Effects of fallow type and farmer's practices on weed infestation in an upland rice based swidden system in northern Thailand

Broadleaved pantropical annual herbs were the dominant weeds found during an on-farm agronomic survey in first and second year upland rice fields, regardless of the previous fallow type, burning intensity or soil preparation technique. Weed communities did not show distinct differences in species composition after clearing and upland rice cultivation, the majority of the plots being dominated by Ageratum conyzoides L. Grasses never represented more than 20% of the weed population after a rice cropping cycle, but could become dominant in corn fields and fallows. The growth of broadleaved annuals was suppressed during the fallow period, but seeds remained viable in the soil seed bank. Perennial grasses were almost absent in bamboo fallows, but prominent in grassshrub areas and open canopy woodland. If the latter fallow types were opened for upland rice cultivation, deep hoe tillage proved effective to control the grasses, whereas infestation by broadleaved herbs was not affected by soil preparation. This observation was confirmed by a simultaneously conducted on-station trial. Burning decreased the number of broadleaved annuals significantly in bamboo clearings. Prolific growth of Ageratum could be drastically reduced by salt (NaCl) spraying, a local innovation as alternative to the burden of hand weeding. Dense stands of early maturing rice varieties had lower weed infestation levels at panicle initiation than sparse stands of late maturing rice varieties. A sequential red kidney bean crop (Phaseolus vulgaris) led to fewer weeds in the dry season fallow period. (Résumé d'auteur