A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 2000

World rice production is promoted by herbicides by mitigating rice-weed competition. Recently, the agriculturists have been alarmed by the development of herbicide resistance in barnyard grass (Echinocloa crus-gulli L ), a noxious weed in lowland rice, to propanil, butachlor and thiobencarb. Information survey and laboratory and field experiments were conducted to determine the resistance in barnyard grass [a few biotypes from Pathum Thani, Sing Buri, Lop Buri and Chai Nat provinces and susceptible biotypes from USA, Germany and Lop Buri (Thailand)] for selected rice herbicides (viz. butachlor, molinate, thiobencarb and propanil) using seedling bioassay. The field investigation revealed that size of land holding, water availability, number cropping of seasons, crop establishment method etc. determine the use of herbicide. Biotypes from Pathum Thani, Sing Buri and Chai Nat showed resistance to all herbicides. The concentrations of butachlor, molinate and thiobencarb for maximum inhibition of shoot elongation were greater than that required for corresponding susceptible biotypes with resistant indices above 1.0. The exception was Pathum Thani - 4, which was as susceptible as Lopburi - 5 for molinate. The propanil concentrations from 9,450 to 20,000 mg/L were required to kill seedlings in the field study, and 7,500 mg/L required for the susceptible biotype Germany.