A survey of rice - field weeds in Ratnapura and Kurunegala districts of Sri Lanka

A survey of rice-field weeds was carried out in the Districts of Ratnapura and Kurunegala during the 'Yala' and 'Maha' seasons of 1986. Two hundred and thirty four fields of the Ratnapura District and 196 fields of the Kurunegala District were sampled at a time when the rice was at heading to flowering and farmers had carried out weeding or herbicide applications for weed control. One hundred and forty seven species of weeds belonging to 95 genera and 37 plant families were recorded from the two districts, indicating a very rich diversity in the weed-flora. Of these, 38 species occurred in less than 10% of the fields; 65 species in 10-20% of the fields and 44 species in 20% or more fields. By far the commonest and problematic weeds were monocotylcdons of the families Poaceae (Gramineae) and Cyperaceae. Three grass species Ecbinochloa crus-galli (L.) Beauv., Echinochloa cotanurn (L.) Link. and Ischaemum rugousm Salisb. emerged as the most abundant species in both districts, both in terms of frequency of occurrence and levels of infestation. Fimbristylis miliacea (L.)Vahl, Cyperus pilosus Vahl and Cyperus iria L. of the Cyperaceae were also very common in both districts. Dicotyledonous weeds, although common in occurrence, generally did not present major problems to farmers, except Ludwigia byssopifolia (G. Don) Exell in the Ratnapura District. No major differences in the weed-flora of the two Districts were seen, despite the fact that the climate in Kurunegala was much drier than in Ratnapura. Farmer interviews during the survey revealed that the farmer. populations in the areas surveyed lacked adequate knowledge of weeds, problems caused by weeds and effective means of weed control.