Resistance to very-long-chain fatty-acid (VLCFA)- inhibiting herbicides in multiple field-selected rigid ryegrass( Lolium rigidum) populations

Abstract

Five populations of rigid ryegrass (LoliumrigidumGaudin) fromfields across cropping regions in southern Australia were suspected of having resistance to thiocarbamates, chloroacetamides, and sulfonylisoxazoline herbicides. Resistant (R) populations 375-14, 198-15, 16.2, EP162, RAC1, and A18 and two susceptible (S) populations (SLR4 and VLR1) were included in a dose–response study. All suspected R populations expressed resistance to one or all herbicides (thiocarbamates, chloroacetamides, and pyroxasulfone). Population 198-15 exhibited the highest LD50 to triallate (44.7-fold), prosulfocarb (45.7-fold), S-metolachlor (31.5-fold), and metazachlor (27.2-fold) compared with the S populations. Populations 198-15 and 375-14 were also resistant to pyroxasulfone (13.5- and 14.9-fold) compared with the S populations, as was population EP162. This study documents the first case of field-evolved resistance to thiocarbamate, chloroacetamide, and sulfonylisoxazoline herbicides in L. rigidum.