PERFORMANCE OF CONTRASTING RICE CULTIVARS SELECTED FOR RAIN-FED 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 growi ng conditions. Two experiments were conducted with very infertile, san dy soils in Northeast Thailand to determine if cultivars identified to be drought resistant under favourable soil conditions can also produc e higher yield than drought susceptible cultivars under poor soil envi ronments. Eight or six contrasting cultivars were grown with or withou t manure application under rainfed and irrigated lowland conditions. L eaf nitrogen concentration at flowering was mostly less than 1% even w ith manure application, indicating that the crops in all treatments ex perienced severe nitrogen deficiency during the later growth stages. T ime to flowering was reduced in most cultivars when manure or irrigati on was applied. When no manure was applied, grain yield was less than 800 kg ha(-1) and there were no significant cultivar differences in ei ther experiment. Application of manure at 6.25 t ha(-1) increased yiel d by up to 900 kg ha(-1), even when the plants were water stressed dur ing later growth stages. Also, there were significant cultivar differe nces with early flowering cultivars producing higher yield under both rainfed and irrigated conditions when manure was applied. Irrigation i ncreased yield by up to 500 kg ha(-1), both with and without manure ap plication. Total plant dry matter at maturity responded strongly to ma nure application and slightly to irrigation, but cultivar differences were rather small. Cultivar differences in grain yield were thus mostl y associated with differences in harvest index. It is concluded that d rought resistant cultivars may not express their potential advantages when plant growth is severely limited by adverse soil conditions and y ield level is less than 1000 kg ha(-1). Selection for drought resistan ce should be conducted in areas where soil conditions, particularly fe rtility are representative of the region and genotypes should be compa red for drought resistance within each phenology group. Early maturing cultivars are suitable for the drought prone environments with low so il fertility particularly if the adverse effect of very low soil ferti lity is reduced.