Effects of fertiliser application and irrigation on grain yield of rice cultivars grown under lowland conditions

Two lowland experiments were conducted to determine the effects of fertiliser application and irrigation on grain yield of six contrasting rice cultivars at Sanpatong, Northern Thailand. These cultivars were also compared in three other experiments to determine genotype-by-environment interaction for yield of lowland rice. Without fertiliser application and irrigation, mean yield in each experiment was 1.3-1.6 t/ha. Yield responded to application of both farmyard manure and chemical fertiliser, and the effect of fertiliser was greater in rainfed than irrigated conditions. Despite no obvious water-stress symptoms in rainfed conditions, where free water level was sometimes below the soil surface, the yield almost doubled with irrigation, and the effect of irrigation was greater when no fertiliser was applied. Among the 13 environments where the same four cultivars were grown, mean grain yield varied from just over 1.0 to 4.5 t/ha and significant genotypic variation was obtained in eight environments. Generally early-flowering cultivars (IR20 and NSG19) produced lower yield than medium-to late-flowering cultivars (IR57514-PMI-5-B-1-2 and KDML105). Late-flowering cultivars (E-Pad and Chiangsaen) generally performed better than the early-flowering cultivars. These differences among phenology groups were caused by the fact that the early-flowering cultivars produced lower total plant dry matter and a higher proportion of unfilled grain. There was, however, significant genotype-by-environment interaction. Generally, there were larger differences in yield between cultivars in high-yielding environments where irrigation and fertiliser were applied, whereas the genotypic variation was much smaller in environments where there was no application of irrigation and fertiliser.