Performance of contrasting rice cultivars selected for rainfed lowland conditions in relation to soil fertility and water availability

Rainfed lowland rice often experiences severe drought and nutritional problems, and cultivars may respond differently to these adverse growing conditions. Two experiments were conducted with very infertile, sandy soils in Northeast Thailand to determine if cultivars identified to be drought resistant under favourable soil conditions can also produce higher yield than drought susceptible cultivars under poor soil environments. Eight or six contrasting cultivars were grown with or without manure application under rainfed and irrigated lowland conditions. Leaf nitrogen concentration at flowering was mostly less than 1% even with manure application, indicating that the crops in all treatments experienced severe nitrogen deficiency during the later growth stages. Time to flowering was reduced in most cultivars when manure or irrigation was applied. When no manure was applied, grain yield was less than 800 kg ha(-1) and there were no significant cultivar differences in either experiment. Application of manure at 6.25 t ha(-1) increased yield by up to 900 kg ha(-1), even when the plants were water stressed during later growth stages. Also, there were significant cultivar differences with early flowering cultivars producing higher yield under both rainfed and irrigated conditions when manure was applied. Irrigation increased yield by up to 500 kg ha(-1), both with and without manure application. Total plant dry matter at maturity responded strongly to manure application and slightly to irrigation, but cultivar differences were rather small. Cultivar differences in grain yield were thus mostly associated with differences in harvest index. It is concluded that drought resistant cultivars may not express their potential advantages when plant growth is severely limited by adverse soil conditions and yield level is less than 1000 kg ha(-1). Selection for drought resistance should be conducted in areas where soil conditions, particularly fertility are representative of the region and genotypes should be compared for drought resistance within each phenology group. Early maturing cultivars are suitable for the drought prone environments with low soil fertility particularly if the adverse effect of very low soil fertility is reduced.