Altered acetyl‐coenzyme A carboxylase confers resistance to clethodim, fluazifop and sethoxydim in Setaria faberi and Digitaria sanguinalis

Summary Populations of Setaria faberi and Digitaria sanguinalis cross‐resistant to sethoxydim and fluazifop‐P‐butyl were identified in a vegetable cropping system in Wisconsin, USA, in 1991 and 1992 respectively. Experiments were conducted with partially purified acetyl‐CoA carboxylase (ACCase) to determine whether resistance to sethoxydim and other ACCase inhibitors in S. faberi and D. sanguinalis resulted from altered enzyme activity. Based on I50 values (the herbicide dose that inhibited ACCase activity by 50% compared with untreated ACCase), ACCase of the resistant accession of S. faberi was 4.8‐, 10.6‐ and 319‐fold resistant to clethodim, fluazifop‐P acid and sethoxydim, respectively, compared with that of the susceptible accession. Similarly, ACCase of the resistant accession of D. sanguinalis was 5.8‐, 10.3‐ and 66‐fold resistant to clethodim, fluazifop‐P acid and sethoxydim respectively. These results indicated that resistance to ACCase inhibitors in these accessions of S. faberi and D. sanguinalis resulted from an altered ACCase enzyme that confers a very high level of resistance to sethoxydim.