Weed dominance and status of herbicide-resistance in Ischaemum rugosum in direct-seeded rice in low country dry zone

Weed species-dominance significantly contributes to yield losses in Direct-Seeded Rice (DSR) production systems. This study aimed at determining the dominance nature of weeds and development of herbicides resistance in I. rugosum populations found in paddy fields. Studies were carried out in 40 farmer fields at the Hambantota district in Sri Lanka during 2017/2018 Maha (wet season; October to February) and 2018 Yala (dry season; March-September) seasons using 5 m² sampling size per field. Data were collected from each site at 8 weeks after crop establishment. The contribution of each weed species to the weed community in paddy fields was determined using the relative weed density and relative dry weight. Dose responses experiments were conducted using I. rugosum against the pre-mix herbicide formulations; Bispyribac sodium 40 g/1 + Metamifop 100 g/l SC, Clomazone 200 g + Propanil 400 g/l and Thiobencarb 400 g/l +Propanil 200 g/l EC. Results showed that the C4 grasses dominated the paddy fields in both seasons, and they were more productive in the relatively dry Yala season compared to that of wet-Maha season. The C4 grasses E. crus-galli and I. rugosum were the dominant weeds in 2017/18 Maha and 2018 Yala Season, respectively. Three I. rugosum populations exhibited possible multiple resistance development to the pre-mix herbicide formulation Bispyribac sodium 40 g/1 + Metamifop 100 g/1 SC, containing herbicides with two different modes of action (ALS inhibitor and ACCase inhibitor, respectively), showing a resistance index (RI) of 1.8, 2.8 and 1.68. Continuous use of herbicide influences the level of dominance of weed species in DSR and hence, weed management practices should be implemented based on the nature of weed dominance.