Response of 2, 4-D resistant biotype of Fimbristylis miliacea (L.) Vahl. to 2, 4-D dimethylamine and its distribution in the Muda plain, Peninsular Malaysia

Fimbristylis miliacea (L.) Vahl. is one of the most common annual sedges in direct seeded rice fields in the Muda plain, the largest double cropping area in Peninsular Malaysia. In 1989, a 2, 4-D resistant biotype was observed in a farmer's field in Kampung (Kpg.=village) Gulau where 2, 4-D had seasonally been applied as a single compound since 1975. Response of the resistant plants to 2, 4-D dimethylamine was compared to susceptible plants in a pot experiment (Fig. 1). Resistant plants recovered after the application of 2, 4-D dimethylamine at rates up to 2.78g a. i./m2, equivalent to 32 times the recommended rate, whereas the growth of susceptible plants was strongly affected at a rate of 0.087g a. i./m2, the recommended rate, and completely controlled at 0.17g a. i./m2 (two times the recommended rate) or higher (Fig. 2, Fig. 3). The 50% lethal dose (LD50) of 2, 4-D dimethylamine to resistant plants was 1.89g a. i./m2, which was 29 times higher than the LD50 to susceptible plants. Resistant plants were not distinguishable from susceptible plants in morphological characters of panicle size, spikelet length or form of nut (Table 1). Distribution of resistant plants, detected by treatment of 2, 4-D dimethylamine at a rate of 0.17g a. i./m2, was investigated in 1993 using soil samples from 100 rice fields which were heavily infested with F. miliacea in the Muda plain; resistant plants were detected in five rice fields, but not in the other 95 fields (Fig. 4). It was suggested that 2, 4-D resistant F. miliacea had not dominated in that area. Proportion of resistant plants in individual fields ranged between 0 to 96% in Kpg. Gulau which was not correlated with the degree of F. miliacea infestation (Fig. 5). Proportion of resistant plants decreased from 85.6% in 1992 to 1.7% in 1994 in a monitored field where herbicide 2, 4-D had not been applied since 1992.