Weed Control Using Imazamox in Imidazolinone-Resistant Wheat (Triticum aestivum L.).

     A Clearfield wheat cultivar, AGS CL7, has been developed by the University of Georgia small grain breeding program.  UGA Clearfield wheat varieties are not GMOs because they were developed using traditional breeding techniques.  No foreign DNA was inserted during development.  This process began in 2000 using traditional backcrossing methods with yearly herbicide screening and selections.  In 2007, AGS CL7 was identified for release and seed increase occurred.  Clearfield wheat cultivars are herbicide resistant for use with imazamox (Beyond^®) herbicide.  Imazamox is a member of the imidazolinone family of herbicides, along with imazapic (Cadre^®), imazethapyr (Pursuit^®), and several other herbicides that are used in Clearfield crop production.  Currently Clearfield corn, rice, canola, wheat, and sunflower are cropping systems used in the US.  However no Clearfield wheat cultivars adapted to the southeastern US exist so the use of this technology has not been available to farmers of this region. 
     The introduction of AGS CL7 wheat will soon provide options to farmers to incorporate newer technologies into their cropping system.  Currently little information is available about the spectrum of weeds controlled with imazamox herbicide in wheat for the southeastern US.  Additionally, other ALS herbicides may also be used in Clearfield wheat, but no information is available as to their effect on growth, development, or yield on this cultivar.  In order to evaluate the use of Clearfield wheat systems, field trials that emphasize weed control and yield as a standard to measure potential should be conducted.  Therefore, a study was conducted using AGS CL7 to establish tolerance, weed control, and yield using current wheat herbicides in 2007-2008 and 2008-2009.
     Herbicides treatments were diclofop (Hoelon^®) at 250 or 480 g ai/ha, imazamox at 35 or 70 g/ha, or mesosulfuron (Osprey^®) at 15 or 30 g/ha to wheat at either Feekes stage two (FS2), or three (FS3).  Additional treatments included sequential applications of diclofop at FS2 followed by either imazamox, mesosulfuron, or diclofop at FS3.  The experiment was conducted as a randomized complete block with four replications and included a nontreated control for comparison. Plots were 1.8 wide by 9.0 m long.  All applications were made with a back pack boom sprayer set to deliver 140 L/ha.  Imazamox and mesosulfuron were applied in non-ionic surfactant (0.25 to 0.5%) plus urea ammonium nitrate (5 L/ha).  AGS CL7 wheat injury ratings (chlorosis/stunting) were made periodically after each treatment and prior to harvest.  Italian ryegrass was over seeded at wheat planting to 2 m of each plot at 20 plants/m².
     Both rates of mesosulfuron caused 16 to 20% injury 7 days after treatment (DAT) for the FS2 treatment.  Imazamox at 70 g/ha caused up to 13% injury 20 DAT to FS3 wheat.  However, all injury was transient and was not observable by harvest.
Diclofop at either rate or timing (FS2 or FS3) controlled Italian ryegrass 75% or greater at 195 days after planting (DAP).  In contrast, mesosulfuron and imazamox at either rate or timing controlled Italian ryegrass 60% or less at 195 DAP, which was unacceptable.  Sequential applications of diclofop at FS2 followed by imazamox, mesosulfuron, or diclofop FS3 provided 75% or greater control of Italian ryegrass.  These data indicate that while Italian ryegrass control was acceptable with diclofop, the incidence of diclofop-resistant Italian ryegrass will necessitate the use of herbicides with alternative modes of action.  Imazamox and mesosulfuron have ALS modes of action, but neither in 2007-2008 was efficacious enough to provide season long Italian ryegrass when applied POST at FS2 or FS3.  Thus, combinations of herbicides may be required for acceptable control.
     Wheat yield was indicative of Italian ryegrass control.  Yield was 5580 kg/ha for the nontreated control.  Wheat yield ranged from 5550 to 6020 kg/ha for the imazamox and mesosulfuron treatments applied at FS2 or FS3.  For any treatment containing diclofop either alone, or with a sequential, wheat yielded 5840 to 6190 kg/ha.

     These data indicate that AGS CL7 wheat was tolerant to imazamox, mesosulfuron, and diclofop.  Therefore, the Clearfield system of wheat production could be incorporated into Georgia with this cultivar and others in the future.  Future research should examine the control of wild radish and other winter annuals using the Clearfield system.