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In a 5-year experiment, weed species composition in rice was recorded at different positions along the toposequence, from the rain-fed upland to the irrigated valley bottom. A total of 107 species was recorded, 70 of which occurred in more than 1% of the total of 465 samples. Canonical correspondence analysis was used to explore the changes in weed flora over time in the different ricegrowing environments. There were substantial differences in the composition of weed flora between the differing environments and years. On the upland sites, there was a pronounced shift from perennial grasses, including Imperata cylindrica (L.) Raeuschel and Pennisetum purpureum Schum., to annual grass weeds, such as Digitaria horizontalis Willd., Chloris pilosa Schum. and Eleusine indica (L.) Gaertn. The forbs Boerhavia spp. and Euphorbia heterophylla L. also increased in abundance, whereas others, such as Aspilia bussei Hoffm. & Muschl. and Laggera aurita (L.f.) Sch. Bip. ex C. B. Clarke, declined. The total number of species in the upland site declined from 30 to 11 between 1992 and 1996 compared with 11 to eight species in the same period in the irrigated lowlands. Although the numbers of species recorded in the lowlands were lower and more stable over time, there was an increase in problem grass weeds, such as Echinochloa crus-pavonis (Kunth.) Schult. in the irrigated lowlands and Leersia hexandra Sw. in the rain-fed lowlands.

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