[ 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 quinclerac, and the EC50 value 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 ECho values 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 throe biotypes hod high level of resistance to quinclo- rac. EC50 values 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 hod produced resistance to quinclorac. EC50 values of other 10 biotypes were ranged from 98.032 6 to 138. 660 9 g a. i./hm2, and the resistance in- dexes 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.
