises concerns about the resilience of upland rice agro-systems in the face of changes in climatic conditions or in pathogen populations. To identify key varietal adaptive traits that should be taken into account in the breeding of new upland rice varieties in this context, we compared a panel of improved varieties including CD under low input management over two cropping seasons. A strong positive correlation between leaf area index (LAI) before harvest and yield was observed. Varieties with the highest yield, including CD, are of long duration, produce high biomass yields, have a high LAI and the highest harvest index. A significant negative correlation was observed between the LAI of rice varieties and that of weeds, showing that a high LAI also helps limit competition for resources by weeds. In the context of subsistence family farming and in the cold conditions of Madagascar's tropical highlands, new improved varieties should combine high LAI, high harvest index and long duration, as these traits help compete with weeds and contribute to high yield potential under low input management, on the one hand, and resistance to blast, cold tolerance and reduced crop duration, traits that prevent yield losses, on the other hand. The trade-off between crop duration and the risk of cold-induced sterility specific to the cold conditions of Madagascar highlands has to be taken into account. (Résumé d'auteur