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 the Pacific Northwest states of Washington, Idaho, and Oregon. Winter annual grass weeds such as jointed goatgrass (Aegilops cylindrica), downy brome (Bromus tectorum), feral rye (Secale cereale), wild oat (Avena fatua) and Italian ryegrass (Lolium multiflorum) have the same life-cycle as winter wheat and are difficult to control in conventional wheat production systems. These weeds annually account for millions of dollars of lost wheat production and reduced quality (i.e. discount by impurities). There has been only moderate success in controlling winter annual grasses in wheat by utilizing multiple-year crop rotations with spring crops and fallow periods, and with chemical control. Selective herbicides have been available for chemical control of downy brome, Italian ryegrass, and wild oat. However, before use of imazamox herbicide with imidazolinone-resistant (CLEARFIELD^*) wheat, there was no herbicide that could selectively control jointed goatgrass, feral rye, or volunteer cereals in winter wheat. The first commercial release of an imidazolinone-resistant winter wheat variety in the Pacific Northwest was made during the 2003 growing season. Plant breeders in the Pacific Northwest are continuing to develop imidazolinone-resistant winter wheat varieties adapted to a range of Pacific Northwest production regions One issue of concern for wheat varietal development is that single-gene tolerance to imazamox in CLEARFIELD^* varieties can sometimes show visible crop injury, and possibly, yield reductions in response to herbicide applications. Crop tolerance can vary with time of herbicide application relative to wheat stage of growth, environmental conditions that reduce the wheat plant’s ability to metabolize imazamox and, possibly, with specific wheat varieties. A multiple year and location study 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-gene CLEARFIELD^* varieties differed slightly in their relative tolerance to imazamox. The most important determinant of crop tolerance was related to herbicide application rate and timing