[Objective]The paper was to study the resistance of barnyard grass to quinclorac in double-harvest rice area in Hunan Province of China.[Method]Using whole-plant determination method,the resistance of 36 biotypes of barnyard grass to quinclorac in double-harvest rice area in Hunan Province was studied.[Result]The biotype collected from Lijiaping Town of Shaoyang County was the most susceptible barnyard grass to quinclorac,and the EC50value was 70.253 1 g a.i./hm2,so it was recognized as the sensitive biotype.The biotypes collected from Caowei Town of Yuanjiang County(HN3 and HN4 from direct seeding rice fields)and Zhongyukou Town of Nanxian County presented very high resistance to quinclorac;the EC50values were 1 999.122 5,1 664.544 6 and 1 022.739 5 g a.i./hm2,and their resistance indexes reached 28.46,23.69 and 14.56,respectively,indicating that these three biotypes had high level of resistance to quinclorac.EC50values of about 22 biotypes were ranged from 144.837 3 to 408.289 4 g a.i./hm2,and resistance indexes were ranged from 2.06 to 5.81,indicating that these biotypes had produced resistance to quinclorac.EC50values of other 10 biotypes were ranged from 98.032 6 to 138.660 9 g a.i./hm2,and the resistance indexes were ranged from 1.40 to 1.97,indicating that the sensitivity of barnyard grass in these places was decreasing.[Conclusion]The paper provided guidance for scientific and rational use of quinclorac in paddy fields.
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