Herbicide Tolerance in Imidazolinone-Resistant Wheat for Weed Management in the Pacific Northwest U.S.A.

Abstract

Winter wheat ranks high in importance as an agricultural crop in thePacific Northwest states of Washington, Idaho, and Oregon. Winter annualgrass weeds such as jointed goatgrass (Aegilops cylindrica), downy brome(Bromus tcetorum), feral rye (Secale cereale), wild oat (Avena fatua)and Italian ryegrass (Lolium multiflorum) have the same life-cycle aswinter wheat and are difficult to control in conventional wheatproduction systems. These weeds annually account for millions of dollarsof lost wheat production and reduced quality (i.e. discount byimpurities). There has been only moderate success in controlling winterannual grasses in wheat by utilizing multiple-year crop rotations withspring crops and fallow periods, and with chemical control. Selectiveherbicides have been available for chemical control of downy brome,Italian ryegrass, and wild oat. However, before use of imazamoxherbicide with imidazolinone-resistant (CLEARFIELD{*}) wheat, there wasno herbicide that could selectively control jointed goatgrass, feralrye, or volunteer cereals in winter wheat. The first commercial releaseof an imidazolinone-resistant winter wheat variety in the PacificNorthwest was made during the 2003 growing season. Plant breeders in thePacific Northwest are continuing to develop imidazolinone-resistantwinter wheat varieties adapted to a range of Pacific Northwestproduction regionsOne issue of concern for wheat varietal development is that single-genetolerance to imazarnox in CLEARFIELD{*} varieties can sometimes showvisible crop injury, and possibly, yield reductions in response toherbicide applications. Crop tolerance can vary with time of herbicideapplication relative to wheat stage of growth, environmental conditionsthat reduce the wheat plant's ability to metabolize imazamox and,possibly, with specific wheat varieties. A multiple year and locationstudy was conducted to evaluate imazamox tolerance in CLEARFIELD{*}wheat lines being developed for Pacific Northwest production regions.Results indicate that during specific years and locations, single-geneCLEARFIELD{*} varieties differed slightly in their relative tolerance toimazamox. The most important determinant of crop tolerance was relatedto herbicide application rate and timing