SURVEY OF HERBICIDE-RESISTANT ITALIAN RYEGRASS IN OKLAHOMA. Jon-Joseph Armstrong*, Mark C. Boyles, Joshua A. Bushong, Amanda E. Stone, Thomas F. Peeper; Oklahoma State University, Stillwater, OK (56)


Italian ryegrass (Lolium multiflorum) is one of the most widespread and difficult-to-control weeds in winter wheat production in Oklahoma.  In recent years, Oklahoma winter wheat producers have been noticing a lack of control of Italian ryegrass with ALS inhibitor herbicides where they previously had satisfactory control.  To address the issue of potential herbicide-resistance, seed samples from 300 Italian ryegrass populations were collected from winter wheat fields in Oklahoma in 2008 and 2009 and screened in the field for resistance with nine herbicides (chlorsulfuron + metsulfuron, mesosulfuron, pyroxsulam, imazamox, flufenacet + metribuzin, diclofop-methyl, pinoxaden, quizalofop P-ethyl, clethodim, and glyphosate) representing five modes of action (ALS inhibitor, shoot growth inhibitor + PSII inhibitor, ACCase inhibitor, and aromatic amino acid synthesis inhibitor).  Standard field use rates for each herbicide were used.  Visual estimates of weed control were collected and populations were characterized as “susceptible” (80-100% control), “suppressed” (51-79% control), or “resistant” (50% control).  In 2008 and 2009, though control varied among individual herbicides, only 28-51% of the populations tested were classified as controlled with ALS inhibitor herbicides, indicating that ALS-resistant Italian ryegrass is prevalent throughout Oklahoma.   All ryegrass populations collected in 2008 and 2009 were susceptible to flufenacet + metribuzin, quizalofop P-ethyl, clethodim, and glyphosate.  Though ACCase-resistant Italian ryegrass is not thought to be present in Oklahoma, three populations collected in 2009 were controlled at less than 50% with diclofop-methyl and pinoxaden.  Herbicide-resistance testing efforts will continue to monitor the development and spread of ACCase- and glyphosate-resistant Italian ryegrass.