| Abstract: | Experiments were conducted to (i) evaluate the efficacy of propanil formulations available in Sri Lanka in controlling Echinochloa crus‐galli; (ii) study the seedling growth of propanil‐resistant (R) and ‐susceptible (S) biotypes of the weed under different temperatures; (iii) quantify the level of resistance in R biotypes and; (iv) to suggest alternative control measures for R biotypes. Field studies showed that retail propanil formulations (36% a.i., EC) applied at 2.7 kg a.i. ha?1 gave less than 30% control of E. crus‐galli collected from several locations of the north dry zone of Sri Lanka. Chemical analysis revealed that there was no adulteration of propanil formulations at the retailer level. Growth studies conducted in controlled environments indicated that per cent germination and seedling growth of R and S biotypes were similar at the day/night temperature regimes imposed. However, per cent germination for plants grown under a 34/31°C (day/night) regime was 27–29% higher compared to those grown at 28/24°C. At the higher temperature regime, R and S biotypes reached the 2–3 leaf stage five days earlier, and the 4–5 leaf stage seven days earlier. The ED50 values from the dose–response experiments indicated that the R biotype was four times more resistant to propanil than susceptible ones. The resistance index (RI) did not vary significantly under different temperature regimes. Quinclorac (25% a.i., SC) applied at 200 g a.i. ha?1 and bispyribac‐sodium (10% a.i., SC) applied at 30 g a.i. ha?1 (recommended dosages) successfully controlled propanil‐resistant biotypes of E. crus‐galli. Conversely, oxadiazon and propanil (8% and 23% a.i., EC, respectively) applied at 280 + 805 g a.i. ha?1 did not result in satisfactory control. |
